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

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Message-ID: <ebf97543-ee25-4944-9990-c0c21c180213@redhat.com>
Date: Fri, 21 Mar 2025 22:07:01 +1000
From: Gavin Shan <gshan@...hat.com>
To: Vlastimil Babka <vbabka@...e.cz>, David Hildenbrand <david@...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 3/21/25 9:33 PM, Vlastimil Babka wrote:
> On 3/21/25 12:25, Gavin Shan wrote:
>> 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.
> 
> Thanks for confirming, please state that in the commit log/cover letter too.
> 

Yes, the commit log and cover letter has been improved for this in v2.

>> I just found the issue by code inspection. Lets drop the fix tags
>> in v2.
> 
> Fixes: tag is fine and correct, just Cc: stable is unnecessary.
> 

Thanks for the hints. The 'Cc: stable' tag has been dropped, but the
'Fixes:' tag is kept in v2, which was posted.

https://lore.kernel.org/linux-mm/20250321120222.1456770-1-gshan@redhat.com/T/#t

>>>>
>>>> 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