linux-kernel - Re: Mapcount of subpages

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Message-ID: <77b59314-5593-1a2e-293c-b66e8235ad@google.com>
Date:   Thu, 23 Sep 2021 16:48:41 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Zi Yan <ziy@...dia.com>
cc:     Yang Shi <shy828301@...il.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        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, 23 Sep 2021, Zi Yan wrote:
> On 23 Sep 2021, at 17:54, Yang Shi wrote:
> > On Thu, Sep 23, 2021 at 2:10 PM Hugh Dickins <hughd@...gle.com> wrote:
> >>
> >> NR_FILE_MAPPED being used for /proc/meminfo's "Mapped:" and a couple
> >> of other such stats files, and for a reclaim heuristic in mm/vmscan.c.
> >>
> >> Allow ourselves more slack in NR_FILE_MAPPED accounting (either count
> >> each pte as if it mapped the whole THP, or don't count a THP's ptes
> >> at all - you opted for the latter in the "Mlocked:" accounting),
> >> and I suspect subpage _mapcount could be abandoned.
> >
> > AFAIK, partial THP unmap may need the _mapcount information of every
> > subpage otherwise the deferred split can't know what subpages could be
> > freed.

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.

> 
> Could we just scan page tables of a THP during deferred split process
> instead? Deferred split is a slow path already, so maybe it can afford
> the extra work.

But unless I misunderstand, actually carrying out the deferred split
already unmaps, uses migration entries, and remaps the remaining ptes:
needing no help from subpage _mapcounts to do those, and free the rest.

Hugh