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From: Jan Kara <jack@suse.cz>
To: Dave Chinner <david@fromorbit.com>
Cc: Christoph Hellwig <hch@lst.de>,
	ira.weiny@intel.com, linux-kernel@vger.kernel.org,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Dan Williams <dan.j.williams@intel.com>,
	"Theodore Y. Ts'o" <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
	linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH V4 07/13] fs: Add locking for a dynamic address space operations state
Date: Wed, 26 Feb 2020 12:17:40 +0100	[thread overview]
Message-ID: <20200226111740.GF10728@quack2.suse.cz> (raw)
In-Reply-To: <20200225000937.GA10776@dread.disaster.area>

On Tue 25-02-20 11:09:37, Dave Chinner wrote:
> /me wonders if the best thing to do is to add a ->fault callout to
> tell the filesystem to lock/unlock the inode right up at the top of
> the page fault path, outside even the mmap_sem.  That means all the
> methods that the page fault calls are protected against S_DAX
> changes, and it gives us a low cost method of serialising page
> faults against DIO (e.g. via inode_dio_wait())....

Well, that's going to be pretty hard. The main problem is: you cannot
lookup VMA until you hold mmap_sem and the inode is inside the VMA. And
this is a fundamental problem because until you hold mmap_sem, the address
space can change and thus the virtual address you are faulting can be
changing inode it is mapped to. So you would have to do some dance like:

lock mmap_sem
lookup vma
get inode reference
drop mmap_sem
tell fs about page fault
lock mmap_sem
is the vma still the same?

And I'm pretty confident the overhead will be visible in page fault
intensive workloads...

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

  parent reply	other threads:[~2020-02-26 11:17 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-21  0:41 [PATCH V4 00/13] Enable per-file/per-directory DAX operations V4 ira.weiny
2020-02-21  0:41 ` [PATCH V4 01/13] fs/xfs: Remove unnecessary initialization of i_rwsem ira.weiny
2020-02-21  1:26   ` Dave Chinner
2020-02-27 17:52     ` Ira Weiny
2020-02-21  0:41 ` [PATCH V4 02/13] fs/xfs: Clarify lockdep dependency for xfs_isilocked() ira.weiny
2020-02-21  1:34   ` Dave Chinner
2020-02-21 23:00     ` Ira Weiny
2020-02-21  0:41 ` [PATCH V4 03/13] fs: Remove unneeded IS_DAX() check ira.weiny
2020-02-21  1:42   ` Dave Chinner
2020-02-21 23:04     ` Ira Weiny
2020-02-21 17:42   ` Christoph Hellwig
2020-02-21  0:41 ` [PATCH V4 04/13] fs/stat: Define DAX statx attribute ira.weiny
2020-02-21  0:41 ` [PATCH V4 05/13] fs/xfs: Isolate the physical DAX flag from enabled ira.weiny
2020-02-21  0:41 ` [PATCH V4 06/13] fs/xfs: Create function xfs_inode_enable_dax() ira.weiny
2020-02-22  0:28   ` Darrick J. Wong
2020-02-23 15:07     ` Ira Weiny
2020-02-21  0:41 ` [PATCH V4 07/13] fs: Add locking for a dynamic address space operations state ira.weiny
2020-02-21 17:44   ` Christoph Hellwig
2020-02-21 22:44     ` Dave Chinner
2020-02-21 23:26       ` Dan Williams
2020-02-24 17:56       ` Christoph Hellwig
2020-02-25  0:09         ` Dave Chinner
2020-02-25 17:36           ` Christoph Hellwig
2020-02-25 19:37             ` Jeff Moyer
2020-02-26  9:28               ` Jonathan Halliday
2020-02-26 11:31                 ` Jan Kara
2020-02-26 11:56                   ` Jonathan Halliday
2020-02-26 16:10                 ` Ira Weiny
2020-02-26 16:46                 ` Dan Williams
2020-02-26 17:20                   ` Jan Kara
2020-02-26 17:54                     ` Dan Williams
2020-02-25 21:03             ` Ira Weiny
2020-02-26 11:17           ` Jan Kara [this message]
2020-02-26 15:57             ` Ira Weiny
2020-02-22  0:33   ` Darrick J. Wong
2020-02-23 15:03     ` Ira Weiny
2020-02-21  0:41 ` [PATCH V4 08/13] fs: Prevent DAX state change if file is mmap'ed ira.weiny
2020-02-21  0:41 ` [PATCH V4 09/13] fs/xfs: Add write aops lock to xfs layer ira.weiny
2020-02-22  0:31   ` Darrick J. Wong
2020-02-23 15:04     ` Ira Weiny
2020-02-24  0:34   ` Dave Chinner
2020-02-24 19:57     ` Ira Weiny
2020-02-24 22:32       ` Dave Chinner
2020-02-25 21:12         ` Ira Weiny
2020-02-25 22:59           ` Dave Chinner
2020-02-26 18:02             ` Ira Weiny
2020-02-21  0:41 ` [PATCH V4 10/13] fs/xfs: Clean up locking in dax invalidate ira.weiny
2020-02-21 17:45   ` Christoph Hellwig
2020-02-21 18:06     ` Ira Weiny
2020-02-21  0:41 ` [PATCH V4 11/13] fs/xfs: Allow toggle of effective DAX flag ira.weiny
2020-02-21  0:41 ` [PATCH V4 12/13] fs/xfs: Remove xfs_diflags_to_linux() ira.weiny
2020-02-21  0:41 ` [PATCH V4 13/13] Documentation/dax: Update Usage section ira.weiny
2020-02-26 22:48 ` [PATCH V4 00/13] Enable per-file/per-directory DAX operations V4 Jeff Moyer
2020-02-27  2:43   ` Ira Weiny

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