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

[Baolin Wang]

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.