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From:   Yang Shi <shy828301@gmail.com>
Date:   Tue, 2 Nov 2021 09:41:43 -0700
Message-ID: <CAHbLzkrwEsTgwmaMsOFHRbM6TEvBZA965N+FJwe-ZwSzbrL-VQ@mail.gmail.com>
Subject: Re: [v5 PATCH 6/6] mm: hwpoison: handle non-anonymous THP correctly
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Jue Wang <juew@google.com>, Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        =?UTF-8?B?SE9SSUdVQ0hJIE5BT1lBKOWggOWPoyDnm7TkuZ8p?= 
        <naoya.horiguchi@nec.com>, Oscar Salvador <osalvador@suse.de>,
        Peter Xu <peterx@redhat.com>
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On Mon, Nov 1, 2021 at 8:50 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, Nov 01, 2021 at 01:11:33PM -0700, Yang Shi wrote:
> > On Mon, Nov 1, 2021 at 12:38 PM Jue Wang <juew@google.com> wrote:
> > >
> > > A related bug but whose fix may belong to a separate series:
> > >
> > > split_huge_page fails when invoked concurrently on the same THP page.
> > >
> > > It's possible that multiple memory errors on the same THP get consumed
> > > by multiple threads and come down to split_huge_page path easily.
> >
> > Yeah, I think it should be a known problem since the very beginning.
> > The THP split requires to pin the page and does check if the refcount
> > is expected or not and freezes the refcount if it is expected. So if
> > two concurrent paths try to split the same THP, one will fail due to
> > the pin from the other path, but the other one will succeed.
>
> No, they can both fail, if the timing is just right, because they each
> have a refcount, so neither will succeed.

Oh, yes, if there is race between unlock_page and put_page.