On Sun, Sep 13 2020 at 8:46pm -0400,
Damien Le Moal <Damien.LeMoal@xxxxxxx> wrote:
> On 2020/09/12 6:53, Mike Snitzer wrote:
> > blk_queue_get_max_sectors() has been trained for REQ_OP_WRITE_SAME and
> > REQ_OP_WRITE_ZEROES yet blk_rq_get_max_sectors() didn't call it for
> > those operations.
> >
> > Also, there is no need to avoid blk_max_size_offset() if
> > 'chunk_sectors' isn't set because it falls back to 'max_sectors'.
> >
> > Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
> > ---
> > include/linux/blkdev.h | 19 +++++++++++++------
> > 1 file changed, 13 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > index bb5636cc17b9..453a3d735d66 100644
> > --- a/include/linux/blkdev.h
> > +++ b/include/linux/blkdev.h
> > @@ -1070,17 +1070,24 @@ static inline unsigned int blk_rq_get_max_sectors(struct request *rq,
> > sector_t offset)
> > {
> > struct request_queue *q = rq->q;
> > + int op;
> > + unsigned int max_sectors;
> >
> > if (blk_rq_is_passthrough(rq))
> > return q->limits.max_hw_sectors;
> >
> > - if (!q->limits.chunk_sectors ||
> > - req_op(rq) == REQ_OP_DISCARD ||
> > - req_op(rq) == REQ_OP_SECURE_ERASE)
> > - return blk_queue_get_max_sectors(q, req_op(rq));
> > + op = req_op(rq);
> > + max_sectors = blk_queue_get_max_sectors(q, op);
> >
> > - return min(blk_max_size_offset(q, offset),
> > - blk_queue_get_max_sectors(q, req_op(rq)));
> > + switch (op) {
> > + case REQ_OP_DISCARD:
> > + case REQ_OP_SECURE_ERASE:
> > + case REQ_OP_WRITE_SAME:
> > + case REQ_OP_WRITE_ZEROES:
> > + return max_sectors;
> > + }
>
> Doesn't this break md devices ? (I think does use chunk_sectors for stride size,
> no ?)
>
> As mentioned in my reply to Ming's email, this will allow these commands to
> potentially cross over zone boundaries on zoned block devices, which would be an
> immediate command failure.
Depending on the implementation it is beneficial to get a large
discard (one not constrained by chunk_sectors, e.g. dm-stripe.c's
optimization for handling large discards and issuing N discards, one per
stripe). Same could apply for other commands.
Like all devices, zoned devices should impose command specific limits in
the queue_limits (and not lean on chunk_sectors to do a
one-size-fits-all).
But that aside, yes I agree I didn't pay close enough attention to the
implications of deferring the splitting of these commands until they
were issued to underlying storage. This chunk_sectors early splitting
override is a bit of a mess... not quite following the logic given we
were supposed to be waiting to split bios as late as possible.
Mike
