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

From: Nikolay Borisov <nborisov@suse.com>
To: Goldwyn Rodrigues <rgoldwyn@suse.de>, linux-btrfs@vger.kernel.org
Cc: hch@infradead.org, darrick.wong@oracle.com, fdmanana@kernel.org,
	dsterba@suse.cz, jthumshirn@suse.de,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 0/8 v6] btrfs direct-io using iomap
Date: Mon, 16 Dec 2019 02:01:27 +0200	[thread overview]
Message-ID: <0cfcbf67-8bca-8d55-6d7e-b79e5e5f66c0@suse.com> (raw)
In-Reply-To: <20191213195750.32184-1-rgoldwyn@suse.de>

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?

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

> - Removed lockdep_assert_held() near callers
> 
> --
> Goldwyn
> 
> 