Re: [PATCH 0/3] Fix some issues when looking up hugetlb page

From: Baolin Wang <baolin.wang@linux.alibaba.com>
To: Mike Kravetz <mike.kravetz@oracle.com>
Cc: akpm@linux-foundation.org, songmuchun@bytedance.com,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] Fix some issues when looking up hugetlb page
Date: Sun, 21 Aug 2022 13:54:37 +0800	[thread overview]
Message-ID: <8b4fc8cc-23c0-8629-1225-eefcbab1f039@linux.alibaba.com> (raw)
In-Reply-To: <YwAl948ny7AZEfT1@monkey>

On 8/20/2022 8:08 AM, Mike Kravetz wrote:
> On 08/19/22 18:12, Baolin Wang wrote:
>> Hi,
>>
>> On ARM64 architecture, it can support CONT-PTE/PMD size hugetlb. When
>> looking up hugetlb page by follow_page(), we will hold the incorrect
>> pte/pmd lock for the CONT-PTE/PMD size hugetlb page, which will make
>> the pte/pmd entry unstable even under the lock and cause some potential
>> race issues. So considering the CONT-PTE/PMD size hugetlb, this patch set
>> changes to use the correct function to get the correct pte/pmd entry lock
>> to make the pte/pmd entry stable.
> 
> Thank you for looking at this.
> 
> I often get confused by arm64 CONT-PTE/PMD layout, so my understanding may be
> wrong.  Can we use the PMD page lock for locking both CONT-PTE and CONT-PMD
> entries?  Again, I may be confused by the CONT-* page table layout, but it
> seems these would all be referenced via that same PMD page of the page table.
> Or, perhaps CONT-PMD can span multiple PMD pages?

Good point. CONT-PMD can not span multiple PMD pages, so I think 
CONT-PMD can use PMD pagetable page lock, but CONT-PTE also can not span 
multiple PTE pagetable page lock, so I think CONT-PTE can use the PTE 
pagetable page lock to get a fine grained lock.

I will add CONT-PTE and CONT-PMD case in huge_pte_lockptr() in next 
version. Thanks for your comment.

> If we can use PMD page for locking, this would be much finer grain that
> lock in the mm.