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From: Jason Gunthorpe <jgg@ziepe.ca>
To: Dan Williams <dan.j.williams@intel.com>
Cc: Joao Martins <joao.m.martins@oracle.com>,
	Linux MM <linux-mm@kvack.org>, Ira Weiny <ira.weiny@intel.com>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Matthew Wilcox <willy@infradead.org>,
	Jane Chu <jane.chu@oracle.com>,
	Muchun Song <songmuchun@bytedance.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps
Date: Tue, 23 Feb 2021 19:07:23 -0400	[thread overview]
Message-ID: <20210223230723.GP2643399@ziepe.ca> (raw)
In-Reply-To: <CAPcyv4gnga5py0j+_1y_9tAxi98+FmYrtOVy7xQTHJ1zJhz2ZA@mail.gmail.com>

On Tue, Feb 23, 2021 at 02:48:20PM -0800, Dan Williams wrote:
> 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.

From the perspective of CPU A it can't tell if CPU B is doing a HW
page table walk or a GUP fast when it invalidates a page table. The
design of gup-fast is supposed to be the same as the design of a HW
page table walk, and the tlb invalidate CPU A does when removing a
page from a page table is supposed to serialize against both a HW page
table walk and gup-fast.

Given that the HW page table walker does not do dev_pagemap stuff, why
does gup-fast?

Can you sketch the exact race this is protecting against?

Jason


  reply	other threads:[~2021-02-23 23:07 UTC|newest]

Thread overview: 80+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-08 17:28 [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps Joao Martins
2020-12-08 17:28 ` [PATCH RFC 1/9] memremap: add ZONE_DEVICE support for compound pages Joao Martins
2020-12-09  5:59   ` John Hubbard
2020-12-09  6:33     ` Matthew Wilcox
2020-12-09 13:12       ` Joao Martins
2021-02-20  1:43     ` Dan Williams
2021-02-22 11:24       ` Joao Martins
2021-02-22 20:37         ` Dan Williams
2021-02-23 15:46           ` Joao Martins
2021-02-23 16:50             ` Dan Williams
2021-02-23 17:18               ` Joao Martins
2021-02-23 18:18                 ` Dan Williams
2021-03-10 18:12           ` Joao Martins
2021-03-12  5:54             ` Dan Williams
2021-02-20  1:24   ` Dan Williams
2021-02-22 11:09     ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 2/9] sparse-vmemmap: Consolidate arguments in vmemmap section populate Joao Martins
2020-12-09  6:16   ` John Hubbard
2020-12-09 13:51     ` Joao Martins
2021-02-20  1:49   ` Dan Williams
2021-02-22 11:26     ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 3/9] sparse-vmemmap: Reuse vmemmap areas for a given mhp_params::align Joao Martins
2020-12-08 17:38   ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 3/9] sparse-vmemmap: Reuse vmemmap areas for a given page size Joao Martins
2021-02-20  3:34   ` Dan Williams
2021-02-22 11:42     ` Joao Martins
2021-02-22 22:40       ` Dan Williams
2021-02-23 15:46         ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 4/9] mm/page_alloc: Reuse tail struct pages for compound pagemaps Joao Martins
2021-02-20  6:17   ` Dan Williams
2021-02-22 12:01     ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 5/9] device-dax: Compound pagemap support Joao Martins
2020-12-08 17:28 ` [PATCH RFC 6/9] mm/gup: Grab head page refcount once for group of subpages Joao Martins
2020-12-08 19:49   ` Jason Gunthorpe
2020-12-09 11:05     ` Joao Martins
2020-12-09 15:15       ` Jason Gunthorpe
2020-12-09 16:02         ` Joao Martins
2020-12-09 16:24           ` Jason Gunthorpe
2020-12-09 17:27             ` Joao Martins
2020-12-09 18:14             ` Matthew Wilcox
2020-12-09 19:08               ` Jason Gunthorpe
2020-12-10 15:43               ` Joao Martins
2020-12-09  4:40   ` John Hubbard
2020-12-09 13:44     ` Joao Martins
2020-12-08 17:28 ` [PATCH RFC 7/9] mm/gup: Decrement head page " Joao Martins
2020-12-08 19:34   ` Jason Gunthorpe
2020-12-09  5:06     ` John Hubbard
2020-12-09 13:43       ` Jason Gunthorpe
2020-12-09 12:17     ` Joao Martins
2020-12-17 19:05     ` Joao Martins
2020-12-17 20:05       ` Jason Gunthorpe
2020-12-17 22:34         ` Joao Martins
2020-12-18 14:25           ` Jason Gunthorpe
2020-12-19  2:06         ` John Hubbard
2020-12-19 13:10           ` Joao Martins
2020-12-08 17:29 ` [PATCH RFC 8/9] RDMA/umem: batch page unpin in __ib_mem_release() Joao Martins
2020-12-08 19:29   ` Jason Gunthorpe
2020-12-09 10:59     ` Joao Martins
2020-12-19 13:15       ` Joao Martins
2020-12-09  5:18   ` John Hubbard
2020-12-08 17:29 ` [PATCH RFC 9/9] mm: Add follow_devmap_page() for devdax vmas Joao Martins
2020-12-08 19:57   ` Jason Gunthorpe
2020-12-09  8:05     ` Christoph Hellwig
2020-12-09 11:19     ` Joao Martins
2020-12-09  5:23   ` John Hubbard
2020-12-09  9:38 ` [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps David Hildenbrand
2020-12-09  9:52 ` [External] " Muchun Song
2021-02-20  1:18 ` Dan Williams
2021-02-22 11:06   ` Joao Martins
2021-02-22 14:32     ` Joao Martins
2021-02-23 16:28   ` Joao Martins
2021-02-23 16:44     ` Dan Williams
2021-02-23 17:15       ` Joao Martins
2021-02-23 18:15         ` Dan Williams
2021-02-23 18:54       ` Jason Gunthorpe
2021-02-23 22:48         ` Dan Williams
2021-02-23 23:07           ` Jason Gunthorpe [this message]
2021-02-24  0:14             ` Dan Williams
2021-02-24  1:00               ` Jason Gunthorpe
2021-02-24  1:32                 ` Dan Williams

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