linux-kernel - Re: [v5 PATCH 6/6] mm: hwpoison: handle non-anonymous THP correctly

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Date:   Tue, 2 Nov 2021 09:41:43 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Jue Wang <juew@...gle.com>, Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>, Oscar Salvador <osalvador@...e.de>,
        Peter Xu <peterx@...hat.com>
Subject: Re: [v5 PATCH 6/6] mm: hwpoison: handle non-anonymous THP correctly

On Mon, Nov 1, 2021 at 8:50 PM Matthew Wilcox <willy@...radead.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@...gle.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.