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From:   David Stevens <stevensd@chromium.org>
Date:   Wed, 15 Feb 2023 10:33:15 +0900
Message-ID: <CAD=HUj76eeScAKkX=uf03KnGcVvyzz_kJ4sYVx8XeFqwTVhDow@mail.gmail.com>
Subject: Re: [PATCH 1/2] mm/khugepaged: set THP as uptodate earlier for shmem
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-mm@kvack.org, Peter Xu <peterx@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Yang Shi <shy828301@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Hugh Dickins <hughd@google.com>, linux-kernel@vger.kernel.org
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On Wed, Feb 15, 2023 at 12:44 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Feb 14, 2023 at 04:57:09PM +0900, David Stevens wrote:
> >       /*
> > -      * At this point the hpage is locked and not up-to-date.
> > -      * It's safe to insert it into the page cache, because nobody would
> > -      * be able to map it or use it in another way until we unlock it.
> > +      * Mark hpage as up-to-date before inserting it into the page cache to
> > +      * prevent it from being mistaken for an fallocated but unwritten page.
> > +      * Inserting the unfinished hpage into the page cache is safe because
> > +      * it is locked, so nobody can map it or use it in another way until we
> > +      * unlock it.
>
> No, that's not true.  The data has to be there before we mark it
> uptodate.  See filemap_get_pages() for example, used as part of
> read().  We don't lock the page unless we need to bring it uptodate
> ourselves.

I've been focusing on the shmem case for collapse_file and forgot to
think about the !is_shmem case. As far as I could tell, shmem doesn't
use filemap_get_pages() and everything else in filemap.c/shmem.c that
checks folio_test_uptodate also locks the folio. But yeah, this would
break the !is_shmem case and is kind of sketchy anyway. I'll put
together a better patch.

-David