Message-ID: <20200305155144.GA5598@lst.de>
Date:   Thu, 5 Mar 2020 16:51:44 +0100
From:   Christoph Hellwig <hch@....de>
To:     ira.weiny@...el.com
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>,
        Christoph Hellwig <hch@....de>,
        "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

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.

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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