Re: [PATCH 01/12] mm/ksm: catch tail page abnormal in page_stable_node

[David Hildenbrand]

On 18.03.24 13:14, alexs@xxxxxxxxxx wrote:
> From: Alex Shi <alexs@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Cc: Izik Eidus <izik.eidus@xxxxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Chris Wright <chrisw@xxxxxxxxxxxx>
> To: linux-kernel@xxxxxxxxxxxxxxx
> To: linux-mm@xxxxxxxxx
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>   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