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From: Dave Chinner <david@fromorbit.com>
To: Christoph Hellwig <hch@lst.de>
Cc: linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	avi@scylladb.com, Dave Chinner <dchinner@redhat.com>
Subject: Re: [PATCH 11/11] xfs: reduce exclusive locking on unaligned dio
Date: Tue, 19 Jan 2021 07:55:21 +1100
Message-ID: <20210118205521.GF78941@dread.disaster.area> (raw)
In-Reply-To: <20210118193516.2915706-12-hch@lst.de>

On Mon, Jan 18, 2021 at 08:35:16PM +0100, Christoph Hellwig wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> Attempt shared locking for unaligned DIO, but only if the the
> underlying extent is already allocated and in written state. On
> failure, retry with the existing exclusive locking.
....
> @@ -590,19 +617,27 @@ xfs_file_dio_write_unaligned(
>  		goto out_unlock;
>  
>  	/*
> -	 * If we are doing unaligned I/O, we can't allow any other overlapping
> -	 * I/O in-flight at the same time or we risk data corruption. Wait for
> -	 * all other I/O to drain before we submit.
> +	 * If we are doing exclusive unaligned IO, we can't allow any other
> +	 * overlapping IO in-flight at the same time or we risk data corruption.
> +	 * Wait for all other IO to drain before we submit.
>  	 */
> -	inode_dio_wait(VFS_I(ip));
> +	if (!(flags & IOMAP_DIO_UNALIGNED))
> +		inode_dio_wait(VFS_I(ip));
>  
> -	/*
> -	 * This must be the only I/O in-flight. Wait on it before we release the
> -	 * iolock to prevent subsequent overlapping I/O.
> -	 */
>  	trace_xfs_file_direct_write(iocb, from);
>  	ret = iomap_dio_rw(iocb, from, &xfs_direct_write_iomap_ops,
> -			   &xfs_dio_write_ops, IOMAP_DIO_FORCE_WAIT);
> +			   &xfs_dio_write_ops, flags);
> +	/*
> +	 * Retry unaligned IO with exclusive blocking semantics if the DIO
> +	 * layer rejected it for mapping or locking reasons. If we are doing
> +	 * nonblocking user IO, propagate the error.
> +	 */
> +	if (ret == -EAGAIN && !(iocb->ki_flags & IOCB_NOWAIT)) {
> +		ASSERT(flags & IOMAP_DIO_UNALIGNED);
> +		xfs_iunlock(ip, iolock);
> +		goto retry_exclusive;
> +	}
> +
>  out_unlock:
>  	if (iolock)
>  		xfs_iunlock(ip, iolock);

Do we ever get here without holding the iolock anymore?

> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 7b9ff824e82d48..dc8c86e98b99bf 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -784,15 +784,30 @@ xfs_direct_write_iomap_begin(
>  		goto allocate_blocks;
>  
>  	/*
> -	 * NOWAIT IO needs to span the entire requested IO with a single map so
> -	 * that we avoid partial IO failures due to the rest of the IO range not
> -	 * covered by this map triggering an EAGAIN condition when it is
> -	 * subsequently mapped and aborting the IO.
> +	 * NOWAIT and unaligned IO needs to span the entire requested IO with a
> +	 * single map so that we avoid partial IO failures due to the rest of
> +	 * the IO range not covered by this map triggering an EAGAIN condition
> +	 * when it is subsequently mapped and aborting the IO.
>  	 */
> -	if ((flags & IOMAP_NOWAIT) &&
> -	    !imap_spans_range(&imap, offset_fsb, end_fsb)) {
> +	if (flags & (IOMAP_NOWAIT | IOMAP_UNALIGNED)) {
>  		error = -EAGAIN;
> -		goto out_unlock;
> +		if (!imap_spans_range(&imap, offset_fsb, end_fsb))
> +			goto out_unlock;
> +	}
> +
> +	/*
> +	 * For unsigned I/O we can't convert an unwritten extents if the I/O is
> +	 * not block size aligned, as such a conversion would have to do
> +	 * sub-block zeroing, and that can only be done under an exclusive
> +	 * IOLOCK. Hence if this is not a written extent, return EAGAIN to tell
> +	 * the caller to try again.
> +	 */

A few typos in that comment :)

	/*
	 * For unaligned IO, we cannot convert unwritten extents without
	 * requiring sub-block zeroing. This can only be done under an exclusive
	 * IOLOCK, hence return -EAGAIN if this is not a written extent to tell
	 * the caller to try again.
	 */

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  reply index

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-18 19:35 reduce sub-block DIO serialisation v2 Christoph Hellwig
2021-01-18 19:35 ` [PATCH 01/11] xfs: factor out a xfs_ilock_iocb helper Christoph Hellwig
2021-01-20 18:41   ` Darrick J. Wong
2021-01-18 19:35 ` [PATCH 02/11] xfs: make xfs_file_aio_write_checks IOCB_NOWAIT-aware Christoph Hellwig
     [not found]   ` <CACz=WeeaqMrGM53pJF0C_Wt2JuavTOnOV26-osPviYLUpqUmFw@mail.gmail.com>
2021-01-20 16:28     ` Christoph Hellwig
2021-01-20 18:42   ` Darrick J. Wong
2021-01-18 19:35 ` [PATCH 03/11] xfs: cleanup the read/write helper naming Christoph Hellwig
2021-01-19 15:23   ` Brian Foster
2021-01-20 18:43   ` Darrick J. Wong
2021-01-18 19:35 ` [PATCH 04/11] xfs: remove the buffered I/O fallback assert Christoph Hellwig
2021-01-19 15:23   ` Brian Foster
2021-01-20 18:43   ` Darrick J. Wong
2021-01-18 19:35 ` [PATCH 05/11] xfs: simplify the read/write tracepoints Christoph Hellwig
2021-01-19 15:23   ` Brian Foster
2021-01-20 18:45   ` Darrick J. Wong
2021-01-18 19:35 ` [PATCH 06/11] xfs: improve the reflink_bounce_dio_write tracepoint Christoph Hellwig
2021-01-19 15:23   ` Brian Foster
2021-01-20 18:45   ` Darrick J. Wong
2021-01-18 19:35 ` [PATCH 07/11] xfs: split unaligned DIO write code out Christoph Hellwig
2021-01-19 15:23   ` Brian Foster
2021-01-20 18:46   ` Darrick J. Wong
2021-01-18 19:35 ` [PATCH 08/11] iomap: rename the flags variable in __iomap_dio_rw Christoph Hellwig
2021-01-18 20:34   ` Dave Chinner
2021-01-19 15:23   ` Brian Foster
2021-01-20 18:46   ` Darrick J. Wong
2021-01-18 19:35 ` [PATCH 09/11] iomap: pass a flags argument to iomap_dio_rw Christoph Hellwig
2021-01-19 15:23   ` Brian Foster
2021-01-20 18:17   ` Darrick J. Wong
2021-01-20 18:35     ` Christoph Hellwig
2021-01-18 19:35 ` [PATCH 10/11] iomap: add a IOMAP_DIO_UNALIGNED flag Christoph Hellwig
2021-01-18 20:45   ` Dave Chinner
2021-01-18 21:41   ` Matthew Wilcox
2021-01-20 16:40     ` Christoph Hellwig
2021-01-20 18:47   ` Darrick J. Wong
2021-01-18 19:35 ` [PATCH 11/11] xfs: reduce exclusive locking on unaligned dio Christoph Hellwig
2021-01-18 20:55   ` Dave Chinner [this message]
2021-01-20 16:36     ` Christoph Hellwig
2021-01-20 18:40   ` Darrick J. Wong
2021-01-20 18:44     ` Christoph Hellwig
2021-01-20 19:58       ` Darrick J. Wong
2021-01-21  8:58 reduce sub-block DIO serialisation v3 Christoph Hellwig
2021-01-21  8:59 ` [PATCH 11/11] xfs: reduce exclusive locking on unaligned dio Christoph Hellwig
2021-01-21  9:35   ` Dave Chinner
2021-01-22 10:20     ` Christoph Hellwig
2021-01-21 13:33   ` Brian Foster
2021-01-21 19:01     ` Darrick J. Wong
2021-01-22 16:20 reduce sub-block DIO serialisation v4 Christoph Hellwig
2021-01-22 16:20 ` [PATCH 11/11] xfs: reduce exclusive locking on unaligned dio Christoph Hellwig
2021-01-22 17:24   ` Darrick J. Wong

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