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From: Zi Yan <ziy@nvidia.com>
To: David Hildenbrand <david@redhat.com>
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 Richard Chang <richardycc@google.com>
Subject: Re: [PATCH v1 01/18] mm: allow for detecting underflows with
 page_mapcount() again
Date: Tue, 09 Apr 2024 16:06:15 -0400
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In-Reply-To: <20240409192301.907377-2-david@redhat.com>
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On 9 Apr 2024, at 15:22, David Hildenbrand wrote:

> Commit 53277bcf126d ("mm: support page_mapcount() on page_has_type()
> pages") made it impossible to detect mapcount underflows by treating
> any negative raw mapcount value as a mapcount of 0.
>
> We perform such underflow checks in zap_present_folio_ptes() and
> zap_huge_pmd(), which would currently no longer trigger.
>
> Let's check against PAGE_MAPCOUNT_RESERVE instead by using
> page_type_has_type(), like page_has_type() would, so we can still catch
> some underflows.
>
> Fixes: 53277bcf126d ("mm: support page_mapcount() on page_has_type() pages")
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  include/linux/mm.h | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ef34cf54c14f..0fb8a40f82dd 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1229,11 +1229,10 @@ static inline void page_mapcount_reset(struct page *page)
>   */
>  static inline int page_mapcount(struct page *page)
>  {
> -	int mapcount = atomic_read(&page->_mapcount) + 1;
> +	int mapcount = atomic_read(&page->_mapcount);
>
>  	/* Handle page_has_type() pages */
> -	if (mapcount < 0)
> -		mapcount = 0;
> +	mapcount = page_type_has_type(mapcount) ? 0 : mapcount + 1;
>  	if (unlikely(PageCompound(page)))
>  		mapcount += folio_entire_mapcount(page_folio(page));

LGTM. This could be picked up separately. Reviewed-by: Zi Yan <ziy@nvidia.com>

--
Best Regards,
Yan, Zi

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