Re: [PATCH 0/8 v6] btrfs direct-io using iomap

Goldwyn Rodrigues
2019-12-16T12:42:02Z

On 2:01 16/12, Nikolay Borisov wrote:
>
>
> On 13.12.19 г. 21:57 ч., Goldwyn Rodrigues wrote:
> > This is an effort to use iomap for direct I/O in btrfs. This would
> > change the call from __blockdev_direct_io() to iomap_dio_rw().
> >
> > The main objective is to lose the buffer head and use bio defined by
> > iomap code, and hopefully to use more of generic-FS codebase.
> >
> > These patches are based and tested on v5.5-rc1. I have tested it against
> > xfstests/btrfs.
> >
> > The tree is available at
> > https://github.com/goldwynr/linux/tree/btrfs-iomap-dio
> >
> > Changes since v1
> > - Incorporated back the efficiency change for inode locking
> > - Review comments about coding style and git comments
> > - Merge related patches into one
> > - Direct read to go through btrfs_direct_IO()
> > - Removal of no longer used function dio_end_io()
> >
> > Changes since v2
> > - aligning iomap offset/length to the position/length of I/O
> > - Removed btrfs_dio_data
> > - Removed BTRFS_INODE_READDIO_NEED_LOCK
> > - Re-incorporating write efficiency changes caused lockdep_assert() in
> > iomap to be triggered, remove that code.
> >
> > Changes since v3
> > - Fixed freeze on generic/095. Use iomap_end() to account for
> > failed/incomplete dio instead of btrfs_dio_data
> >
> > Changes since v4
> > - moved lockdep_assert_held() to functions calling iomap_dio_rw()
> > This may be called immidiately after calling inode lock and
> > may feel not required, but it seems important.
> > - Removed comments which are no longer required
> > - Changed commit comments to make them more appropriate
> >
> > Changes since v5
> > - restore inode_dio_wait() in truncate
>
> I'm confused about this - you no longer call inode_dio_begin after patch
> 4/8 so inode_dio_wait which is left intact in truncate can never trigger
> a wait really. Exclusion is provided by the fact that btrfs_direct_IO is
> called with rwsem held shared and truncate holds it exclusive? So what
> necessitated restoring inode_dio_wait?

The optimization of the write path of direct I/O. The rwsem is
released if write is within i_size. This is also the reason we
removed lockdep_assert_held().

Reference: 38851cc19adb ("Btrfs: implement unlocked dio write")

This was inspired by ext4. However, ext4 after it's switch to
iomap_dio_rw() does not employ this optimization.

>
>
> Another point, I don't see whether you have explicitly addressed
> concerns raised in:
>
> https://lore.kernel.org/linux-btrfs/20191212003043.31093-1-rgoldwyn@suse.de/T/#me7f96506e5a1d921d05b76d01ecf6ea1ebcea594

I did address it by releasing csums (referring io_bio) before calling
bio_endio().

--
Goldwyn