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Date:   Thu, 23 Sep 2021 16:48:41 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Zi Yan <ziy@nvidia.com>
cc:     Yang Shi <shy828301@gmail.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Hugh Dickins <hughd@google.com>,
        Matthew Wilcox <willy@infradead.org>,
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        Johannes Weiner <hannes@cmpxchg.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        David Howells <dhowells@redhat.com>,
        Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: Mapcount of subpages
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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@google.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