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mm: shmem: fix potential livelock issue for shmem direct swapin

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When skipping swapcache for synchronous IO swap devices, swapcache_prepare()
is used to prevent parallel swapin from proceeding with the swap cache flag.
However, on PREEMPT kernels this can lead to a livelock, as reported by Chao[1]:

Thread A starts direct swapin of a shmem folio and calls swapcache_prepare()
to set SWAP_HAS_CACHE. It may then be preempted inside workingset_refault().
Meanwhile, a higher priority thread B also attempts direct swapin of the same
shmem swap entry. Since swapcache_prepare() already marks the entry, thread B
repeatedly gets -EEXIST and busy-loops waiting for thread A to finish. But as
thread B runs at higher priority, thread A cannot preempt it, resulting in
starvation and a livelock.

Fix it by yielding the CPU with schedule_timeout_uninterruptible(1) when
swapcache_prepare() fails, following the same approach used in commit
029c4628b2 ("mm: swap: get rid of livelock in swapin readahead") and
commit 13ddaf26be ("mm/swap: fix race when skipping swapcache").

However, commit 01626a1823 ("mm: avoid unconditional one-tick sleep when
swapcache_prepare fails") found that the unconditional one-tick sleep can
cause UI stuttering on latency-sensitive Android devices. So we can follow
the same approach by adding a waitqueue to wake up tasks when needed,
instead of always sleeping for a full tick.

Note that mainline does not have this potential issue, which has already been
resolved by Kairui's swap refactoring work[2].

[1] https://lore.kernel.org/all/700a2cbf90a2484f979aac858f08f5d4@xiaomi.com/
[2] https://lore.kernel.org/all/20260517-swap-table-p4-v5-0-88ae43e064c7@tencent.com/
Fixes: 1dd44c0af4 ("mm: shmem: skip swapcache for swapin of synchronous swap device")
Reported-by: Ma Chao <machao26@xiaomi.com>
Closes: https://lore.kernel.org/all/700a2cbf90a2484f979aac858f08f5d4@xiaomi.com/
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Baolin Wang
2026-07-18 16:53:25 +02:00
committed by Greg Kroah-Hartman
parent 9818bcae3c
commit b7b2d2ccdb
1 changed files with 13 additions and 1 deletions
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mm/shmem.c
+13 -1
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@@ -2005,11 +2005,14 @@ unlock:
return ERR_PTR(error);
}
static DECLARE_WAIT_QUEUE_HEAD(shmem_swapcache_wq);
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);
DECLARE_WAITQUEUE(wait, current);
int nr_pages = 1 << order;
struct folio *new;
gfp_t alloc_gfp;
@@ -2066,6 +2069,10 @@ retry:
if (swapcache_prepare(entry, nr_pages)) {
folio_put(new);
new = ERR_PTR(-EEXIST);
/* Relax a bit to prevent rapid repeated page faults */
add_wait_queue(&shmem_swapcache_wq, &wait);
schedule_timeout_uninterruptible(1);
remove_wait_queue(&shmem_swapcache_wq, &wait);
/* Try smaller folio to avoid cache conflict */
goto fallback;
}
@@ -2423,6 +2430,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
if (skip_swapcache) {
folio->swap.val = 0;
swapcache_clear(si, swap, nr_pages);
if (waitqueue_active(&shmem_swapcache_wq))
wake_up(&shmem_swapcache_wq);
} else {
swap_cache_del_folio(folio);
}
@@ -2442,8 +2451,11 @@ unlock:
if (folio)
folio_unlock(folio);
failed_nolock:
if (skip_swapcache)
if (skip_swapcache) {
swapcache_clear(si, folio->swap, folio_nr_pages(folio));
if (waitqueue_active(&shmem_swapcache_wq))
wake_up(&shmem_swapcache_wq);
}
if (folio)
folio_put(folio);
put_swap_device(si);