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From:   Lokesh Gidra <lokeshgidra@google.com>
Date:   Thu, 8 Jun 2023 03:05:11 -0700
Message-ID: <CA+EESO6NkpugZa7zh=mj+A0BEqMuS4TKL0cpTm4EWeHgUUVksg@mail.gmail.com>
Subject: Re: RFC for new feature to move pages from one vma to another without split
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Vlastimil Babka <vbabka@suse.cz>, Peter Xu <peterx@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Brian Geffon <bgeffon@google.com>,
        Kalesh Singh <kaleshsingh@google.com>,
        Nicolas Geoffray <ngeoffray@google.com>,
        Jared Duke <jdduke@google.com>,
        android-mm <android-mm@google.com>,
        Blake Caldwell <blake.caldwell@colorado.edu>,
        Mike Rapoport <rppt@kernel.org>
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On Tue, Jun 6, 2023 at 4:18=E2=80=AFPM Suren Baghdasaryan <surenb@google.co=
m> wrote:
>
> On Tue, Jun 6, 2023 at 1:15=E2=80=AFPM Vlastimil Babka <vbabka@suse.cz> w=
rote:
> >
> > On 4/13/23 17:36, Peter Xu wrote:
> > > On Thu, Apr 13, 2023 at 10:10:44AM +0200, David Hildenbrand wrote:
> > >> So instead, we consider the whole address space as a virtual, anon f=
ile,
> > >> starting at offset 0. The pgoff of a VMA is then simply the offset i=
n that
> > >> virtual file (easily computed from the start of the VMA), and VMA me=
rging is
> > >> just the same as for an ordinary file.
> > >
> > > Interesting point, thanks!
> >
> > FYI, I've advised a master thesis exploring how to update page->index d=
uring
> > mremap() to keep things mergeable:
> >
> > https://dspace.cuni.cz/bitstream/handle/20.500.11956/176288/120426800.p=
df
> >
> > I think the last RFC posting was:
> > https://lore.kernel.org/all/20220516125405.1675-1-matenajakub@gmail.com=
/
> >
> > It was really tricky for the general case. Maybe it would be more feasi=
ble
> > for the limited case Lokesh describes, if we could be sure the pages th=
at
> > are moved aren't mapped anywhere else.

It's great that mremap is being improved for mereabilitly. However,
IIUC, it would still cause unnecessary splits and merges in the
private anonymous case. Also, mremap works with mmap_sem exclusively
held, thereby impacting scalability of concurrent mremap calls.

IMHO, Andrea's userfaultfd REMAP patch is a better alternative as it
doesn't have these downsides.

>
> Lokesh asked me to pick up this work and prepare patches for
> upstreaming. I'll start working on them after I finish with per-vma
> lock support for swap and userfaultd (targeting later this week).
> Thanks for all the input folks!

Thanks so much, Suren!