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

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Message-ID: <ad33344d-e289-4c93-a29a-c85b6cdcd204@redhat.com>
Date: Fri, 21 Mar 2025 12:27:43 +0100
From: David Hildenbrand <david@...hat.com>
To: Gavin Shan <gshan@...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 21.03.25 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.
> 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.

I think we should only do that one Willy splits struct folio off from, 
struct page, storing the type elsewhere. For now, we should likely just 
leave it as is.

-- 
Cheers,

David / dhildenb