Re: [PATCH 2/2] dax: move writeback calls into the filesystems

[Dan Williams <dan.j.williams@intel.com>]

On Sat, Feb 6, 2016 at 11:19 PM, Ross Zwisler
<ross.zwisler@linux.intel.com> wrote:
> Previously calls to dax_writeback_mapping_range() for all DAX filesystems
> (ext2, ext4 & xfs) were centralized in filemap_write_and_wait_range().
> dax_writeback_mapping_range() needs a struct block_device, and it used to
> get that from inode->i_sb->s_bdev.  This is correct for normal inodes
> mounted on ext2, ext4 and XFS filesystems, but is incorrect for DAX raw
> block devices and for XFS real-time files.
>
> Instead, call dax_writeback_mapping_range() directly from the filesystem or
> raw block device fsync/msync code so that they can supply us with a valid
> block device.
>
> It should be noted that this will reduce the number of calls to
> dax_writeback_mapping_range() because filemap_write_and_wait_range() is
> called in the various filesystems for operations other than just
> fsync/msync.  Both ext4 & XFS call filemap_write_and_wait_range() outside
> of ->fsync for hole punch, truncate, and block relocation
> (xfs_shift_file_space() && ext4_collapse_range()/ext4_insert_range()).
>
> I don't believe that these extra flushes are necessary in the DAX case.  In
> the page cache case when we have dirty data in the page cache, that data
> will be actively lost if we evict a dirty page cache page without flushing
> it to media first.  For DAX, though, the data will remain consistent with
> the physical address to which it was written regardless of whether it's in
> the processor cache or not - really the only reason I see to flush is in
> response to a fsync or msync so that our data is durable on media in case
> of a power loss.  The case where we could throw dirty data out of the page
> cache and essentially lose writes simply doesn't exist.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> ---
>  fs/block_dev.c      |  7 +++++++
>  fs/dax.c            |  5 ++---
>  fs/ext2/file.c      | 10 ++++++++++
>  fs/ext4/fsync.c     | 10 +++++++++-
>  fs/xfs/xfs_file.c   | 12 ++++++++++--
>  include/linux/dax.h |  4 ++--
>  mm/filemap.c        |  6 ------
>  7 files changed, 40 insertions(+), 14 deletions(-)

This sprinkling of dax specific fixups outside of vm_operations_struct
routines still has me thinking that we are going in the wrong
direction for fsync/msync support.

If an application is both unaware of DAX and doing mmap I/O it is
better served by the page cache where writeback is durable by default.
We expect DAX-aware applications to assume responsibility for cpu
cache management [1].  Making DAX mmap semantics explicit opt-in
solves not only durability support, but also the current problem that
DAX gets silently disabled leaving an app to wonder if it really got a
direct mapping. DAX also silently picks pud, pmd, or pte mappings
which is information an application would really like to know at map
time.

The proposal: make applications explicitly request DAX semantics with
a new MAP_DAX flag and fail if DAX is unavailable.  Document that a
successful MAP_DAX request mandates that the application assumes
responsibility for cpu cache management.  Require that all
applications that mmap the file agree on MAP_DAX.  This also solves
the future problem of DAX support on virtually tagged cache
architectures where it is difficult for the kernel to know what alias
addresses need flushing.

[1]: https://github.com/pmem/nvml

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