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

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Message-ID: <5ec97dd6-6561-4687-ac94-41c63ffc82cf@redhat.com>
Date: Fri, 21 Mar 2025 21:25:27 +1000
From: Gavin Shan <gshan@...hat.com>
To: David Hildenbrand <david@...hat.com>, Vlastimil Babka <vbabka@...e.cz>,
 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 3/21/25 8:11 PM, David Hildenbrand wrote:
> 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).
> 

Yes, it shouldn't cause any visible impacts so far due to the gap.
I just found the issue by code inspection. Lets drop the fix tags
in v2.

>>
>> 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.
> 

+1 to remove page_mapcount_is_type(). After Willy confirms, I can post
an extra series to do it if needed.

Thanks,
Gavin