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: <2409b6e5-ee02-4cb6-abc8-5b821f58f540@redhat.com>
Date: Mon, 30 Jun 2025 09:55:15 +0200
From: David Hildenbrand <david@...hat.com>
To: Muhammad Usama Anjum <usama.anjum@...labora.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] fs/proc/task_mmu: fix PAGE_IS_PFNZERO detection for
 the huge zero folio

On 30.06.25 07:18, Muhammad Usama Anjum wrote:
> On 6/17/25 7:35 PM, David Hildenbrand 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?
>>
>> We should also extend the pagemap_ioctl selftest to cover this case, but I
>> don't have time for that right now. @Muhammad ?
> Currently, we don't have any test case covering zero pfn. I'm trying to write
> a few test cases. But I'm not able to get ZERO PFN. I've tried to allocate a
> read only memory and then read it. Is there a trick to how to create ZERO PFN
> memory from userspace?

You need a MAP_ANON | MAP_PRIVATE mapping and have to make sure that the 
compiler does not optimize out the read.

E.g.,

char *mem =  mmap(...);
char tmp = *mem;

asm volatile("" : "+r" (tmp))

or

char *mem =  mmap(...);

*(volatile char *)mem;


To get the shared huge zero folio, you need a suitably aligned VMA. See 
run_with_huge_zeropage() in in tools/testing/selftests/mm/cow.c as one 
example.

-- 
Cheers,

David / dhildenb