Re: [PATCH] Btrfs: tolerate errors if we have retried successfully

[Liu Bo <bo.li.liu@oracle.com>]

On Fri, May 05, 2017 at 06:52:45PM +0200, David Sterba wrote:
> On Thu, Apr 13, 2017 at 06:11:56PM -0700, Liu Bo wrote:
> > With raid1 profile, dio read isn't tolerating IO errors if read length is
> > less than the stripe length (64K).
> 
> Can you please write more details why this is true? Some pointers to
> code etc, I'm lost. Eg. where the errors is tolerated. Thanks.

Sure.

Our bio didn't get split in btrfs_submit_direct_hook() if (dip->flags &
BTRFS_DIO_ORIG_BIO_SUBMITTED) is true.  If the underlying device returns error
somehow, bio->bi_error has recorded that error.

If we could recover the correct data from another copy in profile raid1/10/5/6,
with btrfs_subio_endio_read() returning 0, bio would have the correct data in
its vector, but bio->bi_error is not updated accordingly so that the following
dio_end_io(dio_bio, bio->bi_error) makes directIO think this read has failed.

Thanks,

-liubo

> 
> > This fixes the problem by setting bio's error to 0 if a good copy has been
> > found.
> > 
> > Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
> > ---
> >  fs/btrfs/inode.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> > index 632b616..4e1398e 100644
> > --- a/fs/btrfs/inode.c
> > +++ b/fs/btrfs/inode.c
> > @@ -8113,8 +8113,11 @@ static void btrfs_endio_direct_read(struct bio *bio)
> >  	struct btrfs_io_bio *io_bio = btrfs_io_bio(bio);
> >  	int err = bio->bi_error;
> >  
> > -	if (dip->flags & BTRFS_DIO_ORIG_BIO_SUBMITTED)
> > +	if (dip->flags & BTRFS_DIO_ORIG_BIO_SUBMITTED) {
> >  		err = btrfs_subio_endio_read(inode, io_bio, err);
> > +		if (!err)
> > +			bio->bi_error = 0;
> > +	}
> >  
> >  	unlock_extent(&BTRFS_I(inode)->io_tree, dip->logical_offset,
> >  		      dip->logical_offset + dip->bytes - 1);
> > -- 
> > 2.5.5
> > 
> > --
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