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From: Ira Weiny <ira.weiny@intel.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
	Theodore Ts'o <tytso@mit.edu>, John Hubbard <jhubbard@nvidia.com>,
	Michal Hocko <mhocko@suse.com>,
	Dave Chinner <david@fromorbit.com>,
	linux-xfs@vger.kernel.org, linux-rdma@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-nvdimm@lists.01.org, linux-ext4@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [RFC PATCH v2 16/19] RDMA/uverbs: Add back pointer to system file object
Date: Tue, 13 Aug 2019 13:38:59 -0700	[thread overview]
Message-ID: <20190813203858.GA12695@iweiny-DESK2.sc.intel.com> (raw)
In-Reply-To: <20190813180022.GF29508@ziepe.ca>

On Tue, Aug 13, 2019 at 03:00:22PM -0300, Jason Gunthorpe wrote:
> On Tue, Aug 13, 2019 at 10:41:42AM -0700, Ira Weiny wrote:
> 
> > And I was pretty sure uverbs_destroy_ufile_hw() would take care of (or ensure
> > that some other thread is) destroying all the MR's we have associated with this
> > FD.
> 
> fd's can't be revoked, so destroy_ufile_hw() can't touch them. It
> deletes any underlying HW resources, but the FD persists.

I misspoke.  I should have said associated with this "context".  And of course
uverbs_destroy_ufile_hw() does not touch the FD.  What I mean is that the
struct file which had file_pins hanging off of it would be getting its file
pins destroyed by uverbs_destroy_ufile_hw().  Therefore we don't need the FD
after uverbs_destroy_ufile_hw() is done.

But since it does not block it may be that the struct file is gone before the
MR is actually destroyed.  Which means I think the GUP code would blow up in
that case...  :-(

I was thinking it was the other way around.  And in fact most of the time I
think it is.  But we can't depend on that...

>  
> > > This is why having a back pointer like this is so ugly, it creates a
> > > reference counting cycle
> > 
> > Yep...  I worked through this...  and it was giving me fits...
> > 
> > Anyway, the struct file is the only object in the core which was reasonable to
> > store this information in since that is what is passed around to other
> > processes...
> 
> It could be passed down in the uattr_bundle, once you are in file operations

What is "It"?  The struct file?  Or the file pin information?

> handle the file is guarenteed to exist, and we've now arranged things

I don't understand what you mean by "... once you are in file operations handle... "?

> so the uattr_bundle flows into the umem, as umems can only be
> established under a system call.

"uattr_bundle" == uverbs_attr_bundle right?

The problem is that I don't think the core should be handling
uverbs_attr_bundles directly.  So, I think you are driving at the same idea I
had WRT callbacks into the driver.

The drivers could provide some generic object (in RDMA this could be the
uverbs_attr_bundle) which represents their "context".

The GUP code calls back into the driver with file pin information as it
encounters and pins pages.  The driver, RDMA in this case, associates this
information with the "context".

But for the procfs interface, that context then needs to be associated with any
file which points to it...  For RDMA, or any other "FD based pin mechanism", it
would be up to the driver to "install" a procfs handler into any struct file
which _may_ point to this context.  (before _or_ after memory pins).

Then the procfs code can walk the FD array and if this handler is installed it
knows there is file pin information associated with that struct file and it can
be printed...

This is not impossible.  But I think is a lot harder for drivers to make
right...

Ira



  reply	other threads:[~2019-08-13 20:39 UTC|newest]

Thread overview: 110+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-09 22:58 [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-) ira.weiny
2019-08-09 22:58 ` [RFC PATCH v2 01/19] fs/locks: Export F_LAYOUT lease to user space ira.weiny
2019-08-09 23:52   ` Dave Chinner
2019-08-12 17:36     ` Ira Weiny
2019-08-14  8:05       ` Dave Chinner
2019-08-14 11:21         ` Jeff Layton
2019-08-14 11:38           ` Dave Chinner
2019-08-09 22:58 ` [RFC PATCH v2 02/19] fs/locks: Add Exclusive flag to user Layout lease ira.weiny
2019-08-14 14:15   ` Jeff Layton
2019-08-14 21:56     ` Dave Chinner
2019-08-26 10:41       ` Jeff Layton
2019-08-29 23:34         ` Ira Weiny
2019-09-04 12:52           ` Jeff Layton
2019-09-04 23:12   ` John Hubbard
2019-08-09 22:58 ` [RFC PATCH v2 03/19] mm/gup: Pass flags down to __gup_device_huge* calls ira.weiny
2019-08-09 22:58 ` [RFC PATCH v2 04/19] mm/gup: Ensure F_LAYOUT lease is held prior to GUP'ing pages ira.weiny
2019-08-09 22:58 ` [RFC PATCH v2 05/19] fs/ext4: Teach ext4 to break layout leases ira.weiny
2019-08-09 22:58 ` [RFC PATCH v2 06/19] fs/ext4: Teach dax_layout_busy_page() to operate on a sub-range ira.weiny
2019-08-23 15:18   ` Vivek Goyal
2019-08-29 18:52     ` Ira Weiny
2019-08-09 22:58 ` [RFC PATCH v2 07/19] fs/xfs: Teach xfs to use new dax_layout_busy_page() ira.weiny
2019-08-09 23:30   ` Dave Chinner
2019-08-12 18:05     ` Ira Weiny
2019-08-14  8:04       ` Dave Chinner
2019-08-09 22:58 ` [RFC PATCH v2 08/19] fs/xfs: Fail truncate if page lease can't be broken ira.weiny
2019-08-09 23:22   ` Dave Chinner
2019-08-12 18:08     ` Ira Weiny
2019-08-09 22:58 ` [RFC PATCH v2 09/19] mm/gup: Introduce vaddr_pin structure ira.weiny
2019-08-10  0:06   ` John Hubbard
2019-08-09 22:58 ` [RFC PATCH v2 10/19] mm/gup: Pass a NULL vaddr_pin through GUP fast ira.weiny
2019-08-10  0:06   ` John Hubbard
2019-08-09 22:58 ` [RFC PATCH v2 11/19] mm/gup: Pass follow_page_context further down the call stack ira.weiny
2019-08-10  0:18   ` John Hubbard
2019-08-12 19:01     ` Ira Weiny
2019-08-09 22:58 ` [RFC PATCH v2 12/19] mm/gup: Prep put_user_pages() to take an vaddr_pin struct ira.weiny
2019-08-10  0:30   ` John Hubbard
2019-08-12 20:46     ` Ira Weiny
2019-08-09 22:58 ` [RFC PATCH v2 13/19] {mm,file}: Add file_pins objects ira.weiny
2019-08-09 22:58 ` [RFC PATCH v2 14/19] fs/locks: Associate file pins while performing GUP ira.weiny
2019-08-09 22:58 ` [RFC PATCH v2 15/19] mm/gup: Introduce vaddr_pin_pages() ira.weiny
2019-08-10  0:09   ` John Hubbard
2019-08-12 21:00     ` Ira Weiny
2019-08-12 21:20       ` John Hubbard
2019-08-11 23:07   ` John Hubbard
2019-08-12 21:01     ` Ira Weiny
2019-08-12 12:28   ` Jason Gunthorpe
2019-08-12 21:48     ` Ira Weiny
2019-08-13 11:47       ` Jason Gunthorpe
2019-08-13 17:46         ` Ira Weiny
2019-08-13 17:56           ` John Hubbard
2019-08-09 22:58 ` [RFC PATCH v2 16/19] RDMA/uverbs: Add back pointer to system file object ira.weiny
2019-08-12 13:00   ` Jason Gunthorpe
2019-08-12 17:28     ` Ira Weiny
2019-08-12 17:56       ` Jason Gunthorpe
2019-08-12 21:15         ` Ira Weiny
2019-08-13 11:48           ` Jason Gunthorpe
2019-08-13 17:41             ` Ira Weiny
2019-08-13 18:00               ` Jason Gunthorpe
2019-08-13 20:38                 ` Ira Weiny [this message]
2019-08-14 12:23                   ` Jason Gunthorpe
2019-08-14 17:50                     ` Ira Weiny
2019-08-14 18:15                       ` Jason Gunthorpe
2019-09-04 22:25                     ` Ira Weiny
2019-09-11  8:19                       ` Jason Gunthorpe
2019-08-09 22:58 ` [RFC PATCH v2 17/19] RDMA/umem: Convert to vaddr_[pin|unpin]* operations ira.weiny
2019-08-09 22:58 ` [RFC PATCH v2 18/19] {mm,procfs}: Add display file_pins proc ira.weiny
2019-08-09 22:58 ` [RFC PATCH v2 19/19] mm/gup: Remove FOLL_LONGTERM DAX exclusion ira.weiny
2019-08-14 10:17 ` [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-) Jan Kara
2019-08-14 18:08   ` Ira Weiny
2019-08-15 13:05     ` Jan Kara
2019-08-16 19:05       ` Ira Weiny
2019-08-16 23:20         ` [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ; -) Ira Weiny
2019-08-19  6:36           ` Jan Kara
2019-08-17  2:26         ` [RFC PATCH v2 00/19] RDMA/FS DAX truncate proposal V1,000,002 ;-) Dave Chinner
2019-08-19  6:34           ` Jan Kara
2019-08-19  9:24             ` Dave Chinner
2019-08-19 12:38               ` Jason Gunthorpe
2019-08-19 21:53                 ` Ira Weiny
2019-08-20  1:12                 ` Dave Chinner
2019-08-20 11:55                   ` Jason Gunthorpe
2019-08-21 18:02                     ` Ira Weiny
2019-08-21 18:13                       ` Jason Gunthorpe
2019-08-21 18:22                         ` John Hubbard
2019-08-21 18:57                         ` Ira Weiny
2019-08-21 19:06                           ` Ira Weiny
2019-08-21 19:48                           ` Jason Gunthorpe
2019-08-21 20:44                             ` Ira Weiny
2019-08-21 23:49                               ` Jason Gunthorpe
2019-08-23  3:23                               ` Dave Chinner
2019-08-23 12:04                                 ` Jason Gunthorpe
2019-08-24  0:11                                   ` Dave Chinner
2019-08-24  5:08                                     ` Ira Weiny
2019-08-26  5:55                                       ` Dave Chinner
2019-08-29  2:02                                         ` Ira Weiny
2019-08-29  3:27                                           ` John Hubbard
2019-08-29 16:16                                             ` Ira Weiny
2019-09-02 22:26                                           ` Dave Chinner
2019-09-04 16:54                                             ` Ira Weiny
2019-08-25 19:39                                     ` Jason Gunthorpe
2019-08-24  4:49                                 ` Ira Weiny
2019-08-25 19:40                                   ` Jason Gunthorpe
2019-08-23  0:59                       ` Dave Chinner
2019-08-23 17:15                         ` Ira Weiny
2019-08-24  0:18                           ` Dave Chinner
2019-08-20  0:05               ` John Hubbard
2019-08-20  1:20                 ` Dave Chinner
2019-08-20  3:09                   ` John Hubbard
2019-08-20  3:36                     ` Dave Chinner
2019-08-21 18:43                       ` John Hubbard
2019-08-21 19:09                         ` Ira Weiny

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