On Wed, 26 Feb 2025, Zi Yan wrote:
This is a preparation patch, both added functions are not used yet.
The added __split_unmapped_folio() is able to split a folio with its mapping removed in two manners: 1) uniform split (the existing way), and 2) buddy allocator like split.
The added __split_folio_to_order() can split a folio into any lower order. For uniform split, __split_unmapped_folio() calls it once to split the given folio to the new order. For buddy allocator split, __split_unmapped_folio() calls it (folio_order - new_order) times and each time splits the folio containing the given page to one lower order.
Signed-off-by: Zi Yan ziy@nvidia.com
Sorry, I'm tired and don't really want to be writing this yet, but the migrate "hotfix" has tipped my hand, and I need to get this out to you before more days pass.
I'd been unable to complete even a single iteration of my "kernel builds on huge tmpfs while swapping to SSD" testing during this current 6.14-rc mm.git cycle (6.14-rc itself fine) - until the last week, when some important fixes have come in, so I'm no longer getting I/O errors from ext4-on-loop0-on-huge-tmpfs, and "Huh VM_FAULT_OOM leaked" warnings: good.
But I still can't get beyond a few iterations, a few minutes: there's some corruption of user data, which usually manifests as a kernel build failing because fixdep couldn't find some truncated-on-the-left pathname.
While it definitely bisected to your folio_split() series, it's quite possible that you're merely exposing an existing bug to wider use.
I've spent the last few days trying to track this down, but still not succeeded: I'm still getting much the same corruption. But have been folding in various fixes as I found them, even though they have not solved the main problem at all. I'll return to trying to debug the corruption "tomorrow".
I think (might be wrong, I'm in a rush) my mods are all to this "add two new (not yet used) functions for folio_split()" patch: please merge them in if you agree.
1. From source inspection, it looks like a folio_set_order() was missed.
2. Why is swapcache only checked when folio_test_anon? I can see that you've just copied that over from the old __split_huge_page(), but it seems wrong to me here and there - I guess a relic from before shmem could swap out a huge page.
3. Doing folio_next() inside the for(;;) is unsafe in those configs which have to look up zone etc, I got an oops from the "new_folio" loop; didn't hit an oops from the "release" loop but fixed that too.
4. While correcting anon versus mapping versus swap_cache, shortened the lines by avoiding origin_folio->mapping and &release->page.
Signed-off-by: Hugh Dickins hughd@google.com --- mm/huge_memory.c | 39 ++++++++++++++++++++------------------- 1 file changed, 20 insertions(+), 19 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 0e45937c0d91..9ce3906672b9 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3612,7 +3612,9 @@ static void __split_folio_to_order(struct folio *folio, int new_order) folio_xchg_last_cpupid(new_folio, folio_last_cpupid(folio)); }
- if (!new_order) + if (new_order) + folio_set_order(folio, new_order); + else ClearPageCompound(&folio->page); }
@@ -3682,7 +3684,9 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, int ret = 0; bool stop_split = false;
- if (folio_test_anon(folio) && folio_test_swapcache(folio)) { + if (folio_test_swapcache(folio)) { + VM_BUG_ON(mapping); + /* a swapcache folio can only be uniformly split to order-0 */ if (!uniform_split || new_order != 0) return -EINVAL; @@ -3750,9 +3754,8 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, * is new_order, since the folio will be worked on in next * iteration. */ - for (release = folio, next = folio_next(folio); - release != end_folio; - release = next, next = folio_next(next)) { + for (release = folio; release != end_folio; release = next) { + next = folio_next(release); /* * for buddy allocator like split, the folio containing * page will be split next and should not be released, @@ -3784,32 +3787,31 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, lru_add_page_tail(origin_folio, &release->page, lruvec, list);
- /* Some pages can be beyond EOF: drop them from page cache */ + /* Some pages can be beyond EOF: drop them from cache */ if (release->index >= end) { - if (shmem_mapping(origin_folio->mapping)) + if (shmem_mapping(mapping)) nr_dropped += folio_nr_pages(release); else if (folio_test_clear_dirty(release)) folio_account_cleaned(release, - inode_to_wb(origin_folio->mapping->host)); + inode_to_wb(mapping->host)); __filemap_remove_folio(release, NULL); folio_put(release); - } else if (!folio_test_anon(release)) { - __xa_store(&origin_folio->mapping->i_pages, - release->index, &release->page, 0); + } else if (mapping) { + __xa_store(&mapping->i_pages, + release->index, release, 0); } else if (swap_cache) { __xa_store(&swap_cache->i_pages, swap_cache_index(release->swap), - &release->page, 0); + release, 0); } } }
unlock_page_lruvec(lruvec);
- if (folio_test_anon(origin_folio)) { - if (folio_test_swapcache(origin_folio)) - xa_unlock(&swap_cache->i_pages); - } else + if (swap_cache) + xa_unlock(&swap_cache->i_pages); + if (mapping) xa_unlock(&mapping->i_pages);
/* Caller disabled irqs, so they are still disabled here */ @@ -3828,9 +3830,8 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, * For buddy allocator like split, the first after-split folio is left * for caller to unlock. */ - for (new_folio = origin_folio, next = folio_next(origin_folio); - new_folio != next_folio; - new_folio = next, next = folio_next(next)) { + for (new_folio = origin_folio; new_folio != next_folio; new_folio = next) { + next = folio_next(new_folio); if (new_folio == page_folio(lock_at)) continue;