From: Matthew Wilcox <willy@infradead.org>
To: Jerome Glisse <jglisse@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>, Jan Kara <jack@suse.cz>,
John Hubbard <jhubbard@nvidia.com>,
Dan Williams <dan.j.williams@intel.com>,
John Hubbard <john.hubbard@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Linux MM <linux-mm@kvack.org>,
tom@talpey.com, Al Viro <viro@zeniv.linux.org.uk>,
benve@cisco.com, Christoph Hellwig <hch@infradead.org>,
Christopher Lameter <cl@linux.com>,
"Dalessandro, Dennis" <dennis.dalessandro@intel.com>,
Doug Ledford <dledford@redhat.com>,
Jason Gunthorpe <jgg@ziepe.ca>, Michal Hocko <mhocko@kernel.org>,
mike.marciniszyn@intel.com, rcampbell@nvidia.com,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions
Date: Mon, 17 Dec 2018 11:51:51 -0800 [thread overview]
Message-ID: <20181217195150.GP10600@bombadil.infradead.org> (raw)
In-Reply-To: <20181217194759.GB3341@redhat.com>
On Mon, Dec 17, 2018 at 02:48:00PM -0500, Jerome Glisse wrote:
> On Mon, Dec 17, 2018 at 10:34:43AM -0800, Matthew Wilcox wrote:
> > On Mon, Dec 17, 2018 at 01:11:50PM -0500, Jerome Glisse wrote:
> > > On Mon, Dec 17, 2018 at 08:58:19AM +1100, Dave Chinner wrote:
> > > > Sure, that's a possibility, but that doesn't close off any race
> > > > conditions because there can be DMA into the page in progress while
> > > > the page is being bounced, right? AFAICT this ext3+DIF/DIX case is
> > > > different in that there is no 3rd-party access to the page while it
> > > > is under IO (ext3 arbitrates all access to it's metadata), and so
> > > > nothing can actually race for modification of the page between
> > > > submission and bouncing at the block layer.
> > > >
> > > > In this case, the moment the page is unlocked, anyone else can map
> > > > it and start (R)DMA on it, and that can happen before the bio is
> > > > bounced by the block layer. So AFAICT, block layer bouncing doesn't
> > > > solve the problem of racing writeback and DMA direct to the page we
> > > > are doing IO on. Yes, it reduces the race window substantially, but
> > > > it doesn't get rid of it.
> > >
> > > So the event flow is:
> > > - userspace create object that match a range of virtual address
> > > against a given kernel sub-system (let's say infiniband) and
> > > let's assume that the range is an mmap() of a regular file
> > > - device driver do GUP on the range (let's assume it is a write
> > > GUP) so if the page is not already map with write permission
> > > in the page table than a page fault is trigger and page_mkwrite
> > > happens
> > > - Once GUP return the page to the device driver and once the
> > > device driver as updated the hardware states to allow access
> > > to this page then from that point on hardware can write to the
> > > page at _any_ time, it is fully disconnected from any fs event
> > > like write back, it fully ignore things like page_mkclean
> > >
> > > This is how it is to day, we allowed people to push upstream such
> > > users of GUP. This is a fact we have to live with, we can not stop
> > > hardware access to the page, we can not force the hardware to follow
> > > page_mkclean and force a page_mkwrite once write back ends. This is
> > > the situation we are inheriting (and i am personnaly not happy with
> > > that).
> > >
> > > >From my point of view we are left with 2 choices:
> > > [C1] break all drivers that do not abide by the page_mkclean and
> > > page_mkwrite
> > > [C2] mitigate as much as possible the issue
> > >
> > > For [C2] the idea is to keep track of GUP per page so we know if we
> > > can expect the page to be written to at any time. Here is the event
> > > flow:
> > > - driver GUP the page and program the hardware, page is mark as
> > > GUPed
> > > ...
> > > - write back kicks in on the dirty page, lock the page and every
> > > thing as usual , sees it is GUPed and inform the block layer to
> > > use a bounce page
> >
> > No. The solution John, Dan & I have been looking at is to take the
> > dirty page off the LRU while it is pinned by GUP. It will never be
> > found for writeback.
> >
> > That's not the end of the story though. Other parts of the kernel (eg
> > msync) also need to be taught to stay away from pages which are pinned
> > by GUP. But the idea is that no page gets written back to storage while
> > it's pinned by GUP. Only when the last GUP ends is the page returned
> > to the list of dirty pages.
> >
> > > - block layer copy the page to a bounce page effectively creating
> > > a snapshot of what is the content of the real page. This allows
> > > everything in block layer that need stable content to work on
> > > the bounce page (raid, stripping, encryption, ...)
> > > - once write back is done the page is not marked clean but stays
> > > dirty, this effectively disable things like COW for filesystem
> > > and other feature that expect page_mkwrite between write back.
> > > AFAIK it is believe that it is something acceptable
> >
> > So none of this is necessary.
>
> With the solution you are proposing we loose GUP fast and we have to
> allocate a structure for each page that is under GUP, and the LRU
> changes too. Moreover by not writing back there is a greater chance
> of data loss.
Why can't you store the hmm_data in a side data structure? Why does it
have to be in struct page?
next prev parent reply other threads:[~2018-12-17 19:51 UTC|newest]
Thread overview: 179+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-04 0:17 [PATCH 0/2] put_user_page*(): start converting the call sites john.hubbard
2018-12-04 0:17 ` [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions john.hubbard
2018-12-04 7:53 ` Mike Rapoport
2018-12-05 1:40 ` John Hubbard
2018-12-04 20:28 ` Dan Williams
2018-12-04 21:56 ` John Hubbard
2018-12-04 23:03 ` Dan Williams
2018-12-05 0:36 ` Jerome Glisse
2018-12-05 0:40 ` Dan Williams
2018-12-05 0:59 ` John Hubbard
2018-12-05 0:58 ` John Hubbard
2018-12-05 1:00 ` Dan Williams
2018-12-05 1:15 ` Matthew Wilcox
2018-12-05 1:44 ` Jerome Glisse
2018-12-05 1:57 ` John Hubbard
2018-12-07 2:45 ` John Hubbard
2018-12-07 19:16 ` Jerome Glisse
2018-12-07 19:26 ` Dan Williams
2018-12-07 19:40 ` Jerome Glisse
2018-12-08 0:52 ` John Hubbard
2018-12-08 2:24 ` Jerome Glisse
2018-12-10 10:28 ` Jan Kara
2018-12-12 15:03 ` Jerome Glisse
2018-12-12 16:27 ` Dan Williams
2018-12-12 17:02 ` Jerome Glisse
2018-12-12 17:49 ` Dan Williams
2018-12-12 19:07 ` John Hubbard
2018-12-12 21:30 ` Jerome Glisse
2018-12-12 21:40 ` Dan Williams
2018-12-12 21:53 ` Jerome Glisse
2018-12-12 22:11 ` Matthew Wilcox
2018-12-12 22:16 ` Jerome Glisse
2018-12-12 23:37 ` Jason Gunthorpe
2018-12-12 23:46 ` John Hubbard
2018-12-12 23:54 ` Dan Williams
2018-12-13 0:01 ` Jerome Glisse
2018-12-13 0:18 ` Dan Williams
2018-12-13 0:44 ` Jerome Glisse
2018-12-13 3:26 ` Jason Gunthorpe
2018-12-13 3:20 ` Jason Gunthorpe
2018-12-13 12:43 ` Jerome Glisse
2018-12-13 13:40 ` Tom Talpey
2018-12-13 14:18 ` Jerome Glisse
2018-12-13 14:51 ` Tom Talpey
2018-12-13 15:18 ` Jerome Glisse
2018-12-13 18:12 ` Tom Talpey
2018-12-13 19:18 ` Jerome Glisse
2018-12-14 10:41 ` Jan Kara
2018-12-14 15:25 ` Jerome Glisse
2018-12-12 21:56 ` John Hubbard
2018-12-12 22:04 ` Jerome Glisse
2018-12-12 22:11 ` John Hubbard
2018-12-12 22:14 ` Jerome Glisse
2018-12-12 22:17 ` John Hubbard
2018-12-12 21:46 ` Dave Chinner
2018-12-12 21:59 ` Jerome Glisse
2018-12-13 0:51 ` Dave Chinner
2018-12-13 2:02 ` Jerome Glisse
2018-12-13 15:56 ` Christopher Lameter
2018-12-13 16:02 ` Jerome Glisse
2018-12-14 6:00 ` Dave Chinner
2018-12-14 15:13 ` Jerome Glisse
2018-12-14 3:52 ` John Hubbard
2018-12-14 5:21 ` Dan Williams
2018-12-14 6:11 ` John Hubbard
2018-12-14 15:20 ` Jerome Glisse
2018-12-14 19:38 ` Dan Williams
2018-12-14 19:48 ` Matthew Wilcox
2018-12-14 19:53 ` Dave Hansen
2018-12-14 20:03 ` Matthew Wilcox
2018-12-14 20:17 ` Dan Williams
2018-12-14 20:29 ` Matthew Wilcox
2018-12-15 0:41 ` John Hubbard
2018-12-17 8:56 ` Jan Kara
2018-12-17 18:28 ` Dan Williams
2018-12-14 15:43 ` Jan Kara
2018-12-16 21:58 ` Dave Chinner
2018-12-17 18:11 ` Jerome Glisse
2018-12-17 18:34 ` Matthew Wilcox
2018-12-17 19:48 ` Jerome Glisse
2018-12-17 19:51 ` Matthew Wilcox [this message]
2018-12-17 19:54 ` Jerome Glisse
2018-12-17 19:59 ` Matthew Wilcox
2018-12-17 20:55 ` Jerome Glisse
2018-12-17 21:03 ` Matthew Wilcox
2018-12-17 21:15 ` Jerome Glisse
2018-12-18 1:09 ` Dave Chinner
2018-12-18 6:12 ` Darrick J. Wong
2018-12-18 9:30 ` Jan Kara
2018-12-18 23:29 ` John Hubbard
2018-12-19 2:07 ` Jerome Glisse
2018-12-19 11:08 ` Jan Kara
2018-12-20 10:54 ` John Hubbard
2018-12-20 16:50 ` Jerome Glisse
2018-12-20 16:57 ` Dan Williams
2018-12-20 16:49 ` Jerome Glisse
2019-01-03 1:55 ` Jerome Glisse
2019-01-03 3:27 ` John Hubbard
2019-01-03 14:57 ` Jerome Glisse
2019-01-03 9:26 ` Jan Kara
2019-01-03 14:44 ` Jerome Glisse
2019-01-11 2:59 ` John Hubbard
2019-01-11 16:51 ` Jerome Glisse
2019-01-12 1:04 ` John Hubbard
2019-01-12 2:02 ` Jerome Glisse
2019-01-12 2:38 ` John Hubbard
2019-01-12 2:46 ` Jerome Glisse
2019-01-12 3:06 ` John Hubbard
2019-01-12 3:25 ` Jerome Glisse
2019-01-12 20:46 ` John Hubbard
2019-01-14 14:54 ` Jan Kara
2019-01-14 17:21 ` Jerome Glisse
2019-01-14 19:09 ` John Hubbard
2019-01-15 8:34 ` Jan Kara
2019-01-15 21:39 ` John Hubbard
2019-01-15 8:07 ` Jan Kara
2019-01-15 17:15 ` Jerome Glisse
2019-01-15 21:56 ` John Hubbard
2019-01-15 22:12 ` Jerome Glisse
2019-01-16 0:44 ` John Hubbard
2019-01-16 1:56 ` Jerome Glisse
2019-01-16 2:01 ` Dan Williams
2019-01-16 2:23 ` Jerome Glisse
2019-01-16 4:34 ` Dave Chinner
2019-01-16 14:50 ` Jerome Glisse
2019-01-16 22:51 ` Dave Chinner
2019-01-16 11:38 ` Jan Kara
2019-01-16 13:08 ` Jerome Glisse
2019-01-17 5:42 ` John Hubbard
2019-01-17 15:21 ` Jerome Glisse
2019-01-18 0:16 ` Dave Chinner
2019-01-18 1:59 ` Jerome Glisse
2019-01-17 9:30 ` Jan Kara
2019-01-17 15:17 ` Jerome Glisse
2019-01-22 15:24 ` Jan Kara
2019-01-22 16:46 ` Jerome Glisse
2019-01-23 18:02 ` Jan Kara
2019-01-23 19:04 ` Jerome Glisse
2019-01-29 0:22 ` John Hubbard
2019-01-29 1:23 ` Jerome Glisse
2019-01-29 6:41 ` John Hubbard
2019-01-29 10:12 ` Jan Kara
2019-01-30 2:21 ` John Hubbard
2019-01-17 5:25 ` John Hubbard
2019-01-17 9:04 ` Jan Kara
2019-01-12 3:14 ` Jerome Glisse
2018-12-18 10:33 ` Jan Kara
2018-12-18 23:42 ` Dave Chinner
2018-12-19 3:03 ` Jason Gunthorpe
2018-12-19 5:26 ` Dan Williams
2018-12-19 11:19 ` Jan Kara
2018-12-19 10:28 ` Dave Chinner
2018-12-19 11:35 ` Jan Kara
2018-12-19 16:56 ` Jason Gunthorpe
2018-12-19 22:33 ` Dave Chinner
2018-12-20 9:07 ` Jan Kara
2018-12-20 16:54 ` Jerome Glisse
2018-12-19 13:24 ` Jan Kara
2018-12-08 5:18 ` Matthew Wilcox
2018-12-12 19:13 ` John Hubbard
2018-12-08 7:16 ` Dan Williams
2018-12-08 16:33 ` Jerome Glisse
2018-12-08 16:48 ` Christoph Hellwig
2018-12-08 17:47 ` Jerome Glisse
2018-12-08 18:26 ` Christoph Hellwig
2018-12-08 18:45 ` Jerome Glisse
2018-12-08 18:09 ` Dan Williams
2018-12-08 18:12 ` Christoph Hellwig
2018-12-11 6:18 ` Dave Chinner
2018-12-05 5:52 ` Dan Williams
2018-12-05 11:16 ` Jan Kara
2018-12-04 0:17 ` [PATCH 2/2] infiniband/mm: convert put_page() to put_user_page*() john.hubbard
2018-12-04 17:10 ` [PATCH 0/2] put_user_page*(): start converting the call sites David Laight
2018-12-05 1:05 ` John Hubbard
2018-12-05 14:08 ` David Laight
2018-12-28 8:37 ` Pavel Machek
2019-02-08 7:56 [PATCH 0/2] mm: put_user_page() call site conversion first john.hubbard
2019-02-08 7:56 ` [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions john.hubbard
2019-02-08 10:32 ` Mike Rapoport
2019-02-08 20:44 ` John Hubbard
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