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Subject: Re: [PATCH v7 1/3] bio: limit bio max size
To:     Changheun Lee <nanich.lee@samsung.com>
Cc:     Johannes.Thumshirn@wdc.com, asml.silence@gmail.com,
        axboe@kernel.dk, damien.lemoal@wdc.com, gregkh@linuxfoundation.org,
        hch@infradead.org, jisoo2146.oh@samsung.com,
        junho89.kim@samsung.com, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, ming.lei@redhat.com,
        mj0123.lee@samsung.com, osandov@fb.com, patchwork-bot@kernel.org,
        seunghwan.hyun@samsung.com, sookwan7.kim@samsung.com,
        tj@kernel.org, tom.leiming@gmail.com, woosung2.lee@samsung.com,
        yt0928.kim@samsung.com
References: <bb8f7127-edff-4a32-2d5c-4343002bda19@acm.org>
 <CGME20210416060827epcas1p39350d45cef64c91be681b76180b63140@epcas1p3.samsung.com>
 <20210416055039.20126-1-nanich.lee@samsung.com>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <9ad8ef3f-905c-543e-d2af-7bf2f43ea04c@acm.org>
Date:   Fri, 16 Apr 2021 08:28:27 -0700
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On 4/15/21 10:50 PM, Changheun Lee wrote:
>> On 4/15/21 3:38 AM, Changheun Lee wrote:
>>> @@ -538,6 +540,8 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
>>>  {
>>>  	unsigned int top, bottom, alignment, ret = 0;
>>>  
>>> +	t->bio_max_bytes = min_not_zero(t->bio_max_bytes, b->bio_max_bytes);
>>> +
>>>  	t->max_sectors = min_not_zero(t->max_sectors, b->max_sectors);
>>>  	t->max_hw_sectors = min_not_zero(t->max_hw_sectors, b->max_hw_sectors);
>>>  	t->max_dev_sectors = min_not_zero(t->max_dev_sectors, b->max_dev_sectors);
>>
>> The above will limit bio_max_bytes for all stacked block devices, which
>> is something we do not want. I propose to set t->bio_max_bytes to
>> UINT_MAX in blk_stack_limits() and to let the stacked driver (e.g.
>> dm-crypt) decide whether or not to lower that value.
> 
> Actually, bio size should be limited in dm-crypt too. Because almost I/O
> from user space will be gone to dm-crypt first. I/O issue timing will be
> delayed if bio size is not limited in dm-crypt.
> Do you have any idea to decide whether takes lower bio max size, or not
> in the stacked driver?
> Add a flag to decide this in driver layer like before?
> Or insert code manually in each stacked driver if it is needed?

There will be fewer stacked drivers for which the bio size has to be
limited than for which the bio size has not to be limited. Hence the
proposal to set t->bio_max_bytes to UINT_MAX in blk_stack_limits() and
to let the stacked driver (e.g. dm-crypt) decide whether or not to lower
that value.

Thanks,

Bart.