On Sat, Sep 12 2020 at 9:52am -0400,
Ming Lei <ming.lei@xxxxxxxxxx> wrote:
> On Fri, Sep 11, 2020 at 05:53:36PM -0400, 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.
>
> Actually WRITE_SAME & WRITE_ZEROS are handled by the following if
> chunk_sectors is set:
>
> return min(blk_max_size_offset(q, offset),
> blk_queue_get_max_sectors(q, req_op(rq)));
Yes, but blk_rq_get_max_sectors() is a bit of a mess structurally. he
duality of imposing chunk_sectors and/or considering offset when
calculating the return is very confused.
> > 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;
> > + }
> > +
> > + return min(blk_max_size_offset(q, offset), max_sectors);
> > }
>
> It depends if offset & chunk_sectors limit for WRITE_SAME & WRITE_ZEROS
> needs to be considered.
Yes, I see that now. But why don't they need to be considered for
REQ_OP_DISCARD and REQ_OP_SECURE_ERASE? Is it because the intent of the
block core is to offer late splitting of bios? If so, then why impose
chunk_sectors so early?
Obviously this patch 1/3 should be dropped. I didn't treat
chunk_sectors with proper priority.
But like I said above, blk_rq_get_max_sectors() vs blk_max_size_offset()
is not at all straight-forward. And the code looks prone to imposing
limits that shouldn't be (or vice-versa).
Also, when falling back to max_sectors, why not consider offset to treat
max_sectors like a granularity? Would allow for much more consistent IO
patterns.
Mike
