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From: Dan Williams <dan.j.williams@intel.com>
Date: Tue, 23 Feb 2021 10:15:12 -0800
Message-ID: <CAPcyv4gdL3p-RVKSVGPfSdBWW+eakTHqCYZp2hqV4QiXCv6KTw@mail.gmail.com>
Subject: Re: [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps
To: Joao Martins <joao.m.martins@oracle.com>
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CC: Linux MM <linux-mm@kvack.org>, linux-nvdimm <linux-nvdimm@lists.01.org>, Matthew Wilcox <willy@infradead.org>, Jason Gunthorpe <jgg@ziepe.ca>, Muchun Song <songmuchun@bytedance.com>, Mike Kravetz <mike.kravetz@oracle.com>, Andrew Morton <akpm@linux-foundation.org>
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On Tue, Feb 23, 2021 at 9:16 AM Joao Martins <joao.m.martins@oracle.com> wrote:
>
> On 2/23/21 4:44 PM, Dan Williams wrote:
> > On Tue, Feb 23, 2021 at 8:30 AM Joao Martins <joao.m.martins@oracle.com> wrote:
> >>
> >> On 2/20/21 1:18 AM, Dan Williams wrote:
> >>> On Tue, Dec 8, 2020 at 9:32 AM Joao Martins <joao.m.martins@oracle.com> wrote:
> >>>> Patch 6 - 8: Optimize grabbing/release a page refcount changes given that we
> >>>> are working with compound pages i.e. we do 1 increment/decrement to the head
> >>>> page for a given set of N subpages compared as opposed to N individual writes.
> >>>> {get,pin}_user_pages_fast() for zone_device with compound pagemap consequently
> >>>> improves considerably, and unpin_user_pages() improves as well when passed a
> >>>> set of consecutive pages:
> >>>>
> >>>>                                            before          after
> >>>>     (get_user_pages_fast 1G;2M page size) ~75k  us -> ~3.2k ; ~5.2k us
> >>>>     (pin_user_pages_fast 1G;2M page size) ~125k us -> ~3.4k ; ~5.5k us
> >>>
> >>> Compelling!
> >>>
> >>
> >> BTW is there any reason why we don't support pin_user_pages_fast() with FOLL_LONGTERM for
> >> device-dax?
> >>
> >
> > Good catch.
> >
> > Must have been an oversight of the conversion. FOLL_LONGTERM collides
> > with filesystem operations, but not device-dax.
>
> hmmmm, fwiw, it was unilaterally disabled for any devmap pmd/pud in commit 7af75561e171
> ("mm/gup: add FOLL_LONGTERM capability to GUP fast") and I must only assume that
> by "DAX pages" the submitter was only referring to fs-dax pages.

Agree, that was an fsdax only assumption. Maybe this went unnoticed
because the primary gup-fast case for direct-I/O was not impacted.
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