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

[David Hildenbrand]

On 26.07.24 17:26, Peter Xu wrote:
> On Thu, Jul 25, 2024 at 08:39:55PM +0200, 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.
> >
> > There is one ugly case: powerpc 8xx, whereby we have an 8 MiB hugetlb
> > folio being mapped using two PTE page tables. While hugetlb wants to take
> > the PMD table lock, core-mm would grab the PTE table lock of one of both
> > PTE page tables. In such corner cases, we have to make sure that both
> > locks match, which is (fortunately!) currently guaranteed for 8xx as it
> > does not support SMP.
>
> Do you mean "does not support SPLIT_PMD_PTLOCK" here instead of SMP?

Split PT locks are only enabled once we have NR_CPUS >= 4, so without 
SMP (NR_CPUS == 1), no split PT locks are used. I document that in the 
other series that I just sent out in a better way.

> > Fixes: 9cb28da54643 ("mm/gup: handle hugetlb in the generic follow_page_mask code")
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
>
> Patch looks all right to me:
>
> Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>
>
> Thanks!

Thanks Peter!

--
Cheers,

David / dhildenb