From: John Hubbard <jhubbard@nvidia.com>
To: Jerome Glisse <jglisse@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>, Jan Kara <jack@suse.cz>,
Matthew Wilcox <willy@infradead.org>,
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 <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: Wed, 12 Dec 2018 14:11:58 -0800 [thread overview]
Message-ID: <311cd7a7-6727-a298-964e-ad238a30bdef@nvidia.com> (raw)
In-Reply-To: <20181212220418.GH5037@redhat.com>
On 12/12/18 2:04 PM, Jerome Glisse wrote:
> On Wed, Dec 12, 2018 at 01:56:00PM -0800, John Hubbard wrote:
>> On 12/12/18 1:30 PM, Jerome Glisse wrote:
>>> On Wed, Dec 12, 2018 at 08:27:35AM -0800, Dan Williams wrote:
>>>> On Wed, Dec 12, 2018 at 7:03 AM Jerome Glisse <jglisse@redhat.com> wrote:
>>>>>
>>>>> On Mon, Dec 10, 2018 at 11:28:46AM +0100, Jan Kara wrote:
>>>>>> On Fri 07-12-18 21:24:46, Jerome Glisse wrote:
>>>>>>> Another crazy idea, why not treating GUP as another mapping of the page
>>>>>>> and caller of GUP would have to provide either a fake anon_vma struct or
>>>>>>> a fake vma struct (or both for PRIVATE mapping of a file where you can
>>>>>>> have a mix of both private and file page thus only if it is a read only
>>>>>>> GUP) that would get added to the list of existing mapping.
>>>>>>>
>>>>>>> So the flow would be:
>>>>>>> somefunction_thatuse_gup()
>>>>>>> {
>>>>>>> ...
>>>>>>> GUP(_fast)(vma, ..., fake_anon, fake_vma);
>>>>>>> ...
>>>>>>> }
>>>>>>>
>>>>>>> GUP(vma, ..., fake_anon, fake_vma)
>>>>>>> {
>>>>>>> if (vma->flags == ANON) {
>>>>>>> // Add the fake anon vma to the anon vma chain as a child
>>>>>>> // of current vma
>>>>>>> } else {
>>>>>>> // Add the fake vma to the mapping tree
>>>>>>> }
>>>>>>>
>>>>>>> // The existing GUP except that now it inc mapcount and not
>>>>>>> // refcount
>>>>>>> GUP_old(..., &nanonymous, &nfiles);
>>>>>>>
>>>>>>> atomic_add(&fake_anon->refcount, nanonymous);
>>>>>>> atomic_add(&fake_vma->refcount, nfiles);
>>>>>>>
>>>>>>> return nanonymous + nfiles;
>>>>>>> }
>>>>>>
>>>>>> Thanks for your idea! This is actually something like I was suggesting back
>>>>>> at LSF/MM in Deer Valley. There were two downsides to this I remember
>>>>>> people pointing out:
>>>>>>
>>>>>> 1) This cannot really work with __get_user_pages_fast(). You're not allowed
>>>>>> to get necessary locks to insert new entry into the VMA tree in that
>>>>>> context. So essentially we'd loose get_user_pages_fast() functionality.
>>>>>>
>>>>>> 2) The overhead e.g. for direct IO may be noticeable. You need to allocate
>>>>>> the fake tracking VMA, get VMA interval tree lock, insert into the tree.
>>>>>> Then on IO completion you need to queue work to unpin the pages again as you
>>>>>> cannot remove the fake VMA directly from interrupt context where the IO is
>>>>>> completed.
>>>>>>
>>>>>> You are right that the cost could be amortized if gup() is called for
>>>>>> multiple consecutive pages however for small IOs there's no help...
>>>>>>
>>>>>> So this approach doesn't look like a win to me over using counter in struct
>>>>>> page and I'd rather try looking into squeezing HMM public page usage of
>>>>>> struct page so that we can fit that gup counter there as well. I know that
>>>>>> it may be easier said than done...
>>>>>
>>>>> So i want back to the drawing board and first i would like to ascertain
>>>>> that we all agree on what the objectives are:
>>>>>
>>>>> [O1] Avoid write back from a page still being written by either a
>>>>> device or some direct I/O or any other existing user of GUP.
>>>>> This would avoid possible file system corruption.
>>>>>
>>>>> [O2] Avoid crash when set_page_dirty() is call on a page that is
>>>>> considered clean by core mm (buffer head have been remove and
>>>>> with some file system this turns into an ugly mess).
>>>>>
>>>>> [O3] DAX and the device block problems, ie with DAX the page map in
>>>>> userspace is the same as the block (persistent memory) and no
>>>>> filesystem nor block device understand page as block or pinned
>>>>> block.
>>>>>
>>>>> For [O3] i don't think any pin count would help in anyway. I believe
>>>>> that the current long term GUP API that does not allow GUP of DAX is
>>>>> the only sane solution for now.
>>>>
>>>> No, that's not a sane solution, it's an emergency hack.
>>>>
>>>>> The real fix would be to teach file-
>>>>> system about DAX/pinned block so that a pinned block is not reuse
>>>>> by filesystem.
>>>>
>>>> We already have taught filesystems about pinned dax pages, see
>>>> dax_layout_busy_page(). As much as possible I want to eliminate the
>>>> concept of "dax pages" as a special case that gets sprinkled
>>>> throughout the mm.
>>>
>>> So thinking on O3 issues what about leveraging the recent change i
>>> did to mmu notifier. Add a event for truncate or any other file
>>> event that need to invalidate the file->page for a range of offset.
>>>
>>> Add mmu notifier listener to GUP user (except direct I/O) so that
>>> they invalidate they hardware mapping or switch the hardware mapping
>>> to use a crappy page. When such event happens what ever user do to
>>> the page through that driver is broken anyway. So it is better to
>>> be loud about it then trying to make it pass under the radar.
>>>
>>> This will put the burden on broken user and allow you to properly
>>> recycle your DAX page.
>>>
>>> Think of it as revoke through mmu notifier.
>>>
>>> So patchset would be:
>>> enum mmu_notifier_event {
>>> + MMU_NOTIFY_TRUNCATE,
>>> };
>>>
>>> + Change truncate code path to emit MMU_NOTIFY_TRUNCATE
>>>
>>
>> That part looks good.
>>
>>> Then for each user of GUP (except direct I/O or other very short
>>> term GUP):
>>
>> but, why is there a difference between how we handle long- and
>> short-term callers? Aren't we just leaving a harder-to-reproduce race
>> condition, if we ignore the short-term gup callers?
>>
>> So, how does activity (including direct IO and other short-term callers)
>> get quiesced (stopped, and guaranteed not to restart or continue), so
>> that truncate or umount can continue on?
>
> The fs would delay block reuse to after refcount is gone so it would
> wait for that. It is ok to do that only for short term user in case of
> direct I/O this should really not happen as it means that the application
> is doing something really stupid. So the waiting on short term user
> would be a rare event.
OK, I think that sounds like there are no race conditions left.
>
>
>>> Patch 1: register mmu notifier
>>> Patch 2: listen to MMU_NOTIFY_TRUNCATE and MMU_NOTIFY_UNMAP
>>> when that happens update the device page table or
>>> usage to point to a crappy page and do put_user_page
>>> on all previously held page
>>
>> Minor point, this sequence should be done within a wrapper around existing
>> get_user_pages(), such as get_user_pages_revokable() or something.
>
> No we want to teach everyone to abide by the rules, if we add yet another
> GUP function prototype people will use the one where they don;t have to
> say they abide by the rules. It is time we advertise the fact that GUP
> should not be use willy nilly for anything without worrying about the
> implication it has :)
Well, the best way to do that is to provide a named function call that
implements the rules. That also makes it easy to grep around and see which
call sites still need upgrades, and which don't.
>
> So i would rather see a consolidation in the number of GUP prototype we
> have than yet another one.
We could eventually get rid of the older GUP prototypes, once we're done
converting. Having a new, named function call will *without question* make
the call site conversion go much easier, and the end result is also better:
the common code is in a central function, rather than being at all the call
sites.
thanks,
--
John Hubbard
NVIDIA
next prev parent reply other threads:[~2018-12-12 22:11 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 [this message]
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
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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