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Baolin Wang
b7b2d2ccdb
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 commit029c4628b2("mm: swap: get rid of livelock in swapin readahead") and commit13ddaf26be("mm/swap: fix race when skipping swapcache"). However, commit01626a1823("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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.