Re: [PATCH 2/2] block: virtio_blk: fix handling single range discard request

[Ming Lei <ming.lei@redhat.com>]

On Tue, Aug 11, 2020 at 02:30:44PM +0200, Stefano Garzarella wrote:
> Hi Ming,
> 
> On Tue, Aug 11, 2020 at 05:21:34PM +0800, Ming Lei wrote:
> > 1f23816b8eb8 ("virtio_blk: add discard and write zeroes support") starts
> > to support multi-range discard for virtio-blk. However, the virtio-blk
> > disk may report max discard segment as 1, at least that is exactly what
> > qemu is doing.
> > 
> > So far, block layer switches to normal request merge if max discard segment
> > limit is 1, and multiple bios can be merged to single segment. This way may
> > cause memory corruption in virtblk_setup_discard_write_zeroes().
> > 
> > Fix the issue by handling single max discard segment in straightforward
> > way.
> > 
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > Fixes: 1f23816b8eb8 ("virtio_blk: add discard and write zeroes support")
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Changpeng Liu <changpeng.liu@intel.com>
> > Cc: Daniel Verkamp <dverkamp@chromium.org>
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Cc: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  drivers/block/virtio_blk.c | 23 +++++++++++++++--------
> >  1 file changed, 15 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index 63b213e00b37..05b01903122b 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c'
> > @@ -126,14 +126,21 @@ static int virtblk_setup_discard_write_zeroes(struct request *req, bool unmap)
> >  	if (!range)
> >  		return -ENOMEM;
> 
> We are allocating the 'range' array to contain 'segments' elements.
> When queue_max_discard_segments() returns 1, should we limit 'segments'
> to 1?

That is block layer's responsibility to make sure that 'segments' is <=
1, and we can double check & warn here.

> 
> >  
> > -	__rq_for_each_bio(bio, req) {
> > -		u64 sector = bio->bi_iter.bi_sector;
> > -		u32 num_sectors = bio->bi_iter.bi_size >> SECTOR_SHIFT;
> > -
> > -		range[n].flags = cpu_to_le32(flags);
> > -		range[n].num_sectors = cpu_to_le32(num_sectors);
> > -		range[n].sector = cpu_to_le64(sector);
> > -		n++;
> > +	if (queue_max_discard_segments(req->q) == 1) {
> > +		range[0].flags = cpu_to_le32(flags);
> > +		range[0].num_sectors = cpu_to_le32(blk_rq_sectors(req));
> > +		range[0].sector = cpu_to_le64(blk_rq_pos(req));
> > +		n = 1;
>                 ^
>                 this doesn't seem necessary since we don't use 'n' afterwards.

OK.

Thanks,
Ming