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SubjectRe: [PATCH 01/12] mm/ksm: catch tail page abnormal in page_stable_node
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On 18.03.24 13:14, alexs@kernel.org wrote:
> From: Alex Shi <alexs@kernel.org>
>
> commit 19138349ed59 ("mm/migrate: Add folio_migrate_flags()") change the
> meaning of func page_stable_node() to check the compound head for tail
> 'page' instead of tail page self.
> But seems both semantics are same at results, the func always return NULL
> for tail page. So adding a bug monitor here in case of abnormal.
>
> Signed-off-by: Alex Shi <alexs@kernel.org>
> Cc: Izik Eidus <izik.eidus@ravellosystems.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Chris Wright <chrisw@sous-sol.org>
> To: linux-kernel@vger.kernel.org
> To: linux-mm@kvack.org
> To: Andrew Morton <akpm@linux-foundation.org>
> ---
> mm/ksm.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 8c001819cf10..3ff469961927 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -1091,7 +1091,12 @@ static inline struct ksm_stable_node *folio_stable_node(struct folio *folio)
>
> static inline struct ksm_stable_node *page_stable_node(struct page *page)
> {
> - return folio_stable_node(page_folio(page));
> + struct ksm_stable_node *node;
> +
> + node = folio_stable_node(page_folio(page));
> + VM_BUG_ON_PAGE(PageTail(page) && node, page);

I don't really understand why we would want this.

Only KSM folios can have a node in the stable tree. KSM folios cannot be
large folios. At that is precisely what folio_stable_node() checks.

If we'd have a large folio identify as a KSM folio we'd be in much
bigger trouble.


Besides, I'm sure you read "22) Do not crash the kernel" in
Documentation/process/coding-style.rst

--
Cheers,

David / dhildenb


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