6.16-stable review patch. If anyone has any objections, please let me know.
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From: Kairui Song kasong@tencent.com
commit 5c241ed8d031693dadf33dd98ed2e7cc363e9b66 upstream.
The current swap-in code assumes that, when a swap entry in shmem mapping is order 0, its cached folios (if present) must be order 0 too, which turns out not always correct.
The problem is shmem_split_large_entry is called before verifying the folio will eventually be swapped in, one possible race is:
CPU1 CPU2 shmem_swapin_folio /* swap in of order > 0 swap entry S1 */ folio = swap_cache_get_folio /* folio = NULL */ order = xa_get_order /* order > 0 */ folio = shmem_swap_alloc_folio /* mTHP alloc failure, folio = NULL */ <... Interrupted ...> shmem_swapin_folio /* S1 is swapped in */ shmem_writeout /* S1 is swapped out, folio cached */ shmem_split_large_entry(..., S1) /* S1 is split, but the folio covering it has order > 0 now */
Now any following swapin of S1 will hang: `xa_get_order` returns 0, and folio lookup will return a folio with order > 0. The `xa_get_order(&mapping->i_pages, index) != folio_order(folio)` will always return false causing swap-in to return -EEXIST.
And this looks fragile. So fix this up by allowing seeing a larger folio in swap cache, and check the whole shmem mapping range covered by the swapin have the right swap value upon inserting the folio. And drop the redundant tree walks before the insertion.
This will actually improve performance, as it avoids two redundant Xarray tree walks in the hot path, and the only side effect is that in the failure path, shmem may redundantly reallocate a few folios causing temporary slight memory pressure.
And worth noting, it may seems the order and value check before inserting might help reducing the lock contention, which is not true. The swap cache layer ensures raced swapin will either see a swap cache folio or failed to do a swapin (we have SWAP_HAS_CACHE bit even if swap cache is bypassed), so holding the folio lock and checking the folio flag is already good enough for avoiding the lock contention. The chance that a folio passes the swap entry value check but the shmem mapping slot has changed should be very low.
Link: https://lkml.kernel.org/r/20250728075306.12704-1-ryncsn@gmail.com Link: https://lkml.kernel.org/r/20250728075306.12704-2-ryncsn@gmail.com Fixes: 809bc86517cc ("mm: shmem: support large folio swap out") Signed-off-by: Kairui Song kasong@tencent.com Reviewed-by: Kemeng Shi shikemeng@huaweicloud.com Reviewed-by: Baolin Wang baolin.wang@linux.alibaba.com Tested-by: Baolin Wang baolin.wang@linux.alibaba.com Cc: Baoquan He bhe@redhat.com Cc: Barry Song baohua@kernel.org Cc: Chris Li chrisl@kernel.org Cc: Hugh Dickins hughd@google.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Nhat Pham nphamcs@gmail.com Cc: Dev Jain dev.jain@arm.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/shmem.c | 39 ++++++++++++++++++++++++++++++--------- 1 file changed, 30 insertions(+), 9 deletions(-)
--- a/mm/shmem.c +++ b/mm/shmem.c @@ -884,7 +884,9 @@ static int shmem_add_to_page_cache(struc pgoff_t index, void *expected, gfp_t gfp) { XA_STATE_ORDER(xas, &mapping->i_pages, index, folio_order(folio)); - long nr = folio_nr_pages(folio); + unsigned long nr = folio_nr_pages(folio); + swp_entry_t iter, swap; + void *entry;
VM_BUG_ON_FOLIO(index != round_down(index, nr), folio); VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio); @@ -896,14 +898,25 @@ static int shmem_add_to_page_cache(struc
gfp &= GFP_RECLAIM_MASK; folio_throttle_swaprate(folio, gfp); + swap = radix_to_swp_entry(expected);
do { + iter = swap; xas_lock_irq(&xas); - if (expected != xas_find_conflict(&xas)) { - xas_set_err(&xas, -EEXIST); - goto unlock; + xas_for_each_conflict(&xas, entry) { + /* + * The range must either be empty, or filled with + * expected swap entries. Shmem swap entries are never + * partially freed without split of both entry and + * folio, so there shouldn't be any holes. + */ + if (!expected || entry != swp_to_radix_entry(iter)) { + xas_set_err(&xas, -EEXIST); + goto unlock; + } + iter.val += 1 << xas_get_order(&xas); } - if (expected && xas_find_conflict(&xas)) { + if (expected && iter.val - nr != swap.val) { xas_set_err(&xas, -EEXIST); goto unlock; } @@ -2327,7 +2340,7 @@ static int shmem_swapin_folio(struct ino error = -ENOMEM; goto failed; } - } else if (order != folio_order(folio)) { + } else if (order > folio_order(folio)) { /* * Swap readahead may swap in order 0 folios into swapcache * asynchronously, while the shmem mapping can still stores @@ -2352,15 +2365,23 @@ static int shmem_swapin_folio(struct ino
swap = swp_entry(swp_type(swap), swp_offset(swap) + offset); } + } else if (order < folio_order(folio)) { + swap.val = round_down(swap.val, 1 << folio_order(folio)); + index = round_down(index, 1 << folio_order(folio)); }
alloced: - /* We have to do this with folio locked to prevent races */ + /* + * We have to do this with the folio locked to prevent races. + * The shmem_confirm_swap below only checks if the first swap + * entry matches the folio, that's enough to ensure the folio + * is not used outside of shmem, as shmem swap entries + * and swap cache folios are never partially freed. + */ folio_lock(folio); if ((!skip_swapcache && !folio_test_swapcache(folio)) || - folio->swap.val != swap.val || !shmem_confirm_swap(mapping, index, swap) || - xa_get_order(&mapping->i_pages, index) != folio_order(folio)) { + folio->swap.val != swap.val) { error = -EEXIST; goto unlock; }