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Mon, 14 Sep 2020 14:49:38 +0000 (UTC) Received: from localhost (unknown [10.18.25.174]) by smtp.corp.redhat.com (Postfix) with ESMTPS id D037481C4D; Mon, 14 Sep 2020 14:49:29 +0000 (UTC) Date: Mon, 14 Sep 2020 10:49:28 -0400 From: Mike Snitzer To: Ming Lei Cc: Vijayendra Suman , dm-devel@redhat.com, linux-block@vger.kernel.org Subject: Re: [PATCH 1/3] block: fix blk_rq_get_max_sectors() to flow more carefully Message-ID: <20200914144928.GA14410@redhat.com> References: <20200911215338.44805-1-snitzer@redhat.com> <20200911215338.44805-2-snitzer@redhat.com> <20200912135252.GA210077@T590> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200912135252.GA210077@T590> User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Sat, Sep 12 2020 at 9:52am -0400, Ming Lei 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 > > --- > > 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