linux-kernel - Re: [PATCH 1/2] mm/khugepaged: set THP as uptodate earlier for shmem

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Message-ID: <Y+1XOS88M+Pigqtq@x1n>
Date:   Wed, 15 Feb 2023 17:05:45 -0500
From:   Peter Xu <peterx@...hat.com>
To:     David Stevens <stevensd@...omium.org>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Yang Shi <shy828301@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/khugepaged: set THP as uptodate earlier for shmem

On Wed, Feb 15, 2023 at 10:33:15AM +0900, David Stevens wrote:
> On Wed, Feb 15, 2023 at 12:44 AM Matthew Wilcox <willy@...radead.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.

AFAIU we lock the page iff !uptodate and we want to wait it to be uptodate,
or as Matthew said when we want to modify !uptodate->uptodate.

Take the same example of folio_seek_hole_data() that you mentioned:

	if (xa_is_value(folio) || folio_test_uptodate(folio))
		return seek_data ? start : end;

-- 
Peter Xu