linux-kernel - Re: [PATCH 01/12] mm/ksm: catch tail page abnormal in page_stable

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Message-ID: <7a0d7d0e-d241-4942-b6c1-e60d597b3dd6@redhat.com>
Date: Mon, 18 Mar 2024 13:25:50 +0100
From: David Hildenbrand <david@...hat.com>
To: alexs@...nel.org, kasong@...cent.com,
 Andrew Morton <akpm@...ux-foundation.org>,
 "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
 open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/12] mm/ksm: catch tail page abnormal in
 page_stable_node

On 18.03.24 13:14, alexs@...nel.org wrote:
> From: Alex Shi <alexs@...nel.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@...nel.org>
> Cc: Izik Eidus <izik.eidus@...ellosystems.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Chris Wright <chrisw@...s-sol.org>
> To: linux-kernel@...r.kernel.org
> To: linux-mm@...ck.org
> To: Andrew Morton <akpm@...ux-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