Re: [PATCH V2] block: correctly fallback for zeroout

[Shaohua Li <shli@fb.com>]

On Tue, Jun 07, 2016 at 05:50:49AM +0100, Sitsofe Wheeler wrote:
> On Mon, Jun 06, 2016 at 03:33:58PM -0700, Shaohua Li wrote:
> > blkdev_issue_zeroout try discard/writesame first, if they fail, zeroout
> > fallback to regular write. The problem is discard/writesame doesn't
> > return error for -EOPNOTSUPP, then zeroout can't do fallback and leave
> > disk data not changed. zeroout should have guaranteed zero-fill
> > behavior.
> > 
> > https://bugzilla.kernel.org/show_bug.cgi?id=118581
> > 
> > V2: move the return value policy to blkdev_issue_discard and
> > delete the policy for blkdev_issue_write_same (Martin)
> > 
> > Cc: Sitsofe Wheeler <sitsofe@yahoo.com>
> > Cc: Mike Snitzer <snitzer@redhat.com>
> > Cc: Jens Axboe <axboe@fb.com>
> > Cc: Martin K. Petersen <martin.petersen@oracle.com>
> > Signed-off-by: Shaohua Li <shli@fb.com>
> > ---
> >  block/blk-lib.c | 49 +++++++++++++++++++++++++++++++------------------
> >  1 file changed, 31 insertions(+), 18 deletions(-)
> > 
> > diff --git a/block/blk-lib.c b/block/blk-lib.c
> > index 23d7f30..a3a26c8 100644
> > --- a/block/blk-lib.c
> > +++ b/block/blk-lib.c
> > @@ -84,6 +84,28 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
> >  }
> >  EXPORT_SYMBOL(__blkdev_issue_discard);
> >  
> > +static int do_blkdev_issue_discard(struct block_device *bdev, sector_t sector,
> > +		sector_t nr_sects, gfp_t gfp_mask, unsigned long flags,
> > +		int *io_err)
> > +{
> > +	int type = REQ_WRITE | REQ_DISCARD;
> > +	struct bio *bio = NULL;
> > +	struct blk_plug plug;
> > +	int ret;
> > +
> > +	if (flags & BLKDEV_DISCARD_SECURE)
> > +		type |= REQ_SECURE;
> > +
> > +	blk_start_plug(&plug);
> > +	ret = __blkdev_issue_discard(bdev, sector, nr_sects, gfp_mask, type,
> > +			&bio);
> > +	if (!ret && bio)
> > +		*io_err = submit_bio_wait(type, bio);
> > +	blk_finish_plug(&plug);
> > +
> > +	return ret;
> > +}
> > +
> >  /**
> >   * blkdev_issue_discard - queue a discard
> >   * @bdev:	blockdev to issue discard for
> > @@ -98,23 +120,12 @@ EXPORT_SYMBOL(__blkdev_issue_discard);
> >  int blkdev_issue_discard(struct block_device *bdev, sector_t sector,
> >  		sector_t nr_sects, gfp_t gfp_mask, unsigned long flags)
> >  {
> > -	int type = REQ_WRITE | REQ_DISCARD;
> > -	struct bio *bio = NULL;
> > -	struct blk_plug plug;
> > -	int ret;
> > +	int ret, io_err;
> >  
> > -	if (flags & BLKDEV_DISCARD_SECURE)
> > -		type |= REQ_SECURE;
> > -
> > -	blk_start_plug(&plug);
> > -	ret = __blkdev_issue_discard(bdev, sector, nr_sects, gfp_mask, type,
> > -			&bio);
> > -	if (!ret && bio) {
> > -		ret = submit_bio_wait(type, bio);
> > -		if (ret == -EOPNOTSUPP)
> > -			ret = 0;
> > -	}
> > -	blk_finish_plug(&plug);
> > +	ret = do_blkdev_issue_discard(bdev, sector, nr_sects, gfp_mask,
> > +		flags, &io_err);
> > +	if (!ret && io_err != -EOPNOTSUPP)
> > +		ret = io_err;
> 
> Because io_err is always consulted if ret is not true shouldn't it be
> explicitly initialized to 0 before the call to do_blkdev_issue_discard
> (as do_blkdev_issue_discard will only set io_err if bio returned true)?
> 
> Perhaps there's an argument that do_blkdev_issue_discard should always
> set io_err on all its paths rather than just on errors in case the
> caller hasn't initialized it - is there an existing kernel pattern for
> this)?

I didn't follow. io_err is only and always set when ret == 0. io_err is
meanless if ret != 0, because that means the disk doesn't support discard and
we don't dispatch discard IO. why should we initialized io_err to 0?