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Kairui Song a99f9a4669 mm/shmem, swap: fix race of truncate and swap entry split commit `8a1968bd99` upstream. The helper for shmem swap freeing is not handling the order of swap entries correctly. It uses xa_cmpxchg_irq to erase the swap entry, but it gets the entry order before that using xa_get_order without lock protection, and it may get an outdated order value if the entry is split or changed in other ways after the xa_get_order and before the xa_cmpxchg_irq. And besides, the order could grow and be larger than expected, and cause truncation to erase data beyond the end border. For example, if the target entry and following entries are swapped in or freed, then a large folio was added in place and swapped out, using the same entry, the xa_cmpxchg_irq will still succeed, it's very unlikely to happen though. To fix that, open code the Xarray cmpxchg and put the order retrieval and value checking in the same critical section. Also, ensure the order won't exceed the end border, skip it if the entry goes across the border. Skipping large swap entries crosses the end border is safe here. Shmem truncate iterates the range twice, in the first iteration, find_lock_entries already filtered such entries, and shmem will swapin the entries that cross the end border and partially truncate the folio (split the folio or at least zero part of it). So in the second loop here, if we see a swap entry that crosses the end order, it must at least have its content erased already. I observed random swapoff hangs and kernel panics when stress testing ZSWAP with shmem. After applying this patch, all problems are gone. Link: https://lkml.kernel.org/r/20260120-shmem-swap-fix-v3-1-3d33ebfbc057@tencent.com Fixes: `809bc86517` ("mm: shmem: support large folio swap out") Signed-off-by: Kairui Song <kasong@tencent.com> Reviewed-by: Nhat Pham <nphamcs@gmail.com> Acked-by: Chris Li <chrisl@kernel.org> Cc: Baolin Wang <baolin.wang@linux.alibaba.com> Cc: Baoquan He <bhe@redhat.com> Cc: Barry Song <baohua@kernel.org> Cc: Hugh Dickins <hughd@google.com> Cc: Kemeng Shi <shikemeng@huaweicloud.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>		History 2026-02-06 16:55:47 +01:00
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README

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.