From: Zi Yan ziy@nvidia.com

We do not have non power of two pages, using nr is error prone if nr is not power-of-two. Use page order instead.

Signed-off-by: Zi Yan ziy@nvidia.com --- include/linux/memcontrol.h | 4 ++-- mm/huge_memory.c | 3 ++- mm/memcontrol.c | 3 ++- mm/page_alloc.c | 4 ++-- 4 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index aa69ea98e2d8..e06a61ea4fc1 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -1151,7 +1151,7 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm, rcu_read_unlock(); }

-void split_page_memcg(struct page *head, unsigned int nr); +void split_page_memcg(struct page *head, int order);

unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order, gfp_t gfp_mask, @@ -1588,7 +1588,7 @@ void count_memcg_event_mm(struct mm_struct *mm, enum vm_event_item idx) { }

-static inline void split_page_memcg(struct page *head, unsigned int nr) +static inline void split_page_memcg(struct page *head, int order) { }

diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 81a5689806af..3bb003eb80a3 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2512,10 +2512,11 @@ static void __split_huge_page(struct page *page, struct list_head *list, struct address_space *swap_cache = NULL; unsigned long offset = 0; unsigned int nr = thp_nr_pages(head); + int order = folio_order(folio); int i;

/* complete memcg works before add pages to LRU */ - split_page_memcg(head, nr); + split_page_memcg(head, order);

if (PageAnon(head) && PageSwapCache(head)) { swp_entry_t entry = { .val = page_private(head) }; diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 681e7528a714..cab2828e188d 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3414,11 +3414,12 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size) /* * Because page_memcg(head) is not set on tails, set it now. */ -void split_page_memcg(struct page *head, unsigned int nr) +void split_page_memcg(struct page *head, int order) { struct folio *folio = page_folio(head); struct mem_cgroup *memcg = folio_memcg(folio); int i; + unsigned int nr = 1 << order;

if (mem_cgroup_disabled() || !memcg) return; diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 0767dd6bc5ba..d84b121d1e03 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2781,7 +2781,7 @@ void split_page(struct page *page, unsigned int order) for (i = 1; i < (1 << order); i++) set_page_refcounted(page + i); split_page_owner(page, 1 << order); - split_page_memcg(page, 1 << order); + split_page_memcg(page, order); } EXPORT_SYMBOL_GPL(split_page);

@@ -4997,7 +4997,7 @@ static void *make_alloc_exact(unsigned long addr, unsigned int order, struct page *last = page + nr;

split_page_owner(page, 1 << order); - split_page_memcg(page, 1 << order); + split_page_memcg(page, order); while (page < --last) set_page_refcounted(last);

From: Zi Yan ziy@nvidia.com

It sets memcg information for the pages after the split. A new parameter new_order is added to tell the order of subpages in the new page, always 0 for now. It prepares for upcoming changes to support split huge page to any lower order.

Signed-off-by: Zi Yan ziy@nvidia.com --- include/linux/memcontrol.h | 4 ++-- mm/huge_memory.c | 2 +- mm/memcontrol.c | 11 ++++++----- mm/page_alloc.c | 4 ++-- 4 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h index e06a61ea4fc1..1633c00fe393 100644 --- a/include/linux/memcontrol.h +++ b/include/linux/memcontrol.h @@ -1151,7 +1151,7 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm, rcu_read_unlock(); }

-void split_page_memcg(struct page *head, int order); +void split_page_memcg(struct page *head, int old_order, int new_order);

unsigned long mem_cgroup_soft_limit_reclaim(pg_data_t *pgdat, int order, gfp_t gfp_mask, @@ -1588,7 +1588,7 @@ void count_memcg_event_mm(struct mm_struct *mm, enum vm_event_item idx) { }

-static inline void split_page_memcg(struct page *head, int order) +static inline void split_page_memcg(struct page *head, int old_order, int new_order) { }

diff --git a/mm/huge_memory.c b/mm/huge_memory.c index a21921c90b21..106cde74d933 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2516,7 +2516,7 @@ static void __split_huge_page(struct page *page, struct list_head *list, int i;

/* complete memcg works before add pages to LRU */ - split_page_memcg(head, order); + split_page_memcg(head, order, 0);

if (PageAnon(head) && PageSwapCache(head)) { swp_entry_t entry = { .val = page_private(head) }; diff --git a/mm/memcontrol.c b/mm/memcontrol.c index cab2828e188d..93ae37f90c84 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3414,23 +3414,24 @@ void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size) /* * Because page_memcg(head) is not set on tails, set it now. */ -void split_page_memcg(struct page *head, int order) +void split_page_memcg(struct page *head, int old_order, int new_order) { struct folio *folio = page_folio(head); struct mem_cgroup *memcg = folio_memcg(folio); int i; - unsigned int nr = 1 << order; + unsigned int old_nr = 1 << old_order; + unsigned int new_nr = 1 << new_order;

if (mem_cgroup_disabled() || !memcg) return;

- for (i = 1; i < nr; i++) + for (i = new_nr; i < old_nr; i += new_nr) folio_page(folio, i)->memcg_data = folio->memcg_data;

if (folio_memcg_kmem(folio)) - obj_cgroup_get_many(__folio_objcg(folio), nr - 1); + obj_cgroup_get_many(__folio_objcg(folio), old_nr / new_nr - 1); else - css_get_many(&memcg->css, nr - 1); + css_get_many(&memcg->css, old_nr / new_nr - 1); }

#ifdef CONFIG_SWAP diff --git a/mm/page_alloc.c b/mm/page_alloc.c index d537828bc4be..ef559795525b 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2781,7 +2781,7 @@ void split_page(struct page *page, unsigned int order) for (i = 1; i < (1 << order); i++) set_page_refcounted(page + i); split_page_owner(page, order); - split_page_memcg(page, order); + split_page_memcg(page, order, 0); } EXPORT_SYMBOL_GPL(split_page);

@@ -4997,7 +4997,7 @@ static void *make_alloc_exact(unsigned long addr, unsigned int order, struct page *last = page + nr;

split_page_owner(page, order); - split_page_memcg(page, order); + split_page_memcg(page, order, 0); while (page < --last) set_page_refcounted(last);

From: Zi Yan ziy@nvidia.com

To split a THP to any lower order pages, we need to reform THPs on subpages at given order and add page refcount based on the new page order. Also we need to reinitialize page_deferred_list after removing the page from the split_queue, otherwise a subsequent split will see list corruption when checking the page_deferred_list again.

It has many uses, like minimizing the number of pages after truncating a huge pagecache page. For anonymous THPs, we can only split them to order-0 like before until we add support for any size anonymous THPs.

Signed-off-by: Zi Yan ziy@nvidia.com --- include/linux/huge_mm.h | 10 ++-- mm/huge_memory.c | 102 +++++++++++++++++++++++++++++----------- 2 files changed, 81 insertions(+), 31 deletions(-)

diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h index 20284387b841..32c91e1b59cd 100644 --- a/include/linux/huge_mm.h +++ b/include/linux/huge_mm.h @@ -147,10 +147,11 @@ void prep_transhuge_page(struct page *page); void free_transhuge_page(struct page *page);

bool can_split_folio(struct folio *folio, int *pextra_pins); -int split_huge_page_to_list(struct page *page, struct list_head *list); +int split_huge_page_to_list_to_order(struct page *page, struct list_head *list, + unsigned int new_order); static inline int split_huge_page(struct page *page) { - return split_huge_page_to_list(page, NULL); + return split_huge_page_to_list_to_order(page, NULL, 0); } void deferred_split_folio(struct folio *folio);

@@ -297,7 +298,8 @@ can_split_folio(struct folio *folio, int *pextra_pins) return false; } static inline int -split_huge_page_to_list(struct page *page, struct list_head *list) +split_huge_page_to_list_to_order(struct page *page, struct list_head *list, + unsigned int new_order) { return 0; } @@ -397,7 +399,7 @@ static inline bool thp_migration_supported(void) static inline int split_folio_to_list(struct folio *folio, struct list_head *list) { - return split_huge_page_to_list(&folio->page, list); + return split_huge_page_to_list_to_order(&folio->page, list, 0); }

static inline int split_folio(struct folio *folio) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index f8a8a72b207d..619d25278340 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2359,11 +2359,13 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,

static void unmap_folio(struct folio *folio) { - enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD | - TTU_SYNC; + enum ttu_flags ttu_flags = TTU_RMAP_LOCKED | TTU_SYNC;

VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);

+ if (folio_test_pmd_mappable(folio)) + ttu_flags |= TTU_SPLIT_HUGE_PMD; + /* * Anon pages need migration entries to preserve them, but file * pages can simply be left unmapped, then faulted back on demand. @@ -2395,7 +2397,6 @@ static void lru_add_page_tail(struct page *head, struct page *tail, struct lruvec *lruvec, struct list_head *list) { VM_BUG_ON_PAGE(!PageHead(head), head); - VM_BUG_ON_PAGE(PageCompound(tail), head); VM_BUG_ON_PAGE(PageLRU(tail), head); lockdep_assert_held(&lruvec->lru_lock);

@@ -2416,7 +2417,7 @@ static void lru_add_page_tail(struct page *head, struct page *tail, }

static void __split_huge_page_tail(struct page *head, int tail, - struct lruvec *lruvec, struct list_head *list) + struct lruvec *lruvec, struct list_head *list, unsigned int new_order) { struct page *page_tail = head + tail;

@@ -2483,10 +2484,15 @@ static void __split_huge_page_tail(struct page *head, int tail, * which needs correct compound_head(). */ clear_compound_head(page_tail); + if (new_order) { + prep_compound_page(page_tail, new_order); + prep_transhuge_page(page_tail); + }

/* Finally unfreeze refcount. Additional reference from page cache. */ - page_ref_unfreeze(page_tail, 1 + (!PageAnon(head) || - PageSwapCache(head))); + page_ref_unfreeze(page_tail, 1 + ((!PageAnon(head) || + PageSwapCache(head)) ? + thp_nr_pages(page_tail) : 0));

if (page_is_young(head)) set_page_young(page_tail); @@ -2504,7 +2510,7 @@ static void __split_huge_page_tail(struct page *head, int tail, }

static void __split_huge_page(struct page *page, struct list_head *list, - pgoff_t end) + pgoff_t end, unsigned int new_order) { struct folio *folio = page_folio(page); struct page *head = &folio->page; @@ -2512,11 +2518,12 @@ static void __split_huge_page(struct page *page, struct list_head *list, struct address_space *swap_cache = NULL; unsigned long offset = 0; unsigned int nr = thp_nr_pages(head); + unsigned int new_nr = 1 << new_order; int order = folio_order(folio); int i;

/* complete memcg works before add pages to LRU */ - split_page_memcg(head, order, 0); + split_page_memcg(head, order, new_order);

if (PageAnon(head) && PageSwapCache(head)) { swp_entry_t entry = { .val = page_private(head) }; @@ -2531,14 +2538,14 @@ static void __split_huge_page(struct page *page, struct list_head *list,

ClearPageHasHWPoisoned(head);

- for (i = nr - 1; i >= 1; i--) { - __split_huge_page_tail(head, i, lruvec, list); + for (i = nr - new_nr; i >= new_nr; i -= new_nr) { + __split_huge_page_tail(head, i, lruvec, list, new_order); /* Some pages can be beyond EOF: drop them from page cache */ if (head[i].index >= end) { struct folio *tail = page_folio(head + i);

if (shmem_mapping(head->mapping)) - shmem_uncharge(head->mapping->host, 1); + shmem_uncharge(head->mapping->host, new_nr); else if (folio_test_clear_dirty(tail)) folio_account_cleaned(tail, inode_to_wb(folio->mapping->host)); @@ -2548,29 +2555,38 @@ static void __split_huge_page(struct page *page, struct list_head *list, __xa_store(&head->mapping->i_pages, head[i].index, head + i, 0); } else if (swap_cache) { + /* + * split anonymous THPs (including swapped out ones) to + * non-zero order not supported + */ + VM_WARN_ONCE(new_order, + "Split swap-cached anon folio to non-0 order not supported"); __xa_store(&swap_cache->i_pages, offset + i, head + i, 0); } }

- ClearPageCompound(head); + if (!new_order) + ClearPageCompound(head); + else + set_compound_order(head, new_order); unlock_page_lruvec(lruvec); /* Caller disabled irqs, so they are still disabled here */

- split_page_owner(head, order, 0); + split_page_owner(head, order, new_order);

/* See comment in __split_huge_page_tail() */ if (PageAnon(head)) { /* Additional pin to swap cache */ if (PageSwapCache(head)) { - page_ref_add(head, 2); + page_ref_add(head, 1 + new_nr); xa_unlock(&swap_cache->i_pages); } else { page_ref_inc(head); } } else { /* Additional pin to page cache */ - page_ref_add(head, 2); + page_ref_add(head, 1 + new_nr); xa_unlock(&head->mapping->i_pages); } local_irq_enable(); @@ -2583,7 +2599,15 @@ static void __split_huge_page(struct page *page, struct list_head *list, split_swap_cluster(entry); }

- for (i = 0; i < nr; i++) { + /* + * set page to its compound_head when split to non order-0 pages, so + * we can skip unlocking it below, since PG_locked is transferred to + * the compound_head of the page and the caller will unlock it. + */ + if (new_order) + page = compound_head(page); + + for (i = 0; i < nr; i += new_nr) { struct page *subpage = head + i; if (subpage == page) continue; @@ -2617,29 +2641,31 @@ bool can_split_folio(struct folio *folio, int *pextra_pins) }

/* - * This function splits huge page into normal pages. @page can point to any - * subpage of huge page to split. Split doesn't change the position of @page. + * This function splits huge page into pages in @new_order. @page can point to + * any subpage of huge page to split. Split doesn't change the position of + * @page. * * Only caller must hold pin on the @page, otherwise split fails with -EBUSY. * The huge page must be locked. * * If @list is null, tail pages will be added to LRU list, otherwise, to @list. * - * Both head page and tail pages will inherit mapping, flags, and so on from - * the hugepage. + * Pages in new_order will inherit mapping, flags, and so on from the hugepage. * - * GUP pin and PG_locked transferred to @page. Rest subpages can be freed if - * they are not mapped. + * GUP pin and PG_locked transferred to @page or the compound page @page belongs + * to. Rest subpages can be freed if they are not mapped. * * Returns 0 if the hugepage is split successfully. * Returns -EBUSY if the page is pinned or if anon_vma disappeared from under * us. */ -int split_huge_page_to_list(struct page *page, struct list_head *list) +int split_huge_page_to_list_to_order(struct page *page, struct list_head *list, + unsigned int new_order) { struct folio *folio = page_folio(page); struct deferred_split *ds_queue = get_deferred_split_queue(folio); - XA_STATE(xas, &folio->mapping->i_pages, folio->index); + /* reset xarray order to new order after split */ + XA_STATE_ORDER(xas, &folio->mapping->i_pages, folio->index, new_order); struct anon_vma *anon_vma = NULL; struct address_space *mapping = NULL; int extra_pins, ret; @@ -2649,6 +2675,18 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio); VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);

+ /* Cannot split THP to order-1 (no order-1 THPs) */ + if (new_order == 1) { + VM_WARN_ONCE(1, "Cannot split to order-1 folio"); + return -EINVAL; + } + + /* Split anonymous folio to non-zero order not support */ + if (folio_test_anon(folio) && new_order) { + VM_WARN_ONCE(1, "Split anon folio to non-0 order not support"); + return -EINVAL; + } + is_hzp = is_huge_zero_page(&folio->page); VM_WARN_ON_ONCE_FOLIO(is_hzp, folio); if (is_hzp) @@ -2744,7 +2782,13 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) if (folio_ref_freeze(folio, 1 + extra_pins)) { if (!list_empty(&folio->_deferred_list)) { ds_queue->split_queue_len--; - list_del(&folio->_deferred_list); + /* + * Reinitialize page_deferred_list after removing the + * page from the split_queue, otherwise a subsequent + * split will see list corruption when checking the + * page_deferred_list. + */ + list_del_init(&folio->_deferred_list); } spin_unlock(&ds_queue->split_queue_lock); if (mapping) { @@ -2754,14 +2798,18 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) if (folio_test_swapbacked(folio)) { __lruvec_stat_mod_folio(folio, NR_SHMEM_THPS, -nr); - } else { + } else if (!new_order) { + /* + * Decrease THP stats only if split to normal + * pages + */ __lruvec_stat_mod_folio(folio, NR_FILE_THPS, -nr); filemap_nr_thps_dec(mapping); } }

- __split_huge_page(page, list, end); + __split_huge_page(page, list, end, new_order); ret = 0; } else { spin_unlock(&ds_queue->split_queue_lock);

From: Zi Yan ziy@nvidia.com

It is used to test split_huge_page_to_list_to_order for pagecache THPs. Also add test cases for split_huge_page_to_list_to_order via both debugfs, truncating a file, and punching holes in a file.

Signed-off-by: Zi Yan ziy@nvidia.com --- mm/huge_memory.c | 34 ++- .../selftests/mm/split_huge_page_test.c | 225 +++++++++++++++++- 2 files changed, 242 insertions(+), 17 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 619d25278340..ad5b29558a51 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3023,7 +3023,7 @@ static inline bool vma_not_suitable_for_thp_split(struct vm_area_struct *vma) }

static int split_huge_pages_pid(int pid, unsigned long vaddr_start, - unsigned long vaddr_end) + unsigned long vaddr_end, unsigned int new_order) { int ret = 0; struct task_struct *task; @@ -3085,13 +3085,19 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start, goto next;

total++; - if (!can_split_folio(page_folio(page), NULL)) + /* + * For folios with private, split_huge_page_to_list_to_order() + * will try to drop it before split and then check if the folio + * can be split or not. So skip the check here. + */ + if (!folio_test_private(page_folio(page)) && + !can_split_folio(page_folio(page), NULL)) goto next;

if (!trylock_page(page)) goto next;

- if (!split_huge_page(page)) + if (!split_huge_page_to_list_to_order(page, NULL, new_order)) split++;

unlock_page(page); @@ -3109,7 +3115,7 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start, }

static int split_huge_pages_in_file(const char *file_path, pgoff_t off_start, - pgoff_t off_end) + pgoff_t off_end, unsigned int new_order) { struct filename *file; struct file *candidate; @@ -3148,7 +3154,7 @@ static int split_huge_pages_in_file(const char *file_path, pgoff_t off_start, if (!folio_trylock(folio)) goto next;

- if (!split_folio(folio)) + if (!split_huge_page_to_list_to_order(&folio->page, NULL, new_order)) split++;

folio_unlock(folio); @@ -3173,10 +3179,14 @@ static ssize_t split_huge_pages_write(struct file *file, const char __user *buf, { static DEFINE_MUTEX(split_debug_mutex); ssize_t ret; - /* hold pid, start_vaddr, end_vaddr or file_path, off_start, off_end */ + /* + * hold pid, start_vaddr, end_vaddr, new_order or + * file_path, off_start, off_end, new_order + */ char input_buf[MAX_INPUT_BUF_SZ]; int pid; unsigned long vaddr_start, vaddr_end; + unsigned int new_order = 0;

ret = mutex_lock_interruptible(&split_debug_mutex); if (ret) @@ -3205,29 +3215,29 @@ static ssize_t split_huge_pages_write(struct file *file, const char __user *buf, goto out; }

- ret = sscanf(buf, "0x%lx,0x%lx", &off_start, &off_end); - if (ret != 2) { + ret = sscanf(buf, "0x%lx,0x%lx,%d", &off_start, &off_end, &new_order); + if (ret != 2 && ret != 3) { ret = -EINVAL; goto out; } - ret = split_huge_pages_in_file(file_path, off_start, off_end); + ret = split_huge_pages_in_file(file_path, off_start, off_end, new_order); if (!ret) ret = input_len;

goto out; }

- ret = sscanf(input_buf, "%d,0x%lx,0x%lx", &pid, &vaddr_start, &vaddr_end); + ret = sscanf(input_buf, "%d,0x%lx,0x%lx,%d", &pid, &vaddr_start, &vaddr_end, &new_order); if (ret == 1 && pid == 1) { split_huge_pages_all(); ret = strlen(input_buf); goto out; - } else if (ret != 3) { + } else if (ret != 3 && ret != 4) { ret = -EINVAL; goto out; }

- ret = split_huge_pages_pid(pid, vaddr_start, vaddr_end); + ret = split_huge_pages_pid(pid, vaddr_start, vaddr_end, new_order); if (!ret) ret = strlen(input_buf); out: diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c index b8558c7f1a39..cbb5e6893cbf 100644 --- a/tools/testing/selftests/mm/split_huge_page_test.c +++ b/tools/testing/selftests/mm/split_huge_page_test.c @@ -16,6 +16,7 @@ #include <sys/mount.h> #include <malloc.h> #include <stdbool.h> +#include <time.h> #include "vm_util.h"

uint64_t pagesize; @@ -23,10 +24,12 @@ unsigned int pageshift; uint64_t pmd_pagesize;

#define SPLIT_DEBUGFS "/sys/kernel/debug/split_huge_pages" +#define SMAP_PATH "/proc/self/smaps" +#define THP_FS_PATH "/mnt/thp_fs" #define INPUT_MAX 80

-#define PID_FMT "%d,0x%lx,0x%lx" -#define PATH_FMT "%s,0x%lx,0x%lx" +#define PID_FMT "%d,0x%lx,0x%lx,%d" +#define PATH_FMT "%s,0x%lx,0x%lx,%d"

#define PFN_MASK ((1UL<<55)-1) #define KPF_THP (1UL<<22) @@ -113,7 +116,7 @@ void split_pmd_thp(void)

/* split all THPs */ write_debugfs(PID_FMT, getpid(), (uint64_t)one_page, - (uint64_t)one_page + len); + (uint64_t)one_page + len, 0);

for (i = 0; i < len; i++) if (one_page[i] != (char)i) { @@ -203,7 +206,7 @@ void split_pte_mapped_thp(void)

/* split all remapped THPs */ write_debugfs(PID_FMT, getpid(), (uint64_t)pte_mapped, - (uint64_t)pte_mapped + pagesize * 4); + (uint64_t)pte_mapped + pagesize * 4, 0);

/* smap does not show THPs after mremap, use kpageflags instead */ thp_size = 0; @@ -269,7 +272,7 @@ void split_file_backed_thp(void) }

/* split the file-backed THP */ - write_debugfs(PATH_FMT, testfile, pgoff_start, pgoff_end); + write_debugfs(PATH_FMT, testfile, pgoff_start, pgoff_end, 0);

status = unlink(testfile); if (status) @@ -290,20 +293,232 @@ void split_file_backed_thp(void) printf("file-backed THP split test done, please check dmesg for more information\n"); }

+void create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd, char **addr) +{ + size_t i; + int dummy; + + srand(time(NULL)); + + *fd = open(testfile, O_CREAT | O_RDWR, 0664); + if (*fd == -1) { + perror("Failed to create a file at "THP_FS_PATH); + exit(EXIT_FAILURE); + } + + for (i = 0; i < fd_size; i++) { + unsigned char byte = (unsigned char)i; + + write(*fd, &byte, sizeof(byte)); + } + close(*fd); + sync(); + *fd = open("/proc/sys/vm/drop_caches", O_WRONLY); + if (*fd == -1) { + perror("open drop_caches"); + goto err_out_unlink; + } + if (write(*fd, "3", 1) != 1) { + perror("write to drop_caches"); + goto err_out_unlink; + } + close(*fd); + + *fd = open(testfile, O_RDWR); + if (*fd == -1) { + perror("Failed to open a file at "THP_FS_PATH); + goto err_out_unlink; + } + + *addr = mmap(NULL, fd_size, PROT_READ|PROT_WRITE, MAP_SHARED, *fd, 0); + if (*addr == (char *)-1) { + perror("cannot mmap"); + goto err_out_close; + } + madvise(*addr, fd_size, MADV_HUGEPAGE); + + for (size_t i = 0; i < fd_size; i++) + dummy += *(*addr + i); + + if (!check_huge_file(*addr, fd_size / pmd_pagesize, pmd_pagesize)) { + printf("No pagecache THP generated, please mount a filesystem supporting pagecache THP at "THP_FS_PATH"\n"); + goto err_out_close; + } + return; +err_out_close: + close(*fd); +err_out_unlink: + unlink(testfile); + exit(EXIT_FAILURE); +} + +void split_thp_in_pagecache_to_order(size_t fd_size, int order) +{ + int fd; + char *addr; + size_t i; + const char testfile[] = THP_FS_PATH "/test"; + int err = 0; + + create_pagecache_thp_and_fd(testfile, fd_size, &fd, &addr); + + printf("split %ld kB PMD-mapped pagecache page to order %d ... ", fd_size >> 10, order); + write_debugfs(PID_FMT, getpid(), (uint64_t)addr, (uint64_t)addr + fd_size, order); + + for (i = 0; i < fd_size; i++) + if (*(addr + i) != (char)i) { + printf("%lu byte corrupted in the file\n", i); + err = EXIT_FAILURE; + goto out; + } + + if (!check_huge_file(addr, 0, pmd_pagesize)) { + printf("Still FilePmdMapped not split\n"); + err = EXIT_FAILURE; + goto out; + } + + printf("done\n"); +out: + close(fd); + unlink(testfile); + if (err) + exit(err); +} + +void truncate_thp_in_pagecache_to_order(size_t fd_size, int order) +{ + int fd; + char *addr; + size_t i; + const char testfile[] = THP_FS_PATH "/test"; + int err = 0; + + create_pagecache_thp_and_fd(testfile, fd_size, &fd, &addr); + + printf("truncate %ld kB PMD-mapped pagecache page to size %lu kB ... ", + fd_size >> 10, 4UL << order); + ftruncate(fd, pagesize << order); + + for (i = 0; i < (pagesize << order); i++) + if (*(addr + i) != (char)i) { + printf("%lu byte corrupted in the file\n", i); + err = EXIT_FAILURE; + goto out; + } + + if (!check_huge_file(addr, 0, pmd_pagesize)) { + printf("Still FilePmdMapped not split after truncate\n"); + err = EXIT_FAILURE; + goto out; + } + + printf("done\n"); +out: + close(fd); + unlink(testfile); + if (err) + exit(err); +} + +void punch_hole_in_pagecache_thp(size_t fd_size, off_t offset[], off_t len[], + int n, int num_left_thps) +{ + int fd, j; + char *addr; + size_t i; + const char testfile[] = THP_FS_PATH "/test"; + int err = 0; + + create_pagecache_thp_and_fd(testfile, fd_size, &fd, &addr); + + for (j = 0; j < n; j++) { + printf("punch a hole to %ld kB PMD-mapped pagecache page at addr: %lx, offset %ld, and len %ld ...\n", + fd_size >> 10, (unsigned long)addr, offset[j], len[j]); + fallocate(fd, FALLOC_FL_PUNCH_HOLE|FALLOC_FL_KEEP_SIZE, offset[j], len[j]); + } + + for (i = 0; i < fd_size; i++) { + int in_hole = 0; + + for (j = 0; j < n; j++) + if (i >= offset[j] && i < (offset[j] + len[j])) { + in_hole = 1; + break; + } + + if (in_hole) { + if (*(addr + i)) { + printf("%lu byte non-zero after punch\n", i); + err = EXIT_FAILURE; + goto out; + } + continue; + } + if (*(addr + i) != (char)i) { + printf("%lu byte corrupted in the file\n", i); + err = EXIT_FAILURE; + goto out; + } + } + + if (!check_huge_file(addr, num_left_thps, pmd_pagesize)) { + printf("Still FilePmdMapped not split after punch\n"); + goto out; + } + printf("done\n"); +out: + close(fd); + unlink(testfile); + if (err) + exit(err); +} + int main(int argc, char **argv) { + int i; + size_t fd_size; + off_t offset[2], len[2]; + if (geteuid() != 0) { printf("Please run the benchmark as root\n"); exit(EXIT_FAILURE); }

+ setbuf(stdout, NULL); + pagesize = getpagesize(); pageshift = ffs(pagesize) - 1; pmd_pagesize = read_pmd_pagesize(); + fd_size = 2 * pmd_pagesize;

split_pmd_thp(); split_pte_mapped_thp(); split_file_backed_thp();

+ for (i = 8; i >= 0; i--) + if (i != 1) + split_thp_in_pagecache_to_order(fd_size, i); + + /* + * for i is 1, truncate code in the kernel should create order-0 pages + * instead of order-1 THPs, since order-1 THP is not supported. No error + * is expected. + */ + for (i = 8; i >= 0; i--) + truncate_thp_in_pagecache_to_order(fd_size, i); + + offset[0] = 123; + offset[1] = 4 * pagesize; + len[0] = 200 * pagesize; + len[1] = 16 * pagesize; + punch_hole_in_pagecache_thp(fd_size, offset, len, 2, 1); + + offset[0] = 259 * pagesize + pagesize / 2; + offset[1] = 33 * pagesize; + len[0] = 129 * pagesize; + len[1] = 16 * pagesize; + punch_hole_in_pagecache_thp(fd_size, offset, len, 2, 1); + return 0; }

On Mon, 3 Apr 2023, Zi Yan wrote:

From: Zi Yan ziy@nvidia.com

To split a THP to any lower order pages, we need to reform THPs on subpages at given order and add page refcount based on the new page order. Also we need to reinitialize page_deferred_list after removing the page from the split_queue, otherwise a subsequent split will see list corruption when checking the page_deferred_list again.

It has many uses, like minimizing the number of pages after truncating a huge pagecache page. For anonymous THPs, we can only split them to order-0 like before until we add support for any size anonymous THPs.

Signed-off-by: Zi Yan ziy@nvidia.com

...

@@ -2754,14 +2798,18 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) if (folio_test_swapbacked(folio)) { __lruvec_stat_mod_folio(folio, NR_SHMEM_THPS, -nr);

• 	} else {
  

• 	} else if (!new_order) {
  

• 		/*
  

• 		 * Decrease THP stats only if split to normal
  

• 		 * pages
  

• 		 */
  	__lruvec_stat_mod_folio(folio, NR_FILE_THPS,
  				-nr);
  	filemap_nr_thps_dec(mapping);
  }
  

  }

This part is wrong. The problem I've had is /proc/sys/vm/stat_refresh warning of negative nr_shmem_hugepages (which then gets shown as 0 in vmstat or meminfo, even though there actually are shmem hugepages).

At first I thought that the fix needed (which I'm running with) is:

--- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2797,17 +2797,16 @@ int split_huge_page_to_list_to_order(str int nr = folio_nr_pages(folio);

xas_split(&xas, folio, folio_order(folio)); - if (folio_test_swapbacked(folio)) { - __lruvec_stat_mod_folio(folio, NR_SHMEM_THPS, - -nr); - } else if (!new_order) { - /* - * Decrease THP stats only if split to normal - * pages - */ - __lruvec_stat_mod_folio(folio, NR_FILE_THPS, - -nr); - filemap_nr_thps_dec(mapping); + if (folio_test_pmd_mappable(folio) && + new_order < HPAGE_PMD_ORDER) { + if (folio_test_swapbacked(folio)) { + __lruvec_stat_mod_folio(folio, + NR_SHMEM_THPS, -nr); + } else { + __lruvec_stat_mod_folio(folio, + NR_FILE_THPS, -nr); + filemap_nr_thps_dec(mapping); + } } }

because elsewhere the maintenance of NR_SHMEM_THPS or NR_FILE_THPS is rightly careful to be dependent on folio_test_pmd_mappable() (and, so far as I know, we shall not be seeing folios of order higher than HPAGE_PMD_ORDER yet in mm/huge_memory.c - those would need more thought).

But it may be more complicated than that, given that patch 7/7 appears (I haven't tried) to allow splitting to other orders on a file opened for reading - that might be a bug.

The complication here is that we now have four kinds of large folio in mm/huge_memory.c, and the rules are a bit different for each.

Anonymous THPs: okay, I think I've seen you exclude those with -EINVAL at a higher level (and they wouldn't be getting into this "if (mapping) {" block anyway).

Shmem (swapbacked) THPs: we are only allocating shmem in 0-order or HPAGE_PMD_ORDER at present. I can imagine that in a few months or a year-or-so's time, we shall want to follow Matthew's folio readahead, and generalize to other orders in shmem; but right now I'd really prefer not to have truncation or debugfs introducing the surprise of other orders there. Maybe there's little that needs to be fixed, only the THP_SWPOUT and THP_SWPOUT_FALLBACK statistics have come to mind so far (would need to be limited to folio_test_pmd_mappable()); though I've no idea how well intermediate orders will work with or against THP swapout.

CONFIG_READ_ONLY_THP_FOR_FS=y file THPs: those need special care, and their filemap_nr_thps_dec(mapping) above may not be good enough. So long as it's working as intended, it does exclude the possibility of truncation splitting here; but if you allow splitting via debugfs to reach them, then the accounting needs to be changed - for them, any order higher than 0 has to be counted in nr_thps - so splitting one HPAGE_PMD_ORDER THP into multiple large folios will need to add to that count, not decrement it. Otherwise, a filesystem unprepared for large folios or compound pages is in danger of meeting them by surprise. Better just disable that possibility, along with shmem.

mapping_large_folio_support() file THPs: this category is the one you're really trying to address with this series, they can already come in various orders, and it's fair for truncation to make a different choice of orders - but is what it's doing worth doing? I'll say more on 6/7.

Hugh

On 16 Apr 2023, at 15:25, Hugh Dickins wrote:

On Mon, 3 Apr 2023, Zi Yan wrote:

...

From: Zi Yan ziy@nvidia.com

To split a THP to any lower order pages, we need to reform THPs on subpages at given order and add page refcount based on the new page order. Also we need to reinitialize page_deferred_list after removing the page from the split_queue, otherwise a subsequent split will see list corruption when checking the page_deferred_list again.

It has many uses, like minimizing the number of pages after truncating a huge pagecache page. For anonymous THPs, we can only split them to order-0 like before until we add support for any size anonymous THPs.

Signed-off-by: Zi Yan ziy@nvidia.com

...

...

@@ -2754,14 +2798,18 @@ int split_huge_page_to_list(struct page *page, struct list_head *list) if (folio_test_swapbacked(folio)) { __lruvec_stat_mod_folio(folio, NR_SHMEM_THPS, -nr);

• 	} else {
  

• 	} else if (!new_order) {
  

• 		/*
  

• 		 * Decrease THP stats only if split to normal
  

• 		 * pages
  

• 		 */
  	__lruvec_stat_mod_folio(folio, NR_FILE_THPS,
  				-nr);
  	filemap_nr_thps_dec(mapping);
  }
  

  }

This part is wrong. The problem I've had is /proc/sys/vm/stat_refresh warning of negative nr_shmem_hugepages (which then gets shown as 0 in vmstat or meminfo, even though there actually are shmem hugepages).

At first I thought that the fix needed (which I'm running with) is:

--- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2797,17 +2797,16 @@ int split_huge_page_to_list_to_order(str int nr = folio_nr_pages(folio);

	xas_split(&xas, folio, folio_order(folio));

• 	if (folio_test_swapbacked(folio)) {
  

• 		__lruvec_stat_mod_folio(folio, NR_SHMEM_THPS,
  

• 					-nr);
  

• 	} else if (!new_order) {
  

• 		/*
  

• 		 * Decrease THP stats only if split to normal
  

• 		 * pages
  

• 		 */
  

• 		__lruvec_stat_mod_folio(folio, NR_FILE_THPS,
  

• 					-nr);
  

• 		filemap_nr_thps_dec(mapping);
  

• 	if (folio_test_pmd_mappable(folio) &&
  

• 	    new_order < HPAGE_PMD_ORDER) {
  

• 		if (folio_test_swapbacked(folio)) {
  

• 			__lruvec_stat_mod_folio(folio,
  

• 					NR_SHMEM_THPS, -nr);
  

• 		} else {
  

• 			__lruvec_stat_mod_folio(folio,
  

• 					NR_FILE_THPS, -nr);
  

• 			filemap_nr_thps_dec(mapping);
  

• 		}
  }
  

  }

because elsewhere the maintenance of NR_SHMEM_THPS or NR_FILE_THPS is rightly careful to be dependent on folio_test_pmd_mappable() (and, so far as I know, we shall not be seeing folios of order higher than HPAGE_PMD_ORDER yet in mm/huge_memory.c - those would need more thought).

But it may be more complicated than that, given that patch 7/7 appears (I haven't tried) to allow splitting to other orders on a file opened for reading - that might be a bug.

The complication here is that we now have four kinds of large folio in mm/huge_memory.c, and the rules are a bit different for each.

Anonymous THPs: okay, I think I've seen you exclude those with -EINVAL at a higher level (and they wouldn't be getting into this "if (mapping) {" block anyway).

Shmem (swapbacked) THPs: we are only allocating shmem in 0-order or HPAGE_PMD_ORDER at present. I can imagine that in a few months or a year-or-so's time, we shall want to follow Matthew's folio readahead, and generalize to other orders in shmem; but right now I'd really prefer not to have truncation or debugfs introducing the surprise of other orders there. Maybe there's little that needs to be fixed, only the THP_SWPOUT and THP_SWPOUT_FALLBACK statistics have come to mind so far (would need to be limited to folio_test_pmd_mappable()); though I've no idea how well intermediate orders will work with or against THP swapout.

CONFIG_READ_ONLY_THP_FOR_FS=y file THPs: those need special care, and their filemap_nr_thps_dec(mapping) above may not be good enough. So long as it's working as intended, it does exclude the possibility of truncation splitting here; but if you allow splitting via debugfs to reach them, then the accounting needs to be changed - for them, any order higher than 0 has to be counted in nr_thps - so splitting one HPAGE_PMD_ORDER THP into multiple large folios will need to add to that count, not decrement it. Otherwise, a filesystem unprepared for large folios or compound pages is in danger of meeting them by surprise. Better just disable that possibility, along with shmem.

OK. I will disable for these two cases. I will come back to them once I figure the details out.

mapping_large_folio_support() file THPs: this category is the one you're really trying to address with this series, they can already come in various orders, and it's fair for truncation to make a different choice of orders - but is what it's doing worth doing? I'll say more on 6/7.

Hugh

-- Best Regards, Yan, Zi