linux-kernel - Re: [PATCH 0/2] Fix parameter passed to page_mapcount_is

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Message-ID: <b9e1269e-e79b-446b-9483-4fdbc1ee42f4@suse.cz>
Date: Fri, 21 Mar 2025 10:23:18 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Gavin Shan <gshan@...hat.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
 willy@...radead.org, david@...hat.com, osalvador@...e.de, gehao@...inos.cn,
 shan.gavin@...il.com
Subject: Re: [PATCH 0/2] Fix parameter passed to page_mapcount_is_type()

On 3/21/25 06:31, Gavin Shan wrote:
> Found by code inspection. There are two places where the parameter
> passed to page_mapcount_is_type() is (page->__mapcount), which is
> correct since it should be one more than the value, as explained in
> the comments to page_mapcount_is_type(): (a) page_has_type() in
> page-flags.h (b) __dump_folio() in mm/debug.c

IIUC you are right. Luckily thanks to the the PGTY_mapcount_underflow limit,
this off-by-one error doesn't currently cause visible issues i.e.
misclassifications legitimate mapcount as page type and vice versa, right?
We'd have to have a mapcount underflown severely right to the limit to make
that off-by-one error cross it?

I wonder if a more future-proof solution would be to redefine
page_mapcount_is_type() instead to not subtract. But I'll leave that to willy.

> PATCH[1] fixes the parameter for (a)
> PATCH[2] fixes the parameter for (b)
> 
> Gavin Shan (2):
>   mm: Fix parameter passed to page_mapcount_is_type()
>   mm/debug: Fix parameter passed to page_mapcount_is_type()
> 
>  include/linux/page-flags.h | 2 +-
>  mm/debug.c                 | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
>