Re: [PATCH v1 01/18] mm: allow for detecting underflows with page_mapcount() again

On 09.04.24 21:22, 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 <[email protected]>
> ---
>   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));
>   

 From b49849001f3d2aad0af93cf2098065d7cbd9a959 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <[email protected]>
Date: Wed, 24 Apr 2024 10:50:09 +0200
Subject: [PATCH] !fixup: mm: allow for detecting underflows with
  page_mapcount() again

Let's make page_mapcount() slighly more efficient by inlining the
page_type_has_type() check.

Signed-off-by: David Hildenbrand <[email protected]>
---
  include/linux/mm.h | 5 +++--
  1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dc33f8269fb52..cf700c5cdd58b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1229,10 +1229,11 @@ static inline void page_mapcount_reset(struct page *page)
   */
  static inline int page_mapcount(struct page *page)
  {
-	int mapcount = atomic_read(&page->_mapcount);
+	int mapcount = atomic_read(&page->_mapcount) + 1;
  
  	/* Handle page_has_type() pages */
-	mapcount = page_type_has_type(mapcount) ? 0 : mapcount + 1;
+	if (mapcount < PAGE_MAPCOUNT_RESERVE + 1)
+		mapcount = 0;
  	if (unlikely(PageCompound(page)))
  		mapcount += folio_entire_mapcount(page_folio(page));
  
-- 
2.44.0


-- 
Cheers,

David / dhildenb