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Re: [PATCH 2/9] ext4: honor the O_SYNC flag for aysnchronous direct I/O

To: "Darrick J. Wong" <darrick.wong@xxxxxxxxxx>
Subject: Re: [PATCH 2/9] ext4: honor the O_SYNC flag for aysnchronous direct I/O requests
From: Jan Kara <jack@xxxxxxx>
Date: Tue, 20 Nov 2012 11:07:51 +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: <20121120074131.24645.38489.stgit@xxxxxxxxxxxxxxxxxxx>
References: <20121120074116.24645.36369.stgit@xxxxxxxxxxxxxxxxxxx> <20121120074131.24645.38489.stgit@xxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Mon 19-11-12 23:41:31, Darrick J. Wong 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.
> 
> From: Jeff Moyer <jmoyer@xxxxxxxxxx>
> Signed-off-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
> [darrick.wong@xxxxxxxxxx: Rework original patch to reflect a subsequent
>                         ext4 reorganization]
> Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>
> ---
>  fs/ext4/ext4.h    |    9 +++++
>  fs/ext4/file.c    |    2 +
>  fs/ext4/inode.c   |    6 +++
>  fs/ext4/page-io.c |   92 
> +++++++++++++++++++++++++++++++++++++++++++++--------
>  fs/ext4/super.c   |   13 +++++++
>  5 files changed, 106 insertions(+), 16 deletions(-)
> 
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 3c20de1..f5a0b89 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -186,6 +186,7 @@ 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;
> @@ -1279,6 +1280,9 @@ 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;
> +
  Umm, I'm not completely decided whether we really need a separate
workqueue. But it doesn't cost too much so I guess it makes some sense -
fsync() is rather heavy so syncing won't starve extent conversion...

> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> index 68e896e..cda013a 100644
> --- a/fs/ext4/page-io.c
> +++ b/fs/ext4/page-io.c
> @@ -84,6 +84,50 @@ void ext4_free_io_end(ext4_io_end_t *io)
>       kmem_cache_free(io_end_cachep, io);
>  }
>  
> +/*
> + * This function is called in the completion path for AIO O_SYNC|O_DIRECT
> + * writes, and also in the fsync path.  The purpose is to ensure that the
> + * disk caches for the journal and data devices are flushed.
> + */
> +static int ext4_end_io_do_flush(ext4_io_end_t *io)
> +{
> +     struct inode *inode = io->inode;
> +     tid_t commit_tid;
> +     bool needs_barrier = false;
> +     journal_t *journal = EXT4_SB(inode->i_sb)->s_journal;
> +     int barriers_enabled = test_opt(inode->i_sb, BARRIER);
> +     int ret = 0;
> +
> +     if (!barriers_enabled)
> +             return 0;
  This is definitely wrong. Even if barriers are disabled, we may need to
push out some buffers or commit a transaction.

> +
> +     /*
> +      * If we are running in nojournal mode, just flush the disk
> +      * cache and return.
> +      */
> +     if (!journal)
> +             return blkdev_issue_flush(inode->i_sb->s_bdev, GFP_NOIO, NULL);
  And this is wrong as well - you need to do work similar to what
ext4_sync_file() does. Actually it would be *much* better if these two
sites used the same helper function. Which also poses an interesting
question about locking - do we need i_mutex or not? Forcing a transaction
commit is definitely OK without it, similarly as grabbing transaction ids
from inode or ext4_should_journal_data() test. __sync_inode() call seems
to be OK without i_mutex as well so I believe we can just get rid of it
(getting i_mutex from the workqueue is a locking nightmare we don't want to
return to).

> +
> +     if (ext4_should_journal_data(inode)) {
> +             ret = ext4_force_commit(inode->i_sb);
> +             goto out;
> +     }
> +
> +     commit_tid = io->iocb->ki_filp->f_flags & __O_SYNC ?
> +             EXT4_I(inode)->i_sync_tid : EXT4_I(inode)->i_datasync_tid;
> +     if (!jbd2_trans_will_send_data_barrier(journal, commit_tid))
> +             needs_barrier = true;
> +
> +     jbd2_log_start_commit(journal, commit_tid);
> +     ret = jbd2_log_wait_commit(journal, commit_tid);
> +
> +     if (!ret && needs_barrier)
> +             ret = blkdev_issue_flush(inode->i_sb->s_bdev, GFP_NOIO, NULL);
> +
> +out:
> +     return ret;
> +}
> +
>  /* check a range of space and convert unwritten extents to written. */
>  static int ext4_end_io(ext4_io_end_t *io)
>  {
...
> @@ -149,8 +209,11 @@ void ext4_add_complete_io(ext4_io_end_t *io_end)
>       struct workqueue_struct *wq;
>       unsigned long flags;
>  
> -     BUG_ON(!(io_end->flag & EXT4_IO_END_UNWRITTEN));
> -     wq = EXT4_SB(io_end->inode->i_sb)->dio_unwritten_wq;
> +     BUG_ON(!ext4_io_end_deferred(io_end));
> +     if (io_end->flag & EXT4_IO_END_UNWRITTEN)
> +             wq = EXT4_SB(io_end->inode->i_sb)->dio_unwritten_wq;
> +     else
> +             wq = EXT4_SB(io_end->inode->i_sb)->aio_dio_flush_wq;
  Umm, I'd prefer if we used aio_dio_flush_wq when EXT4_IO_END_NEEDS_SYNC
is set. That way slow syncing works will be always offloaded to a separate
workqueue.

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

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