Date:   Mon, 9 Mar 2020 10:04:37 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, akpm@...ux-foundation.org,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5

On Thu, Mar 05, 2020 at 04:51:44PM +0100, Christoph Hellwig wrote:
> FYI, I still will fully NAK any series that adds additional locks
> and thus atomic instructions to basically every fs call, and grows
> the inode by a rw_semaphore plus and atomic64_t.  I also think the
> whole idea of switching operation vectors at runtime is fatally flawed
> and we should never add such code, nevermind just for a fringe usecase
> of a fringe feature.

Being new to this area of the kernel I'm not clear on the history...

It was my understanding that the per-file flag support was a requirement to
removing the experimental designation from DAX.  Is this still the case?

Ira

> 
> On Wed, Feb 26, 2020 at 09:24:30PM -0800, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> > 
> > Changes from V4:
> > 	* Open code the aops lock rather than add it to the xfs_ilock()
> > 	  subsystem (Darrick's comments were obsoleted by this change)
> > 	* Fix lkp build suggestions and bugs
> > 
> > Changes from V3:
> > 	* Remove global locking...  :-D
> > 	* put back per inode locking and remove pre-mature optimizations
> > 	* Fix issues with Directories having IS_DAX() set
> > 	* Fix kernel crash issues reported by Jeff
> > 	* Add some clean up patches
> > 	* Consolidate diflags to iflags functions
> > 	* Update/add documentation
> > 	* Reorder/rename patches quite a bit
> > 
> > Changes from V2:
> > 
> > 	* Move i_dax_sem to be a global percpu_rw_sem rather than per inode
> > 		Internal discussions with Dan determined this would be easier,
> > 		just as performant, and slightly less overhead that having it
> > 		in the SB as suggested by Jan
> > 	* Fix locking order in comments and throughout code
> > 	* Change "mode" to "state" throughout commits
> > 	* Add CONFIG_FS_DAX wrapper to disable inode_[un]lock_state() when not
> > 		configured
> > 	* Add static branch for which is activated by a device which supports
> > 		DAX in XFS
> > 	* Change "lock/unlock" to up/down read/write as appropriate
> > 		Previous names were over simplified
> > 	* Update comments/documentation
> > 
> > 	* Remove the xfs specific lock to the vfs (global) layer.
> > 	* Fix i_dax_sem locking order and comments
> > 
> > 	* Move 'i_mapped' count from struct inode to struct address_space and
> > 		rename it to mmap_count
> > 	* Add inode_has_mappings() call
> > 
> > 	* Fix build issues
> > 	* Clean up syntax spacing and minor issues
> > 	* Update man page text for STATX_ATTR_DAX
> > 	* Add reviewed-by's
> > 	* Rebase to 5.6
> > 
> > 	Rename patch:
> > 		from: fs/xfs: Add lock/unlock state to xfs
> > 		to: fs/xfs: Add write DAX lock to xfs layer
> > 	Add patch:
> > 		fs/xfs: Clarify lockdep dependency for xfs_isilocked()
> > 	Drop patch:
> > 		fs/xfs: Fix truncate up
> > 
> > 
> > At LSF/MM'19 [1] [2] we discussed applications that overestimate memory
> > consumption due to their inability to detect whether the kernel will
> > instantiate page cache for a file, and cases where a global dax enable via a
> > mount option is too coarse.
> > 
> > The following patch series enables selecting the use of DAX on individual files
> > and/or directories on xfs, and lays some groundwork to do so in ext4.  In this
> > scheme the dax mount option can be omitted to allow the per-file property to
> > take effect.
> > 
> > The insight at LSF/MM was to separate the per-mount or per-file "physical"
> > capability switch from an "effective" attribute for the file.
> > 
> > At LSF/MM we discussed the difficulties of switching the DAX state of a file
> > with active mappings / page cache.  It was thought the races could be avoided
> > by limiting DAX state flips to 0-length files.
> > 
> > However, this turns out to not be true.[3] This is because address space
> > operations (a_ops) may be in use at any time the inode is referenced and users
> > have expressed a desire to be able to change the DAX state on a file with data
> > in it.  For those reasons this patch set allows changing the DAX state flag on
> > a file as long as it is not current mapped.
> > 
> > Details of when and how DAX state can be changed on a file is included in a
> > documentation patch.
> > 
> > It should be noted that the physical DAX flag inheritance is not shown in this
> > patch set as it was maintained from previous work on XFS.  The physical DAX
> > flag and it's inheritance will need to be added to other file systems for user
> > control. 
> > 
> > As submitted this works on real hardware testing.
> > 
> > 
> > [1] https://lwn.net/Articles/787973/
> > [2] https://lwn.net/Articles/787233/
> > [3] https://lkml.org/lkml/2019/10/20/96
> > [4] https://patchwork.kernel.org/patch/11310511/
> > 
> > 
> > To: linux-kernel@...r.kernel.org
> > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > Cc: "Darrick J. Wong" <darrick.wong@...cle.com>
> > Cc: Dan Williams <dan.j.williams@...el.com>
> > Cc: Dave Chinner <david@...morbit.com>
> > Cc: Christoph Hellwig <hch@....de>
> > Cc: "Theodore Y. Ts'o" <tytso@....edu>
> > Cc: Jan Kara <jack@...e.cz>
> > Cc: linux-ext4@...r.kernel.org
> > Cc: linux-xfs@...r.kernel.org
> > Cc: linux-fsdevel@...r.kernel.org
> > 
> > 
> > Ira Weiny (12):
> >   fs/xfs: Remove unnecessary initialization of i_rwsem
> >   fs: Remove unneeded IS_DAX() check
> >   fs/stat: Define DAX statx attribute
> >   fs/xfs: Isolate the physical DAX flag from enabled
> >   fs/xfs: Create function xfs_inode_enable_dax()
> >   fs: Add locking for a dynamic address space operations state
> >   fs: Prevent DAX state change if file is mmap'ed
> >   fs/xfs: Hold off aops users while changing DAX state
> >   fs/xfs: Clean up locking in dax invalidate
> >   fs/xfs: Allow toggle of effective DAX flag
> >   fs/xfs: Remove xfs_diflags_to_linux()
> >   Documentation/dax: Update Usage section
> > 
> >  Documentation/filesystems/dax.txt | 84 +++++++++++++++++++++++++-
> >  Documentation/filesystems/vfs.rst | 16 +++++
> >  fs/attr.c                         |  1 +
> >  fs/inode.c                        | 16 ++++-
> >  fs/iomap/buffered-io.c            |  1 +
> >  fs/open.c                         |  4 ++
> >  fs/stat.c                         |  5 ++
> >  fs/xfs/xfs_icache.c               |  5 +-
> >  fs/xfs/xfs_inode.h                |  2 +
> >  fs/xfs/xfs_ioctl.c                | 98 +++++++++++++++----------------
> >  fs/xfs/xfs_iops.c                 | 69 +++++++++++++++-------
> >  include/linux/fs.h                | 73 ++++++++++++++++++++++-
> >  include/uapi/linux/stat.h         |  1 +
> >  mm/fadvise.c                      |  7 ++-
> >  mm/filemap.c                      |  4 ++
> >  mm/huge_memory.c                  |  1 +
> >  mm/khugepaged.c                   |  2 +
> >  mm/mmap.c                         | 19 +++++-
> >  mm/util.c                         |  9 ++-
> >  19 files changed, 328 insertions(+), 89 deletions(-)
> > 
> > -- 
> > 2.21.0
> ---end quoted text---

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