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Re: [PATCH] xfs: merge fsync and O_SYNC handling

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfs: merge fsync and O_SYNC handling
From: Felix Blyakher <felixb@xxxxxxx>
Date: Thu, 27 Aug 2009 01:22:15 -0500
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20090827031116.GA6147@xxxxxxxxxxxxx>
References: <20090827031116.GA6147@xxxxxxxxxxxxx>

On Aug 26, 2009, at 10:11 PM, Christoph Hellwig wrote:

The guarantees for O_SYNC are exactly the same as the ones we need to
make for an fsync call (and given that Linux O_SYNC is O_DSYNC the
equivalent is fdadatasync, but we treat both the same in XFS), except
with a range data writeout.  Jan Kara has started unifying these two
path for filesystems using the generic helpers, and I've started to
look at XFS.

The actual transaction commited by xfs_fsync and xfs_write_sync_logforce
has a different transaction number, but actually is exactly the same.
We'll only use the fsync transaction going forward. One major difference
is that xfs_write_sync_logforce never issues a cache flush unless we
commit a transaction causing that as a side-effect, which is an obvious
bug in the O_SYNC handling.  Second all the locking and i_update_size
vs i_update_core changes from 978b7237123d007b9fa983af6e0e2fa8f97f9934
never made it to xfs_write_sync_logforce, so we add them back.

To make xfs_fsync easily usable from the O_SYNC path, the filemap_fdatawait
call is moved up to xfs_file_fsync, so that we don't wait on the whole
file after we already waited for our portion in xfs_write.

We'll also use a plain call to filemap_write_and_wait_range instead
of the previous sync_page_rang which did it in two steps including
an half-hearted inode write out that doesn't help us.

Once we're done with this also remove the now useless i_update_size
tracking.


Signed-off-by: Christoph Hellwig <hch@xxxxxx>

Reviewed-by: Felix Blyakher <felixb@xxxxxxx>

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