Re: [PATCH 1/3] fs,block: Introduce IOCB_ZONE_APPEND and direct-io handling

[Kanchan Joshi]

On Thu, Jun 18, 2020 at 07:16:19AM +0000, Damien Le Moal wrote:
> On 2020/06/18 2:27, Kanchan Joshi wrote:
> > From: Selvakumar S <selvakuma.s1@xxxxxxxxxxx>
> >
> > Introduce IOCB_ZONE_APPEND flag, which is set in kiocb->ki_flags for
> > zone-append. Direct I/O submission path uses this flag to send bio with
> > append op. And completion path uses the same to return zone-relative
> > offset to upper layer.
> >
> > Signed-off-by: SelvaKumar S <selvakuma.s1@xxxxxxxxxxx>
> > Signed-off-by: Kanchan Joshi <joshi.k@xxxxxxxxxxx>
> > Signed-off-by: Nitesh Shetty <nj.shetty@xxxxxxxxxxx>
> > Signed-off-by: Javier Gonzalez <javier.gonz@xxxxxxxxxxx>
> > ---
> >  fs/block_dev.c     | 19 ++++++++++++++++++-
> >  include/linux/fs.h |  1 +
> >  2 files changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/block_dev.c b/fs/block_dev.c
> > index 47860e5..4c84b4d0 100644
> > --- a/fs/block_dev.c
> > +++ b/fs/block_dev.c
> > @@ -185,6 +185,10 @@ static unsigned int dio_bio_write_op(struct kiocb *iocb)
> >  	/* avoid the need for a I/O completion work item */
> >  	if (iocb->ki_flags & IOCB_DSYNC)
> >  		op |= REQ_FUA;
> > +#ifdef CONFIG_BLK_DEV_ZONED
> > +	if (iocb->ki_flags & IOCB_ZONE_APPEND)
> > +		op |= REQ_OP_ZONE_APPEND | REQ_NOMERGE;
> > +#endif
>
> No need for the #ifdef. And no need for the REQ_NOMERGE either since
> REQ_OP_ZONE_APPEND requests are defined as not mergeable already.
>
> >  	return op;
> >  }
> >
> > @@ -295,6 +299,14 @@ static int blkdev_iopoll(struct kiocb *kiocb, bool wait)
> >  	return blk_poll(q, READ_ONCE(kiocb->ki_cookie), wait);
> >  }
> >
> > +#ifdef CONFIG_BLK_DEV_ZONED
> > +static inline long blkdev_bio_end_io_append(struct bio *bio)
> > +{
> > +	return (bio->bi_iter.bi_sector %
> > +		blk_queue_zone_sectors(bio->bi_disk->queue)) << SECTOR_SHIFT;
>
> A zone size is at most 4G sectors as defined by the queue chunk_sectors limit
> (unsigned int). It means that the return value here can overflow due to the
> shift, at least on 32bits arch.
>
> And as Pavel already commented, zone sizes are power of 2 so you can use
> bitmasks instead of costly divisions.
>
> > +}
> > +#endif
> > +
> >  static void blkdev_bio_end_io(struct bio *bio)
> >  {
> >  	struct blkdev_dio *dio = bio->bi_private;
> > @@ -307,15 +319,20 @@ static void blkdev_bio_end_io(struct bio *bio)
> >  		if (!dio->is_sync) {
> >  			struct kiocb *iocb = dio->iocb;
> >  			ssize_t ret;
> > +			long res = 0;
> >
> >  			if (likely(!dio->bio.bi_status)) {
> >  				ret = dio->size;
> >  				iocb->ki_pos += ret;
> > +#ifdef CONFIG_BLK_DEV_ZONED
> > +				if (iocb->ki_flags & IOCB_ZONE_APPEND)
> > +					res = blkdev_bio_end_io_append(bio);
>
> overflow... And no need for the #ifdef.
>
> > +#endif
> >  			} else {
> >  				ret = blk_status_to_errno(dio->bio.bi_status);
> >  			}
> >
> > -			dio->iocb->ki_complete(iocb, ret, 0);
> > +			dio->iocb->ki_complete(iocb, ret, res);
>
> ki_complete interface also needs to be changed to have a 64bit last argument to
> avoid overflow.
>
> And this all does not work anyway because __blkdev_direct_IO() and
> __blkdev_direct_IO_simple() both call bio_iov_iter_get_pages() *before*
> dio_bio_write_op() is called. This means that bio_iov_iter_get_pages() will not
> see that it needs to get the pages for a zone append command and so will not
> call __bio_iov_append_get_pages(). That leads to BIO split and potential
> corruption of the user data as fragments of the kiocb may get reordered.
>
> There is a lot more to do to the block_dev direct IO code for this to work.

Thanks a lot for the great review. Very helpful. We'll fix in V2.