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

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Message-ID: <4a12630b-bd2b-45ec-9a2d-57ed9ac2faa0@redhat.com>
Date: Fri, 26 Jul 2024 17:32:02 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 Muchun Song <muchun.song@...ux.dev>, Oscar Salvador <osalvador@...e.de>,
 stable@...r.kernel.org
Subject: Re: [PATCH v1 2/2] mm/hugetlb: fix hugetlb vs. core-mm PT locking

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@...r.kernel.org>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Patch looks all right to me:
> 
> Reviewed-by: Peter Xu <peterx@...hat.com>
> 
> Thanks!

Thanks Peter!

-- 
Cheers,

David / dhildenb