On Tue, 18 Mar 2025, Sasha Levin wrote:

This is a note to let you know that I've just added the patch titled

mm: shmem: skip swapcache for swapin of synchronous swap device

to the 6.13-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...

The filename of the patch is: mm-shmem-skip-swapcache-for-swapin-of-synchronous-sw.patch and it can be found in the queue-6.13 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.

commit 91c40c9fb0e939508d814e1ac302011d8e8213eb Author: Baolin Wang baolin.wang@linux.alibaba.com Date: Wed Jan 8 10:16:49 2025 +0800

mm: shmem: skip swapcache for swapin of synchronous swap device

[ Upstream commit 1dd44c0af4fa1e80a4e82faa10cbf5d22da40362 ] With fast swap devices (such as zram), swapin latency is crucial to applications. For shmem swapin, similar to anonymous memory swapin, we can skip the swapcache operation to improve swapin latency. Testing 1G shmem sequential swapin without THP enabled, I observed approximately a 6% performance improvement: (Note: I repeated 5 times and took the mean data for each test) w/o patch w/ patch changes 534.8ms 501ms +6.3% In addition, currently, we always split the large swap entry stored in the shmem mapping during shmem large folio swapin, which is not perfect, especially with a fast swap device. We should swap in the whole large folio instead of splitting the precious large folios to take advantage of the large folios and improve the swapin latency if the swap device is synchronous device, which is similar to anonymous memory mTHP swapin. Testing 1G shmem sequential swapin with 64K mTHP and 2M mTHP, I observed obvious performance improvement: mTHP=64K w/o patch w/ patch changes 550.4ms 169.6ms +69% mTHP=2M w/o patch w/ patch changes 542.8ms 126.8ms +77% Note that skipping swapcache requires attention to concurrent swapin scenarios. Fortunately the swapcache_prepare() and shmem_add_to_page_cache() can help identify concurrent swapin and large swap entry split scenarios, and return -EEXIST for retry. [akpm@linux-foundation.org: use IS_ENABLED(), tweak comment grammar] Link: https://lkml.kernel.org/r/3d9f3bd3bc6ec953054baff5134f66feeaae7c1e.173630170... Signed-off-by: Baolin Wang baolin.wang@linux.alibaba.com Cc: David Hildenbrand david@redhat.com Cc: "Huang, Ying" ying.huang@linux.alibaba.com Cc: Hugh Dickins hughd@google.com Cc: Kairui Song kasong@tencent.com Cc: Kefeng Wang wangkefeng.wang@huawei.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Ryan Roberts ryan.roberts@arm.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Stable-dep-of: 058313515d5a ("mm: shmem: fix potential data corruption during shmem swapin") Signed-off-by: Sasha Levin sashal@kernel.org

No, NAK to this one going to stable.

Hugh

diff --git a/mm/shmem.c b/mm/shmem.c index e10d6e0924620..6d30139d3967d 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1903,6 +1903,65 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf, return ERR_PTR(error); } +static struct folio *shmem_swap_alloc_folio(struct inode *inode,

• struct vm_area_struct *vma, pgoff_t index,
  

• swp_entry_t entry, int order, gfp_t gfp)
  

+{

• struct shmem_inode_info *info = SHMEM_I(inode);
• struct folio *new;
• void *shadow;
• int nr_pages;
• /*

• * We have arrived here because our zones are constrained, so don't
  

• * limit chance of success with further cpuset and node constraints.
  

• */
  

• gfp &= ~GFP_CONSTRAINT_MASK;
• if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && order > 0) {

• gfp_t huge_gfp = vma_thp_gfp_mask(vma);
  

• gfp = limit_gfp_mask(huge_gfp, gfp);
  

• }
• new = shmem_alloc_folio(gfp, order, info, index);
• if (!new)

• return ERR_PTR(-ENOMEM);
  

• nr_pages = folio_nr_pages(new);
• if (mem_cgroup_swapin_charge_folio(new, vma ? vma->vm_mm : NULL,

• 			   gfp, entry)) {
  

• folio_put(new);
  

• return ERR_PTR(-ENOMEM);
  

• }
• /*

• * Prevent parallel swapin from proceeding with the swap cache flag.
  

• *
  

• * Of course there is another possible concurrent scenario as well,
  

• * that is to say, the swap cache flag of a large folio has already
  

• * been set by swapcache_prepare(), while another thread may have
  

• * already split the large swap entry stored in the shmem mapping.
  

• * In this case, shmem_add_to_page_cache() will help identify the
  

• * concurrent swapin and return -EEXIST.
  

• */
  

• if (swapcache_prepare(entry, nr_pages)) {

• folio_put(new);
  

• return ERR_PTR(-EEXIST);
  

• }
• __folio_set_locked(new);
• __folio_set_swapbacked(new);
• new->swap = entry;
• mem_cgroup_swapin_uncharge_swap(entry, nr_pages);
• shadow = get_shadow_from_swap_cache(entry);
• if (shadow)

• workingset_refault(new, shadow);
  

• folio_add_lru(new);
• swap_read_folio(new, NULL);
• return new;

+}

/*

• When a page is moved from swapcache to shmem filecache (either by the
• usual swapin of shmem_get_folio_gfp(), or by the less common swapoff of

@@ -2006,7 +2065,8 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp, } static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,

• 			 struct folio *folio, swp_entry_t swap)
  

• 			 struct folio *folio, swp_entry_t swap,
  

• 			 bool skip_swapcache)
  

{ struct address_space *mapping = inode->i_mapping; swp_entry_t swapin_error; @@ -2022,7 +2082,8 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index, nr_pages = folio_nr_pages(folio); folio_wait_writeback(folio);

• delete_from_swap_cache(folio);

• if (!skip_swapcache)

• delete_from_swap_cache(folio);
  

  /*
  • Don't treat swapin error folio as alloced. Otherwise inode->i_blocks
  • won't be 0 when inode is released and thus trigger WARN_ON(i_blocks)

@@ -2126,6 +2187,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index, struct shmem_inode_info *info = SHMEM_I(inode); struct swap_info_struct *si; struct folio *folio = NULL;

• bool skip_swapcache = false; swp_entry_t swap; int error, nr_pages;

@@ -2147,6 +2209,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index, /* Look it up and read it in.. */ folio = swap_cache_get_folio(swap, NULL, 0); if (!folio) {

• int order = xa_get_order(&mapping->i_pages, index);
  

• bool fallback_order0 = false;
  

  int split_order;

/* Or update major stats only when swapin succeeds?? */ @@ -2156,6 +2220,33 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index, count_memcg_event_mm(fault_mm, PGMAJFAULT); }

• /*
  

•  * If uffd is active for the vma, we need per-page fault
  

•  * fidelity to maintain the uffd semantics, then fallback
  

•  * to swapin order-0 folio, as well as for zswap case.
  

•  */
  

• if (order > 0 && ((vma && unlikely(userfaultfd_armed(vma))) ||
  

• 		  !zswap_never_enabled()))
  

• 	fallback_order0 = true;
  

• /* Skip swapcache for synchronous device. */
  

• if (!fallback_order0 && data_race(si->flags & SWP_SYNCHRONOUS_IO)) {
  

• 	folio = shmem_swap_alloc_folio(inode, vma, index, swap, order, gfp);
  

• 	if (!IS_ERR(folio)) {
  

• 		skip_swapcache = true;
  

• 		goto alloced;
  

• 	}
  

• 	/*
  

• 	 * Fallback to swapin order-0 folio unless the swap entry
  

• 	 * already exists.
  

• 	 */
  

• 	error = PTR_ERR(folio);
  

• 	folio = NULL;
  

• 	if (error == -EEXIST)
  

• 		goto failed;
  

• }
  

• /*
  • Now swap device can only swap in order 0 folio, then we
  • should split the large swap entry stored in the pagecache

@@ -2186,9 +2277,10 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index, } } +alloced: /* We have to do this with folio locked to prevent races */ folio_lock(folio);

• if (!folio_test_swapcache(folio) ||

• if ((!skip_swapcache && !folio_test_swapcache(folio)) || folio->swap.val != swap.val || !shmem_confirm_swap(mapping, index, swap)) { error = -EEXIST;

@@ -2224,7 +2316,12 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index, if (sgp == SGP_WRITE) folio_mark_accessed(folio);

• delete_from_swap_cache(folio);

• if (skip_swapcache) {

• folio->swap.val = 0;
  

• swapcache_clear(si, swap, nr_pages);
  

• } else {

• delete_from_swap_cache(folio);
  

• } folio_mark_dirty(folio); swap_free_nr(swap, nr_pages); put_swap_device(si);

@@ -2235,8 +2332,11 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index, if (!shmem_confirm_swap(mapping, index, swap)) error = -EEXIST; if (error == -EIO)

• shmem_set_folio_swapin_error(inode, index, folio, swap);
  

• shmem_set_folio_swapin_error(inode, index, folio, swap,
  

• 			     skip_swapcache);
  

unlock:

• if (skip_swapcache)

• swapcache_clear(si, swap, folio_nr_pages(folio));
  

  if (folio) { folio_unlock(folio); folio_put(folio);