linux-kernel - Re: [PATCH v3] mm/migrate: fix shmem xarray update during migration

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Message-ID: <Z8i0l8apxDsThD9s@casper.infradead.org>
Date: Wed, 5 Mar 2025 20:31:19 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Zi Yan <ziy@...dia.com>
Cc: Liu Shixin <liushixin2@...wei.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Barry Song <baohua@...nel.org>,
	David Hildenbrand <david@...hat.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Lance Yang <ioworker0@...il.com>,
	Ryan Roberts <ryan.roberts@....com>,
	Hugh Dickins <hughd@...gle.com>,
	Charan Teja Kalla <quic_charante@...cinc.com>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v3] mm/migrate: fix shmem xarray update during migration

On Wed, Mar 05, 2025 at 03:04:03PM -0500, Zi Yan wrote:
> A shmem folio can be either in page cache or in swap cache, but not at the
> same time. Namely, once it is in swap cache, folio->mapping should be NULL,
> and the folio is no longer in a shmem mapping.
> 
> In __folio_migrate_mapping(), to determine the number of xarray entries
> to update, folio_test_swapbacked() is used, but that conflates shmem in
> page cache case and shmem in swap cache case. It leads to xarray
> multi-index entry corruption, since it turns a sibling entry to a
> normal entry during xas_store() (see [1] for a userspace reproduction).
> Fix it by only using folio_test_swapcache() to determine whether xarray
> is storing swap cache entries or not to choose the right number of xarray
> entries to update.
> 
> [1] https://lore.kernel.org/linux-mm/Z8idPCkaJW1IChjT@casper.infradead.org/
> 
> Note:
> In __split_huge_page(), folio_test_anon() && folio_test_swapcache() is used
> to get swap_cache address space, but that ignores the shmem folio in swap
> cache case. It could lead to NULL pointer dereferencing when a
> in-swap-cache shmem folio is split at __xa_store(), since
> !folio_test_anon() is true and folio->mapping is NULL. But fortunately,
> its caller split_huge_page_to_list_to_order() bails out early with EBUSY
> when folio->mapping is NULL. So no need to take care of it here.
> 
> Fixes: fc346d0a70a1 ("mm: migrate high-order folios in swap cache correctly")
> Reported-by: Liu Shixin <liushixin2@...wei.com>
> Closes: https://lore.kernel.org/all/28546fb4-5210-bf75-16d6-43e1f8646080@huawei.com/
> Suggested-by: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Zi Yan <ziy@...dia.com>
> Cc: stable@...r.kernel.org

Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>