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From: Dan Williams <dan.j.williams@intel.com>
Date: Tue, 23 Feb 2021 14:48:20 -0800
Message-ID: <CAPcyv4gnga5py0j+_1y_9tAxi98+FmYrtOVy7xQTHJ1zJhz2ZA@mail.gmail.com>
Subject: Re: [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps
To: Jason Gunthorpe <jgg@ziepe.ca>
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CC: Joao Martins <joao.m.martins@oracle.com>, Linux MM <linux-mm@kvack.org>, linux-nvdimm <linux-nvdimm@lists.01.org>, Matthew Wilcox <willy@infradead.org>, 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 10:54 AM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Tue, Feb 23, 2021 at 08:44:52AM -0800, Dan Williams wrote:
>
> > > The downside would be one extra lookup in dev_pagemap tree
> > > for other pgmap->types (P2P, FSDAX, PRIVATE). But just one
> > > per gup-fast() call.
> >
> > I'd guess a dev_pagemap lookup is faster than a get_user_pages slow
> > path. It should be measurable that this change is at least as fast or
> > faster than falling back to the slow path, but it would be good to
> > measure.
>
> What is the dev_pagemap thing doing in gup fast anyhow?
>
> I've been wondering for a while..

It's there to synchronize against dax-device removal. The device will
suspend removal awaiting all page references to be dropped, but
gup-fast could be racing device removal. So gup-fast checks for
pte_devmap() to grab a live reference to the device before assuming it
can pin a page.
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