Re: [PATCH v1 2/2] mm/hugetlb: fix hugetlb vs. core-mm PT locking

[Baolin Wang]

On 2024/7/26 16:04, David Hildenbrand wrote:
> On 26.07.24 04:33, Baolin Wang wrote:
> > On 2024/7/26 02:39, David Hildenbrand wrote:
> > > We recently made GUP's common page table walking code to also walk
> > > hugetlb VMAs without most hugetlb special-casing, preparing for the
> > > future of having less hugetlb-specific page table walking code in the
> > > codebase. Turns out that we missed one page table locking detail: page
> > > table locking for hugetlb folios that are not mapped using a single
> > > PMD/PUD.
> > >
> > > Assume we have hugetlb folio that spans multiple PTEs (e.g., 64 KiB
> > > hugetlb folios on arm64 with 4 KiB base page size). GUP, as it walks the
> > > page tables, will perform a pte_offset_map_lock() to grab the PTE table
> > > lock.
> > >
> > > However, hugetlb that concurrently modifies these page tables would
> > > actually grab the mm->page_table_lock: with USE_SPLIT_PTE_PTLOCKS, the
> > > locks would differ. Something similar can happen right now with hugetlb
> > > folios that span multiple PMDs when USE_SPLIT_PMD_PTLOCKS.
> > >
> > > Let's make huge_pte_lockptr() effectively uses the same PT locks as any
> > > core-mm page table walker would.
> >
> > Thanks for raising the issue again. I remember fixing this issue 2 years
> > ago in commit fac35ba763ed ("mm/hugetlb: fix races when looking up a
> > CONT-PTE/PMD size hugetlb page"), but it seems to be broken again.
>
> Ah, right! We fixed it by rerouting to hugetlb code that we then removed :D
>
> Did we have a reproducer back then that would make my live easier?

I don't have any reproducers right now. I remember I added some ugly 
hack code (adding delay() etc.) in kernel to analyze this issue, and not 
easy to reproduce. :(