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

David Hildenbrand
2024-04-24T09:39:01Z

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 <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));
>

From b49849001f3d2aad0af93cf2098065d7cbd9a959 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
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 <david@redhat.com>
---
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