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Re: [PATCH 9/9] blkdev: Fix up AIO+DIO+O_SYNC to do the sync part correc

To: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
Subject: Re: [PATCH 9/9] blkdev: Fix up AIO+DIO+O_SYNC to do the sync part correctly
From: Jan Kara <jack@xxxxxxx>
Date: Tue, 20 Nov 2012 11:15:59 +0100
Cc: axboe@xxxxxxxxx, tytso@xxxxxxx, david@xxxxxxxxxxxxx, jmoyer@xxxxxxxxxx, bpm@xxxxxxx, viro@xxxxxxxxxxxxxxxxxx, jack@xxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, hch@xxxxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20121120075115.25270.62451.stgit@xxxxxxxxxxxxxxxxxxx>
References: <20121120074116.24645.36369.stgit@xxxxxxxxxxxxxxxxxxx> <20121120075115.25270.62451.stgit@xxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Mon 19-11-12 23:51:15, Darrick J. Wong wrote:
> When performing O_SYNC+AIO+DIO writes to block devices, use the 
> DIO_SYNC_WRITES
> flag so that flushes are issued /after/ the write completes, not before.
> 
> Note, however, that for block devices, the DIO setup code ensures that a flush
> wq is attached to the superblock of the bdevfs filesystem, not the filesystem
> that the device node happens to reside in.  This means that unlike regular
> files, iocb->ki_filp->f_mapping->host->i_sb != inode->i_sb.  Therefore, adjust
> Jeff's earlier patch to keep the pointer use consistent and avoid a NULL 
> deref.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
>  fs/block_dev.c |    5 +++--
>  fs/direct-io.c |    3 ++-
>  2 files changed, 5 insertions(+), 3 deletions(-)
> 
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 1a1e5e3..05ff33a 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -235,7 +235,8 @@ blkdev_direct_IO(int rw, struct kiocb *iocb, const struct 
> iovec *iov,
>       struct inode *inode = file->f_mapping->host;
>  
>       return __blockdev_direct_IO(rw, iocb, inode, I_BDEV(inode), iov, offset,
> -                                 nr_segs, blkdev_get_blocks, NULL, NULL, 0);
> +                                 nr_segs, blkdev_get_blocks, NULL, NULL,
> +                                 DIO_SYNC_WRITES);
>  }
>  
>  int __sync_blockdev(struct block_device *bdev, int wait)
> @@ -1631,7 +1632,7 @@ ssize_t blkdev_aio_write(struct kiocb *iocb, const 
> struct iovec *iov,
>       percpu_down_read(&bdev->bd_block_size_semaphore);
>  
>       ret = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos);
> -     if (ret > 0 || ret == -EIOCBQUEUED) {
> +     if (ret > 0) {
>               ssize_t err;
>  
>               err = generic_write_sync(file, pos, ret);
> diff --git a/fs/direct-io.c b/fs/direct-io.c
> index b7391d4..c626c43 100644
> --- a/fs/direct-io.c
> +++ b/fs/direct-io.c
> @@ -258,7 +258,8 @@ void generic_dio_end_io(struct kiocb *iocb, loff_t 
> offset, ssize_t bytes,
>               work->ret = ret;
>               work->offset = offset;
>               work->len = bytes;
> -             queue_work(inode->i_sb->s_dio_flush_wq, &work->work);
> +             queue_work(iocb->ki_filp->f_mapping->host->i_sb->s_dio_flush_wq,
> +                        &work->work);
  This should be folded into the original patch introducing the
s_dio_flush_wq. And please add a comment before this line saying that block
devices need a dereference exactly like this... Otherwise the patch looks
good so you can add:
  Reviewed-by: Jan Kara <jack@xxxxxxx>

                                                                Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR

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