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From: Dan Williams <dan.j.williams@intel.com>
To: Ira Weiny <ira.weiny@intel.com>
Cc: Dave Chinner <david@fromorbit.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Christoph Hellwig <hch@lst.de>,
	"Theodore Y. Ts'o" <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH v3 07/12] fs: Add locking for a dynamic DAX state
Date: Tue, 11 Feb 2020 12:59:40 -0800
Message-ID: <CAPcyv4hLTg+1dx2bE2S7sLRK17EP_gT5kutwhyUfVB7Ad8khgw@mail.gmail.com> (raw)
In-Reply-To: <20200211201430.GE12866@iweiny-DESK2.sc.intel.com>

On Tue, Feb 11, 2020 at 12:14 PM Ira Weiny <ira.weiny@intel.com> wrote:
>
> On Tue, Feb 11, 2020 at 07:00:35PM +1100, Dave Chinner wrote:
> > On Sat, Feb 08, 2020 at 11:34:40AM -0800, ira.weiny@intel.com wrote:
> > > From: Ira Weiny <ira.weiny@intel.com>
> > >
> > > DAX requires special address space operations but many other functions
> > > check the IS_DAX() state.
> > >
> > > While DAX is a property of the inode we perfer a lock at the super block
> > > level because of the overhead of a rwsem within the inode.
> > >
> > > Define a vfs per superblock percpu rs semaphore to lock the DAX state
> >
> > ????
>
> oops...  I must have forgotten to update the commit message when I did the
> global RW sem.  I implemented the per-SB, percpu rwsem first but it was
> suggested that the percpu nature of the lock combined with the anticipated
> infrequent use of the write side made using a global easier.
>
> But before I proceed on V4 I'd like to get consensus on which of the 2 locking
> models to go with.
>
>         1) percpu per superblock lock
>         2) per inode rwsem
>
> Depending on my mood I can convince myself of both being performant but the
> percpu is very attractive because I don't anticipate many changes of state
> during run time.  OTOH concurrent threads accessing the same file at run time
> may also be low so there is likely to be little read contention across CPU's on
> the per-inode lock?
>
> Opinions?

As one who thought a global lock would be reasonable relative to how
often dax address_space_operations are swapped out (on the order of
taking cgroup_threadgroup_rwsem for write), I think a per-superblock
lock is also an ok starting point. We can always go to finer grained
locking in the future if we see evidence that the benefits of percpu
are lost to stopping the world at the superblock level.

  reply index

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-08 19:34 [PATCH v3 00/12] Enable per-file/directory DAX operations V3 ira.weiny
2020-02-08 19:34 ` [PATCH v3 01/12] fs/stat: Define DAX statx attribute ira.weiny
2020-02-08 19:34 ` [PATCH v3 02/12] fs/xfs: Isolate the physical DAX flag from effective ira.weiny
2020-02-08 19:34 ` [PATCH v3 03/12] fs/xfs: Separate functionality of xfs_inode_supports_dax() ira.weiny
2020-02-11  5:47   ` Dave Chinner
2020-02-11 16:13     ` Ira Weiny
2020-02-08 19:34 ` [PATCH v3 04/12] fs/xfs: Clean up DAX support check ira.weiny
2020-02-11  5:57   ` Dave Chinner
2020-02-11 16:28     ` Ira Weiny
2020-02-11 20:38       ` Dave Chinner
2020-02-08 19:34 ` [PATCH v3 05/12] fs: remove unneeded IS_DAX() check ira.weiny
2020-02-11  5:34   ` Dave Chinner
2020-02-11 16:38     ` Ira Weiny
2020-02-11 20:41       ` Dave Chinner
2020-02-12 16:04         ` Ira Weiny
2020-02-08 19:34 ` [PATCH v3 06/12] fs/xfs: Check if the inode supports DAX under lock ira.weiny
2020-02-11  6:16   ` Dave Chinner
2020-02-11 17:55     ` Ira Weiny
2020-02-11 20:42       ` Dave Chinner
2020-02-12 16:10         ` Ira Weiny
2020-02-08 19:34 ` [PATCH v3 07/12] fs: Add locking for a dynamic DAX state ira.weiny
2020-02-11  8:00   ` Dave Chinner
2020-02-11 20:14     ` Ira Weiny
2020-02-11 20:59       ` Dan Williams [this message]
2020-02-11 21:49       ` Dave Chinner
2020-02-12  6:31         ` Darrick J. Wong
2020-02-08 19:34 ` [PATCH v3 08/12] fs/xfs: Clarify lockdep dependency for xfs_isilocked() ira.weiny
2020-02-08 19:34 ` [PATCH v3 09/12] fs/xfs: Add write DAX lock to xfs layer ira.weiny
2020-02-08 19:34 ` [PATCH v3 10/12] fs: Prevent DAX state change if file is mmap'ed ira.weiny
2020-02-08 19:34 ` [PATCH v3 11/12] fs/xfs: Clean up locking in dax invalidate ira.weiny
2020-02-08 19:34 ` [PATCH v3 12/12] fs/xfs: Allow toggle of effective DAX flag ira.weiny
2020-02-10 15:15 ` [PATCH v3 00/12] Enable per-file/directory DAX operations V3 Jeff Moyer
2020-02-11 20:17   ` Ira Weiny
2020-02-12 19:49     ` Jeff Moyer
2020-02-13 19:01       ` Ira Weiny
2020-02-13 19:05         ` Ira Weiny
2020-02-13 19:58           ` Darrick J. Wong
2020-02-13 23:29             ` Ira Weiny
2020-02-14  0:16               ` Dan Williams
2020-02-14 20:06                 ` Ira Weiny
2020-02-14 21:23                   ` Jeff Moyer
2020-02-14 21:58                     ` Ira Weiny
2020-02-14 22:06                       ` Jeff Moyer
2020-02-14 22:58                         ` Jeff Moyer
2020-02-14 23:03                           ` Jeff Moyer
2020-02-18  2:35                           ` Ira Weiny
2020-02-18 14:22                             ` Jeff Moyer
2020-02-18 23:54                               ` Ira Weiny
2020-02-20 16:20                                 ` Ira Weiny
2020-02-20 16:30                                   ` Darrick J. Wong
2020-02-20 16:49                                     ` Ira Weiny
2020-02-20 17:00                                       ` Darrick J. Wong

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