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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: <2e54f27a-ae4c-af65-34ba-18b43bd4815d@acm.org>
 <CGME20210415105608epcas1p269bae87b8a7dab133753f7916420251e@epcas1p2.samsung.com>
 <20210415103820.23272-1-nanich.lee@samsung.com>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <bb8f7127-edff-4a32-2d5c-4343002bda19@acm.org>
Date:   Thu, 15 Apr 2021 12:18:52 -0700
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Precedence: bulk

On 4/15/21 3:38 AM, Changheun Lee wrote:
> @@ -167,6 +168,7 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
>  	max_sectors = round_down(max_sectors,
>  				 limits->logical_block_size >> SECTOR_SHIFT);
>  	limits->max_sectors = max_sectors;
> +	limits->bio_max_bytes = max_sectors << SECTOR_SHIFT;
>  
>  	q->backing_dev_info->io_pages = max_sectors >> (PAGE_SHIFT - 9);
>  }

Can the new shift operation overflow? If so, how about using
check_shl_overflow()?

> @@ -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.

> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index d0246c92a6e8..e5add63da3af 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -106,6 +106,8 @@ static inline void *bio_data(struct bio *bio)
>  	return NULL;
>  }
>  
> +extern unsigned int bio_max_size(struct bio *bio);

You may want to define bio_max_size() as an inline function in bio.h
such that no additional function calls are introduced in the hot path.

Thanks,

Bart.