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References: <20210610083549.386085-1-aneesh.kumar@linux.ibm.com>
 <20210610083549.386085-7-aneesh.kumar@linux.ibm.com>
In-Reply-To: <20210610083549.386085-7-aneesh.kumar@linux.ibm.com>
From: Jann Horn <jannh@google.com>
Date: Fri, 11 Jun 2021 10:11:19 +0200
Message-ID: <CAG48ez3=gDRSB02aL84-A54jmPTja3BS0imje-PQywgidRuPxA@mail.gmail.com>
Subject: Re: [PATCH 6/6] mm/mremap: hold the rmap lock in write mode when
 moving page table entries.
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
 Hugh Dickins <hughd@google.com>, Nicholas Piggin <npiggin@gmail.com>,
 Linux-MM <linux-mm@kvack.org>, Kalesh Singh <kaleshsingh@google.com>,
 "Joel Fernandes \(Google\)" <joel@joelfernandes.org>,
 "Kirill A . Shutemov" <kirill@shutemov.name>,
 Andrew Morton <akpm@linux-foundation.org>,
 linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
 "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Thu, Jun 10, 2021 at 10:35 AM Aneesh Kumar K.V
<aneesh.kumar@linux.ibm.com> wrote:
> To avoid a race between rmap walk and mremap, mremap does take_rmap_locks().
> The lock was taken to ensure that rmap walk don't miss a page table entry due to
> PTE moves via move_pagetables(). The kernel does further optimization of
> this lock such that if we are going to find the newly added vma after the
> old vma, the rmap lock is not taken. This is because rmap walk would find the
> vmas in the same order and if we don't find the page table attached to
> older vma we would find it with the new vma which we would iterate later.
[...]
> Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions")
> Fixes: c49dd3401802 ("mm: speedup mremap on 1GB or larger regions")

probably also "Cc: stable@vger.kernel.org"?