Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230111133351.807024-1-jannh@google.com> <20230112085649.gvriasb2t5xwmxkm@box.shutemov.name>
 <CAHbLzkpAVTys9dBSodHBB3ovKhwP8imUsyXy=aPqY5SyXCT7ww@mail.gmail.com> <20230113001037.adqha6lxj7lh75at@box.shutemov.name>
In-Reply-To: <20230113001037.adqha6lxj7lh75at@box.shutemov.name>
From:   Yang Shi <shy828301@gmail.com>
Date:   Thu, 12 Jan 2023 19:22:46 -0800
Message-ID: <CAHbLzkpM3mpOfo_NcH2roJf_WCJZno6CqLvM+FtjLG8d0_fGkA@mail.gmail.com>
Subject: Re: [PATCH] mm/khugepaged: Fix ->anon_vma race
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "Zach O'Keefe" <zokeefe@google.com>, linux-kernel@vger.kernel.org,
        David Hildenbrand <david@redhat.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Thu, Jan 12, 2023 at 4:10 PM Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> On Thu, Jan 12, 2023 at 10:12:53AM -0800, Yang Shi wrote:
> > > This is totally wrong direction. Or I don't understand the race.
> > >
> > > At this point we already paid nearly all price of of pagetable retraction.
> > > I don't see any correctness reason to stop here, except for the assert.
> >
> > Isn't it possible that collapse_and_free_pmd() clear the pmd which may
> > point to a PTE which maps the COW'ed anon page if this race happens?
>
> No. At this point we have huge page in the place in the page cache and it
> is locked. COW fault would serialize on the page lock.

Aha, yeah, I missed the page lock. So IIUC even though anon_vma is set
there should be not any COW'ed anon pages since cow fault would be
blocked by the page lock. If so I tend to agree with you the lockdep
assert seems unnecessary.

>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov