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

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Message-ID: <0d096764-302f-4b80-a867-22f5302b8045@redhat.com>
Date: Fri, 21 Mar 2025 11:11:46 +0100
From: David Hildenbrand <david@...hat.com>
To: Vlastimil Babka <vbabka@...e.cz>, Gavin Shan <gshan@...hat.com>,
 linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
 willy@...radead.org, osalvador@...e.de, gehao@...inos.cn,
 shan.gavin@...il.com
Subject: Re: [PATCH 0/2] Fix parameter passed to page_mapcount_is_type()

On 21.03.25 10:23, Vlastimil Babka wrote:
> 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?

Agreed. Likely not stable material because it isn't actually fixing 
anything (because of the safety gaps).

> 
> 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.

With upcoming changes around that, likely best to leave that alone. I 
expect page_mapcount_is_type() to completely vanish.

-- 
Cheers,

David / dhildenb