linux-kernel - Re: [PATCH 03/17] vfs: Remove syncing from generic_file_direct_write() and generic_file_buffered

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Date:	Wed, 19 Aug 2009 12:18:53 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>, hch@...radead.org,
	ocfs2-devel@....oracle.com, Joel Becker <joel.becker@...cle.com>,
	Felix Blyakher <felixb@....com>, xfs@....sgi.com
Subject: Re: [PATCH 03/17] vfs: Remove syncing from
	generic_file_direct_write() and generic_file_buffered_write()

On Wed, Aug 19, 2009 at 06:04:30PM +0200, Jan Kara wrote:
> generic_file_direct_write() and generic_file_buffered_write() called
> generic_osync_inode() if it was called on O_SYNC file or IS_SYNC inode. But
> this is superfluous since generic_file_aio_write() does the syncing as well.
> Also XFS and OCFS2 which call these functions directly handle syncing
> themselves. So let's have a single place where syncing happens:
> generic_file_aio_write().

Yeah, this is something that never made any sense to me.

> @@ -2187,20 +2187,7 @@ generic_file_direct_write(struct kiocb *iocb, const struct iovec *iov,
>  		}
>  		*ppos = end;
>  	}
> -
> -	/*
> -	 * Sync the fs metadata but not the minor inode changes and
> -	 * of course not the data as we did direct DMA for the IO.
> -	 * i_mutex is held, which protects generic_osync_inode() from
> -	 * livelocking.  AIO O_DIRECT ops attempt to sync metadata here.
> -	 */
>  out:
> -	if ((written >= 0 || written == -EIOCBQUEUED) &&
> -	    ((file->f_flags & O_SYNC) || IS_SYNC(inode))) {
> -		int err = generic_osync_inode(inode, mapping, OSYNC_METADATA);
> -		if (err < 0)
> -			written = err;
> -	}
>  	return written;

Here we check (written >= 0 || written == -EIOCBQUEUED), but
generic_file_aio_write only cares about positive return values.  We
defintively do have a change here for partial AIO requests.  The
question is if the previous behaviour made in sense.  If do have an
O_SYNC aio+dio request we would have to flush out the metadata after the
request has completed and not here.

> @@ -2343,16 +2328,6 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
>  	if (likely(status >= 0)) {
>  		written += status;
>  		*ppos = pos + status;
> -
> -		/*
> -		 * For now, when the user asks for O_SYNC, we'll actually give
> -		 * O_DSYNC
> -		 */
> -		if (unlikely((file->f_flags & O_SYNC) || IS_SYNC(inode))) {
> -			if (!a_ops->writepage || !is_sync_kiocb(iocb))
> -				status = generic_osync_inode(inode, mapping,
> -						OSYNC_METADATA|OSYNC_DATA);
> -		}
>    	}

No problem with -EIOCBQUEUED here, but we change from doing
generic_osync_inode with OSYNC_DATA which does a full writeout of the
data to sync_page_range which only does the range writeout here.  That
should be fine (as we only need to sync that range), but should probably
be documented in the patch description.
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