linux-kernel - Re: Mapcount of subpages

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Message-ID: <CAHbLzkpTXxzR7=5mYAOFfu0M2jN5cnvbQMHu3bTba8PSdRDSXg@mail.gmail.com>
Date:   Thu, 23 Sep 2021 20:26:22 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Hugh Dickins <hughd@...gle.com>, Zi Yan <ziy@...dia.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Kent Overstreet <kent.overstreet@...il.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        David Howells <dhowells@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: Mapcount of subpages

On Thu, Sep 23, 2021 at 6:32 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Thu, Sep 23, 2021 at 06:11:19PM -0700, Yang Shi wrote:
> > On Thu, Sep 23, 2021 at 4:49 PM Hugh Dickins <hughd@...gle.com> wrote:
> > > I believe Yang Shi is right insofar as the decision on whether it's worth
> > > queuing for deferred split is being done based on those subpage _mapcounts.
> > > That is a use I had not considered, and I've given no thought to how
> > > important or not it is.
> >
> > Anyway deferred split is anon THP specific. We don't have to worry
> > about this for file THP. So your suggestion about just counting total
> > mapcount seems feasible to me for file THP at least.
>
> But I think we probably *should* do deferred split for file THP.
> At the moment, when we truncate to the middle of a shmem THP, we try
> a few times to split it and then just give up.  We should probably try
> once and then queue it for deferred split.

Yes, probably. Anyway this doesn't need _mapcount of subpages.