From: John Hubbard <jhubbard@nvidia.com>
To: Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>
Cc: Jerome Glisse <jglisse@redhat.com>,
Dave Chinner <david@fromorbit.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: Tue, 18 Dec 2018 15:29:34 -0800 [thread overview]
Message-ID: <9f43d124-2386-7bfd-d90b-4d0417f51ccd@nvidia.com> (raw)
In-Reply-To: <20181218093017.GB18032@quack2.suse.cz>
On 12/18/18 1:30 AM, Jan Kara wrote:
> On Mon 17-12-18 10:34:43, 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.
>
> We've been through this in:
>
> https://lore.kernel.org/lkml/20180709194740.rymbt2fzohbdmpye@quack2.suse.cz/
>
> back in July. You cannot just skip pages for fsync(2). So as I wrote above -
> memory cleaning writeback can skip pinned pages. Data integrity writeback
> must be able to write pinned pages. And bouncing is one reasonable way how
> to do that.
>
> This writeback decision is pretty much independent from the mechanism by
> which we are going to identify pinned pages. Whether that's going to be
> separate counter in struct page, using page->_mapcount, or separately
> allocated data structure as you know promote.
>
> I currently like the most the _mapcount suggestion from Jerome but I'm not
> really attached to any solution as long as it performs reasonably and
> someone can make it working :) as I don't have time to implement it at
> least till January.
>
OK, so let's take another look at Jerome's _mapcount idea all by itself (using
*only* the tracking pinned pages aspect), given that it is the lightest weight
solution for that.
So as I understand it, this would use page->_mapcount to store both the real
mapcount, and the dma pinned count (simply added together), but only do so for
file-backed (non-anonymous) pages:
__get_user_pages()
{
...
get_page(page);
if (!PageAnon)
atomic_inc(page->_mapcount);
...
}
put_user_page(struct page *page)
{
...
if (!PageAnon)
atomic_dec(&page->_mapcount);
put_page(page);
...
}
...and then in the various consumers of the DMA pinned count, we use page_mapped(page)
to see if any mapcount remains, and if so, we treat it as DMA pinned. Is that what you
had in mind?
--
thanks,
John Hubbard
NVIDIA
next prev parent reply other threads:[~2018-12-18 23:29 UTC|newest]
Thread overview: 213+ 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
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 [this message]
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 2:59 ` John Hubbard
2019-01-11 16:51 ` Jerome Glisse
2019-01-11 16:51 ` Jerome Glisse
2019-01-12 1:04 ` John Hubbard
2019-01-12 1:04 ` John Hubbard
2019-01-12 2:02 ` Jerome Glisse
2019-01-12 2:02 ` Jerome Glisse
2019-01-12 2:38 ` John Hubbard
2019-01-12 2:38 ` John Hubbard
2019-01-12 2:46 ` Jerome Glisse
2019-01-12 2:46 ` Jerome Glisse
2019-01-12 3:06 ` John Hubbard
2019-01-12 3:06 ` John Hubbard
2019-01-12 3:25 ` Jerome Glisse
2019-01-12 3:25 ` Jerome Glisse
2019-01-12 20:46 ` John Hubbard
2019-01-12 20:46 ` John Hubbard
2019-01-14 14:54 ` Jan Kara
2019-01-14 14:54 ` Jan Kara
2019-01-14 17:21 ` Jerome Glisse
2019-01-14 17:21 ` Jerome Glisse
2019-01-14 19:09 ` John Hubbard
2019-01-14 19:09 ` John Hubbard
2019-01-15 8:34 ` Jan Kara
2019-01-15 8:34 ` Jan Kara
2019-01-15 21:39 ` John Hubbard
2019-01-15 21:39 ` John Hubbard
2019-01-15 8:07 ` Jan Kara
2019-01-15 8:07 ` Jan Kara
2019-01-15 17:15 ` Jerome Glisse
2019-01-15 17:15 ` Jerome Glisse
2019-01-15 21:56 ` John Hubbard
2019-01-15 21:56 ` John Hubbard
2019-01-15 22:12 ` Jerome Glisse
2019-01-15 22:12 ` Jerome Glisse
2019-01-16 0:44 ` John Hubbard
2019-01-16 0:44 ` John Hubbard
2019-01-16 1:56 ` Jerome Glisse
2019-01-16 1:56 ` Jerome Glisse
2019-01-16 2:01 ` Dan Williams
2019-01-16 2:01 ` Dan Williams
2019-01-16 2:23 ` Jerome Glisse
2019-01-16 2:23 ` Jerome Glisse
2019-01-16 4:34 ` Dave Chinner
2019-01-16 4:34 ` Dave Chinner
2019-01-16 14:50 ` Jerome Glisse
2019-01-16 14:50 ` Jerome Glisse
2019-01-16 22:51 ` Dave Chinner
2019-01-16 22:51 ` Dave Chinner
2019-01-16 11:38 ` Jan Kara
2019-01-16 11:38 ` Jan Kara
2019-01-16 13:08 ` Jerome Glisse
2019-01-16 13:08 ` Jerome Glisse
2019-01-17 5:42 ` John Hubbard
2019-01-17 5:42 ` John Hubbard
2019-01-17 15:21 ` Jerome Glisse
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 9:30 ` Jan Kara
2019-01-17 15:17 ` Jerome Glisse
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 5:25 ` John Hubbard
2019-01-17 9:04 ` Jan Kara
2019-01-17 9:04 ` Jan Kara
2019-01-12 3:14 ` Jerome Glisse
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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