linux-ext4 - Re: [RFC 1/2] iomap: direct-io: Move inode_dio_begin before filemap_write_and_wait

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Message-ID: <20200114171934.GB22081@quack2.suse.cz>
Date:   Tue, 14 Jan 2020 18:19:34 +0100
From:   Jan Kara <jack@...e.cz>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Ritesh Harjani <riteshh@...ux.ibm.com>, linux-ext4@...r.kernel.org,
        tytso@....edu, jack@...e.cz
Subject: Re: [RFC 1/2] iomap: direct-io: Move inode_dio_begin before
 filemap_write_and_wait_range

On Tue 14-01-20 08:37:02, Christoph Hellwig wrote:
> Using i_dio_count for any kind of detection is bogus.  If you want to
> pass flags to the writeback code please do so explicitly through
> struct writeback_control.

We want to detect in the writeback path whether there's direct IO (read)
currently running for the inode. Not for the writeback issued from
iomap_dio_rw() but for any arbitrary writeback that iomap_dio_rw() can be
racing with - so struct writeback_control won't help. Now if you want to
see the ugly details why this hack is needed, see my other email to Ritesh
in this thread with details of the race.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR