linux-kernel - Re: [PATCH] block: respect virtual boundary mask in bvecs

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Date:   Tue, 6 Nov 2018 19:11:36 -0800
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Johannes Thumshirn <jthumshirn@...e.de>,
        Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...com>,
        Linux Block Layer Mailinglist <linux-block@...r.kernel.org>,
        Hannes Reinecke <hare@...e.de>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] block: respect virtual boundary mask in bvecs


>> If you mean the real huge page, this patch shouldn't have made a difference
>> because bio_vec->bv_offset is in [0, PAGE_SIZE), and iSer sets virt
>> boundary as 4K - 1.
>>
>> However, things will change after multipage bvec is introduced.
> 
> 
> Hi Ming,
> 
> I've received a blktrace from our customer showing the issue [1].
> 
> In this example trace they've submitted (contiguous) 64K I/Os and
> without this patch, they're seeing a lot of splits as indicated by the
> trace.
> 
> With the patch applied the I/O is directly issued to the LLDD without
> the splits.
> 
> [1] http://beta.suse.com/private/jthumshirn/blktrace.txt
> 

This patch makes sense to me. However I agree that we need to clarify
that this addresses cases where PAGE_SIZE-1 > queue_virt_boundary(q).

And also, lets replace "like iser" with "drivers that sets a virtual
boundary constraint" as nvme-rdma does the same thing.