Re: [PATCH V5 00/12] Enable per-file/per-directory DAX operations V5

Christoph Hellwig
2020-03-05T15:51:51Z

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@intel.com wrote:
> From: Ira Weiny <ira.weiny@intel.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@vger.kernel.org
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: "Theodore Y. Ts'o" <tytso@mit.edu>
> Cc: Jan Kara <jack@suse.cz>
> Cc: linux-ext4@vger.kernel.org
> Cc: linux-xfs@vger.kernel.org
> Cc: linux-fsdevel@vger.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
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