linux-kernel - Re: [PATCH v1] fs/proc/task_mmu: fix PAGE_IS_PFNZERO detection for the huge zero folio

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Message-ID: <e341d554-073c-4aa2-ab01-f9bdcd51c0f3@redhat.com>
Date: Wed, 18 Jun 2025 10:56:19 +0200
From: David Hildenbrand <david@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 Muhammad Usama Anjum <usama.anjum@...labora.com>
Subject: Re: [PATCH v1] fs/proc/task_mmu: fix PAGE_IS_PFNZERO detection for
 the huge zero folio

On 18.06.25 01:34, Andrew Morton wrote:
> On Tue, 17 Jun 2025 16:35:32 +0200 David Hildenbrand <david@...hat.com> wrote:
> 
>> is_zero_pfn() does not work for the huge zero folio. Fix it by using
>> is_huge_zero_pmd().
>>
>> Found by code inspection.
>>
>> Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Muhammad Usama Anjum <usama.anjum@...labora.com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>
>> Probably we should Cc stable, thoughts?
> 
> Depends on the userspace effects.  I'm thinking these are "This can
> cause the PAGEMAP_SCAN ioctl against /proc/pid/pagemap to omit pages"
> so yup, cc:stable.

I think it will be included as PAGE_IS_PRESENT, but not as 
PAGE_IS_PFNZERO. That makes it a bit harder to judge the impact.

In any case, it's a simple patch and backporting should not really be 
hard (automatic).

-- 
Cheers,

David / dhildenb