The patch titled Subject: mm/shmem, swap: improve cached mTHP handling and fix potential hung has been added to the -mm mm-new branch. Its filename is mm-shmem-swap-improve-cached-mthp-handling-and-fix-potential-hung.patch

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------------------------------------------------------ From: Kairui Song kasong@tencent.com Subject: mm/shmem, swap: improve cached mTHP handling and fix potential hung Date: Wed, 18 Jun 2025 02:35:00 +0800

Patch series "mm/shmem, swap: bugfix and improvement of mTHP swap in".

The current mTHP swapin path have several problems. It may potentially hang, may cause redundant faults due to false positive swap cache lookup, and it will involve at least 4 Xarray tree walks (get order, get order again, confirm swap, insert folio). And for !CONFIG_TRANSPARENT_HUGEPAGE builds, it will performs some mTHP related checks.

This series fixes all of the mentioned issues, and the code should be more robust and prepared for the swap table series. Now tree walks is reduced to twice (get order & confirm, insert folio) and added more sanity checks and comments. !CONFIG_TRANSPARENT_HUGEPAGE build overhead is also minimized, and comes with a sanity check now.

The performance is slightly better after this series, sequential swap in of 24G data from ZRAM, using transparent_hugepage_tmpfs=always (36 samples each):

Before: avg: 11.23s, stddev: 0.06 After patch 1: avg: 10.92s, stddev: 0.05 After patch 2: avg: 10.93s, stddev: 0.15 After patch 3: avg: 10.07s, stddev: 0.09 After patch 4: avg: 10.09s, stddev: 0.08

Each patch improves the performance by a little, which is about ~10% faster in total.

Build kernel test showed very slightly improvement, testing with make -j24 with defconfig in a 256M memcg also using ZRAM as swap, and transparent_hugepage_tmpfs=always (6 samples each):

Before: system time avg: 3945.25s After patch 1: system time avg: 3903.21s After patch 2: system time avg: 3914.76s After patch 3: system time avg: 3907.41s After patch 4: system time avg: 3876.24s

Slightly better than noise level given the number of samples.

Two of the patches in this series come from the swap table series [1], and it is worth noting that the performance gain of this series is independent of the swap table series, we'll see another bigger performance gain and reduction of memory usage after the swap table series.

I found these issues while trying to split the shmem changes out of the swap table series for easier reviewing, and found several more issues while doing stress tests for performance comparision. Barry also mentioned that CONFIG_TRANSPARENT_HUGEPAGE may have redundant checks [2] and I managed to clean them up properly too.

No issue is found with a few days of stress testing.

This patch (of 4):

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 the performance, as it avoided 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/20250617183503.10527-1-ryncsn@gmail.com Link: https://lore.kernel.org/linux-mm/20250514201729.48420-1-ryncsn@gmail.com/ [1] Link: https://lore.kernel.org/linux-mm/CAMgjq7AsKFz7UN+seR5atznE_RBTDC9qjDmwN5saMe... [2] Link: https://lkml.kernel.org/r/20250617183503.10527-2-ryncsn@gmail.com Fixes: 058313515d5a ("mm: shmem: fix potential data corruption during shmem swapin") Fixes: 809bc86517cc ("mm: shmem: support large folio swap out") Signed-off-by: Kairui Song kasong@tencent.com Cc: 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: Kemeng Shi shikemeng@huaweicloud.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Nhat Pham nphamcs@gmail.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/shmem.c | 30 +++++++++++++++++++++--------- 1 file changed, 21 insertions(+), 9 deletions(-)

--- a/mm/shmem.c~mm-shmem-swap-improve-cached-mthp-handling-and-fix-potential-hung +++ a/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 +898,24 @@ static int shmem_add_to_page_cache(struc

gfp &= GFP_RECLAIM_MASK; folio_throttle_swaprate(folio, gfp); + swap = iter = radix_to_swp_entry(expected);

do { 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; } @@ -2323,7 +2335,7 @@ 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 @@ -2348,15 +2360,15 @@ 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(swp_type(swap), folio_order(folio)); }

alloced: /* We have to do this with folio locked to prevent races */ 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; } _

Patches currently in -mm which might be from kasong@tencent.com are

mm-shmem-swap-fix-softlockup-with-mthp-swapin.patch mm-shmem-swap-fix-softlockup-with-mthp-swapin-v3.patch mm-userfaultfd-fix-race-of-userfaultfd_move-and-swap-cache.patch mm-list_lru-refactor-the-locking-code.patch mm-shmem-swap-improve-cached-mthp-handling-and-fix-potential-hung.patch mm-shmem-swap-avoid-redundant-xarray-lookup-during-swapin.patch mm-shmem-swap-improve-mthp-swapin-process.patch mm-shmem-swap-avoid-false-positive-swap-cache-lookup.patch