Re: [PATCH v3] mm/hugetlb: fix races when looking up a CONT-PTE/PMD size hugetlb page

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/2/2022 5:06 AM, Mike Kravetz wrote:
> On 09/01/22 18:41, Baolin Wang wrote:
>> On some architectures (like ARM64), it can support CONT-PTE/PMD size
>> hugetlb, which means it can support not only PMD/PUD size hugetlb
>> (2M and 1G), but also CONT-PTE/PMD size(64K and 32M) if a 4K page size
>> specified.
>>
>> So when looking up a CONT-PTE size hugetlb page by follow_page(), it
>> will use pte_offset_map_lock() to get the pte entry lock for the CONT-PTE
>> size hugetlb in follow_page_pte(). However this pte entry lock is incorrect
>> for the CONT-PTE size hugetlb, since we should use huge_pte_lock() to
>> get the correct lock, which is mm->page_table_lock.
>>
>> That means the pte entry of the CONT-PTE size hugetlb under current
>> pte lock is unstable in follow_page_pte(), we can continue to migrate
>> or poison the pte entry of the CONT-PTE size hugetlb, which can cause
>> some potential race issues, even though they are under the 'pte lock'.
>>
>> For example, suppose thread A is trying to look up a CONT-PTE size
>> hugetlb page by move_pages() syscall under the lock, however antoher
>> thread B can migrate the CONT-PTE hugetlb page at the same time, which
>> will cause thread A to get an incorrect page, if thread A also wants to
>> do page migration, then data inconsistency error occurs.
>>
>> Moreover we have the same issue for CONT-PMD size hugetlb in
>> follow_huge_pmd().
>>
>> To fix above issues, rename the follow_huge_pmd() as follow_huge_pmd_pte()
>> to handle PMD and PTE level size hugetlb, which uses huge_pte_lock() to
>> get the correct pte entry lock to make the pte entry stable.
>>
>> Cc: <stable@vger.kernel.org>
>> Suggested-by: Mike Kravetz <mike.kravetz@oracle.com>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>> Changes from v2:
>>   - Combine PMD and PTE level hugetlb handling into one function.
>>   - Drop unnecessary patches.
>>   - Update the commit message.
> 
> Baolin, were you able to at least exercise the new code paths?  Especially the
> path for CONT_PTE.  Code looks fine to me.

Yes, I've tested CONT-PTE, CONT-PMD and PMD size hugetlb with 
move_pages() syscall, all works well and the lock is expected.

> 
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> 
> It is a little hackish, but this is only for backports.  So, I think it is OK.
> We may want to point out that code cleanup and simplification is going upstream
> that will address these issues in a more elegant manner.
> 
>>
>> Mike, please fold this patch into your series. Thanks.
> 
> If I understand Andrew, this can go in as a separate patch for backport to
> address potential bugs.  I will provide a cleanup/simplification that will
> remove this going forward.
> 
> Andrew also asked for a Fixes tag.
> Support for CONT_PMD/_PTE was added with bb9dd3df8ee9 "arm64: hugetlb: refactor
> find_num_contig()".  Patch series "Support for contiguous pte hugepages", v4.
> However, I do not believe these code paths were executed until migration
> support was added with 5480280d3f2d "arm64/mm: enable HugeTLB migration for
> contiguous bit HugeTLB pages"
> I would go with 5480280d3f2d.

Make sense. And I saw Andrew has helped to add a Fixes tag with your 
suggestion. Thanks Mike and Andrew.