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Re: [PATCH 2/3] ext4: honor the O_SYNC flag for aysnchronous direct I/O

To: Jeff Moyer <jmoyer@xxxxxxxxxx>
Subject: Re: [PATCH 2/3] ext4: honor the O_SYNC flag for aysnchronous direct I/O requests
From: Jan Kara <jack@xxxxxxx>
Date: Thu, 2 Feb 2012 18:31:20 +0100
Cc: linux-ext4@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx
In-reply-to: <1327698949-12616-3-git-send-email-jmoyer@xxxxxxxxxx>
References: <1327698949-12616-1-git-send-email-jmoyer@xxxxxxxxxx> <1327698949-12616-3-git-send-email-jmoyer@xxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
  Hi,

On Fri 27-01-12 16:15:48, Jeff Moyer wrote:
> If a file is opened with O_SYNC|O_DIRECT, the drive cache does not get
> flushed after the write completion.  Instead, it's flushed *before* the
> I/O is sent to the disk (in __generic_file_aio_write).  This patch
> attempts to fix that problem by marking an I/O as requiring a cache
> flush in endio processing.  I'll send a follow-on patch to the
> generic write code to get rid of the bogus generic_write_sync call
> when EIOCBQUEUED is returned.
  Thanks for the patch!

> Signed-off-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
> ---
>  fs/ext4/ext4.h    |    4 ++++
>  fs/ext4/inode.c   |   11 +++++++++--
>  fs/ext4/page-io.c |   39 ++++++++++++++++++++++++++++++++-------
>  fs/ext4/super.c   |   11 +++++++++++
>  4 files changed, 56 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 2d55d7c..4377ed3 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -185,6 +185,7 @@ struct mpage_da_data {
>  #define EXT4_IO_END_ERROR    0x0002
>  #define EXT4_IO_END_QUEUED   0x0004
>  #define EXT4_IO_END_DIRECT   0x0008
> +#define EXT4_IO_END_NEEDS_SYNC       0x0010
>  
>  struct ext4_io_page {
>       struct page     *p_page;
> @@ -1247,6 +1248,9 @@ struct ext4_sb_info {
>       /* workqueue for dio unwritten */
>       struct workqueue_struct *dio_unwritten_wq;
>  
> +     /* workqueue for aio+dio+o_sync disk cache flushing */
> +     struct workqueue_struct *aio_dio_flush_wq;
> +
  Hmm, looking at the patch I'm wondering why did you introduce the new
workqueue? It seems dio_unwritten_wq would be enough? You just need to
rename it to something more appropriate ;)

> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> index 9e1b8eb..d07cd40 100644
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -98,15 +98,40 @@ int ext4_end_io_nolock(ext4_io_end_t *io)
>                  "list->prev 0x%p\n",
>                  io, inode->i_ino, io->list.next, io->list.prev);
>  
> -     ret = ext4_convert_unwritten_extents(inode, offset, size);
> -     if (ret < 0) {
> -             ext4_msg(inode->i_sb, KERN_EMERG,
> -                      "failed to convert unwritten extents to written "
> -                      "extents -- potential data loss!  "
> -                      "(inode %lu, offset %llu, size %zd, error %d)",
> -                      inode->i_ino, offset, size, ret);
> +     if (io->flag & EXT4_IO_END_UNWRITTEN) {
> +
> +             ret = ext4_convert_unwritten_extents(inode, offset, size);
> +             if (ret < 0) {
> +                     ext4_msg(inode->i_sb, KERN_EMERG,
> +                              "failed to convert unwritten extents to "
> +                              "written extents -- potential data loss!  "
> +                              "(inode %lu, offset %llu, size %zd, error %d)",
> +                              inode->i_ino, offset, size, ret);
> +                     goto endio;
> +             }
> +     }
> +
> +     /*
> +      * This function has two callers.  The first is the end_io_work
> +      * routine just below.  This is an asynchronous completion context.
> +      * The second is in the fsync path.  For the latter path, we can't
> +      * return from here until the job is done.  Hence, we issue a
> +      * blocking blkdev_issue_flush call.
> +      */
> +     if (io->flag & EXT4_IO_END_NEEDS_SYNC) {
> +             /*
> +              * Ideally, we'd like to know if the force_commit routine
> +              * actually did send something to disk.  If it didn't,
> +              * then we need to issue the cache flush by hand.  For now,
> +              * play it safe and do both.
> +              */
> +             ret = ext4_force_commit(inode->i_sb);
> +             if (ret)
> +                     goto endio;
> +             ret = blkdev_issue_flush(inode->i_sb->s_bdev, GFP_NOIO, NULL);
  Look at what ext4_sync_file() does. It's more efficient than this.
You need something like:
        commit_tid = file->f_flags & __O_SYNC ? EXT4_I(inode)->i_sync_tid :
                                                EXT4_I(inode)->i_datasync_tid;
        if (journal->j_flags & JBD2_BARRIER &&
            !jbd2_trans_will_send_data_barrier(journal, commit_tid))
                needs_barrier = true;
        jbd2_log_start_commit(journal, commit_tid);
        jbd2_log_wait_commit(journal, commit_tid);
        if (needs_barrier)
                blkdev_issue_flush(inode->i_sb->s_bdev, GFP_NOIO, NULL);

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

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