linux-ext4 - Re: [PATCH] ext4: remove unnecessary ext4_inode_datasync

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Message-ID: <20211103002843.GC418105@dread.disaster.area>
Date:   Wed, 3 Nov 2021 11:28:43 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Zhongwei Cai <sunrise_l@...u.edu.cn>
Cc:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, mingkaidong@...il.com
Subject: Re: [PATCH] ext4: remove unnecessary ext4_inode_datasync_dirty in
 read path

On Tue, Nov 02, 2021 at 10:42:58AM +0800, Zhongwei Cai wrote:
> ext4_inode_datasync_dirty will call read_lock(&journal->j_state_lock) in
> journal mode, which is unnecessary in read path (As far as I know, the
> IOMAP_F_DIRTY flag set in the if branch is only used in write path,
> making it unnecessary in read path. Please correct me if I'm wrong).

IOMAP_F_DIRTY isn't conditional on the type of lookup being done. If
the inode is dirty in a way that O_DSYNC would require it to be
flushed to make the data stable, iomap should be told that it is
dirty, even on read lookups...

e.g. iomap_swapfile_activate() uses IOMAP_REPORT as the flags for
extent mapping iteration passed to iomap_swapfile_iter(). THis then
checks:

	/* No uncommitted metadata or shared blocks. */
	if (iomap->flags & IOMAP_F_DIRTY)
		return iomap_swapfile_fail(isi, "is not committed");

IOWs, we expect the IOMAP_F_DIRTY flag to be set on all types of
iomap mapping calls if the inode is dirty, not just IOMAP_WRITE
calls.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com