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

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Message-Id: <F678B4FD-A806-4BF9-8D8A-8679354A1068@linux.dev>
Date: Fri, 26 Jul 2024 16:18:06 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Linux Memory Management List <linux-mm@...ck.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Peter Xu <peterx@...hat.com>,
 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 Jul 26, 2024, at 02:39, David Hildenbrand <david@...hat.com> 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.
> 
> 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>

Acked-by: Muchun Song <muchun.song@...ux.dev>

Thanks.