linux-kernel - Re: [PATCH 1/1] mm/memory: Fix boundary check for next PFN in folio_pte

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Message-ID: <056a92c5-0cf6-4f71-a93e-d8c43d4eed08@redhat.com>
Date: Tue, 27 Feb 2024 09:46:43 +0100
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, ryan.roberts@....com, 21cnbao@...il.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/memory: Fix boundary check for next PFN in
 folio_pte_batch()

On 27.02.24 09:45, Lance Yang wrote:
> On Tue, Feb 27, 2024 at 4:33 PM David Hildenbrand <david@...hat.com> wrote:
>>
>> On 27.02.24 09:23, Lance Yang wrote:
>>> Hey David,
>>>
>>> Thanks for taking time to review!
>>>
>>> On Tue, Feb 27, 2024 at 3:30 PM David Hildenbrand <david@...hat.com> wrote:
>>>>
>>>> On 27.02.24 08:04, Lance Yang wrote:
>>>>> Previously, in folio_pte_batch(), only the upper boundary of the
>>>>> folio was checked using '>=' for comparison. This led to
>>>>> incorrect behavior when the next PFN exceeded the lower boundary
>>>>> of the folio, especially in corner cases where the next PFN might
>>>>> fall into a different folio.
>>>>
>>>> Which commit does this fix?
>>>>
>>>> The introducing commit (f8d937761d65c87e9987b88ea7beb7bddc333a0e) is
>>>> already in mm-stable, so we would need a Fixes: tag. Unless, Ryan's
>>>> changes introduced a problem.
>>>>
>>>> BUT
>>>>
>>>> I don't see what is broken. :)
>>>>
>>>> Can you please give an example/reproducer?
>>>
>>> For example1:
>>>
>>> PTE0 is present for large folio1.
>>> PTE1 is present for large folio1.
>>> PTE2 is present for large folio1.
>>> PTE3 is present for large folio1.
>>>
>>> folio_nr_pages(folio1) is 4.
>>> folio_nr_pages(folio2) is 4.
>>>
>>> pte = *start_ptep = PTE0;
>>> max_nr = folio_nr_pages(folio2);
>>>
>>> If folio_pfn(folio1) < folio_pfn(folio2),
>>> the return value of folio_pte_batch(folio2, start_ptep, pte, max_nr)
>>> will be 4(Actually it should be 0).
>>>
>>> For example2:
>>>
>>> PTE0 is present for large folio2.
>>> PTE1 is present for large folio1.
>>> PTE2 is present for large folio1.
>>> PTE3 is present for large folio1.
>>>
>>> folio_nr_pages(folio1) is 4.
>>> folio_nr_pages(folio2) is 4.
>>>
>>> pte = *start_ptep = PTE0;
>>> max_nr = folio_nr_pages(folio1);
>>>
>>
>> In both cases, start_ptep does not map the folio.
>>
>> It's a BUG in your caller unless I am missing something important.
> 
> Sorry, I understood.
> 
> Thanks for your clarification!

I'll post some kernel doc as reply to Barry's export patch to clarify that.

-- 
Cheers,

David / dhildenb