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From:   Changheun Lee <nanich.lee@samsung.com>
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Subject: Re: [PATCH v7 1/3] bio: limit bio max size
Date:   Tue, 20 Apr 2021 09:53:44 +0900
Message-Id: <20210420005344.9280-1-nanich.lee@samsung.com>
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Precedence: bulk

> On 4/18/21 10:49 PM, 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()?
> > 
> > Actually, overflow might be not heppen in case of physical device.
> > But I modified as below. feedback about this.
> > 
> > @@ -168,6 +169,9 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
> >  				 limits->logical_block_size >> SECTOR_SHIFT);
> >  	limits->max_sectors = max_sectors;
> >  
> > +	limits->bio_max_bytes = check_shl_overflow(max_sectors, SECTOR_SHIFT,
> > +		&limits->bio_max_bytes) ? UINT_MAX : max_sectors << SECTOR_SHIFT;
> > +
> >  	q->backing_dev_info->io_pages = max_sectors >> (PAGE_SHIFT - 9);
> >  }
> >  EXPORT_SYMBOL(blk_queue_max_hw_sectors);
> 
> If no overflow occurs, check_shl_overflow() stores the result in the
> memory location the third argument points at. So the above expression
> can be simplified into the following:
> 
> if (check_shl_overflow(max_sectors, SECTOR_SHIFT, &limits->bio_max_bytes)) {
> 	limits->bio_max_bytes = UINT_MAX;
> }

OK. No problem. It might be more readable.

> 
> >>> 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.
> > 
> > I tried, but it is not easy. because request_queue structure of blkdev.h
> > should be referred in bio.h. I think it's not good to apply as a inline function.
> 
> Please don't worry about this. Inlining bio_max_size() is not a big
> concern to me.
> 
> Thanks,
> 
> Bart.

I think removing of UINT_MAX setting in blk_stack_limits() might be good.
Because default queue limits for stacking device will be set in
blk_set_stacking_limits(), and blk_set_default_limits() will be called
automatically. So setting of UINT_MAX in blk_stack_limits() is not needed.
And setting in blk_stack_limits() can overwrite bio_max_bytes as a default
after stacking driver set to proper bio_max_bytes value.


Thanks,

Changheun Lee.