The patch titled Subject: mm: shmem: fix potential data corruption during shmem swapin has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-shmem-fix-potential-data-corruption-during-shmem-swapin.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Baolin Wang baolin.wang@linux.alibaba.com Subject: mm: shmem: fix potential data corruption during shmem swapin Date: Tue, 25 Feb 2025 17:52:55 +0800

Alex and Kairui reported some issues (system hang or data corruption) when swapping out or swapping in large shmem folios. This is especially easy to reproduce when the tmpfs is mount with the 'huge=within_size' parameter. Thanks to Kairui's reproducer, the issue can be easily replicated.

The root cause of the problem is that swap readahead may asynchronously swap in order 0 folios into the swap cache, while the shmem mapping can still store large swap entries. Then an order 0 folio is inserted into the shmem mapping without splitting the large swap entry, which overwrites the original large swap entry, leading to data corruption.

When getting a folio from the swap cache, we should split the large swap entry stored in the shmem mapping if the orders do not match, to fix this issue.

Link: https://lkml.kernel.org/r/2fe47c557e74e9df5fe2437ccdc6c9115fa1bf70.174047694... Fixes: 809bc86517cc ("mm: shmem: support large folio swap out") Signed-off-by: Baolin Wang baolin.wang@linux.alibaba.com Reported-by: Alex Xu (Hello71) alex_y_xu@yahoo.ca Reported-by: Kairui Song ryncsn@gmail.com Closes: https://lore.kernel.org/all/1738717785.im3r5g2vxc.none@localhost/ Tested-by: Kairui Song kasong@tencent.com Cc: David Hildenbrand david@redhat.com Cc: Lance Yang ioworker0@gmail.com Cc: Matthew Wilcow willy@infradead.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/shmem.c | 31 +++++++++++++++++++++++++++---- 1 file changed, 27 insertions(+), 4 deletions(-)

--- a/mm/shmem.c~mm-shmem-fix-potential-data-corruption-during-shmem-swapin +++ a/mm/shmem.c @@ -2253,7 +2253,7 @@ static int shmem_swapin_folio(struct ino struct folio *folio = NULL; bool skip_swapcache = false; swp_entry_t swap; - int error, nr_pages; + int error, nr_pages, order, split_order;

VM_BUG_ON(!*foliop || !xa_is_value(*foliop)); swap = radix_to_swp_entry(*foliop); @@ -2272,10 +2272,9 @@ static int shmem_swapin_folio(struct ino

/* Look it up and read it in.. */ folio = swap_cache_get_folio(swap, NULL, 0); + order = xa_get_order(&mapping->i_pages, index); 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?? */ if (fault_type) { @@ -2339,6 +2338,29 @@ static int shmem_swapin_folio(struct ino error = -ENOMEM; goto failed; } + } else if (order != folio_order(folio)) { + /* + * Swap readahead may swap in order 0 folios into swapcache + * asynchronously, while the shmem mapping can still stores + * large swap entries. In such cases, we should split the + * large swap entry to prevent possible data corruption. + */ + split_order = shmem_split_large_entry(inode, index, swap, gfp); + if (split_order < 0) { + error = split_order; + goto failed; + } + + /* + * If the large swap entry has already been split, it is + * necessary to recalculate the new swap entry based on + * the old order alignment. + */ + if (split_order > 0) { + pgoff_t offset = index - round_down(index, 1 << split_order); + + swap = swp_entry(swp_type(swap), swp_offset(swap) + offset); + } }

alloced: @@ -2346,7 +2368,8 @@ alloced: folio_lock(folio); if ((!skip_swapcache && !folio_test_swapcache(folio)) || folio->swap.val != swap.val || - !shmem_confirm_swap(mapping, index, swap)) { + !shmem_confirm_swap(mapping, index, swap) || + xa_get_order(&mapping->i_pages, index) != folio_order(folio)) { error = -EEXIST; goto unlock; } _

Patches currently in -mm which might be from baolin.wang@linux.alibaba.com are

mm-shmem-fix-potential-data-corruption-during-shmem-swapin.patch mm-shmem-drop-the-unused-macro.patch mm-shmem-remove-fadvise-comments.patch mm-shmem-remove-duplicate-error-validation.patch mm-shmem-change-the-return-value-of-shmem_find_swap_entries.patch mm-shmem-factor-out-the-within_size-logic-into-a-new-helper.patch maintainers-add-myself-as-shmem-reviewer.patch