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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:   Mon, 19 Apr 2021 14:49:51 +0900
Message-Id: <20210419054951.6244-1-nanich.lee@samsung.com>
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References: <9ad8ef3f-905c-543e-d2af-7bf2f43ea04c@acm.org>
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Precedence: bulk

> > @@ -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);

> 
> >>> @@ -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.

I see what you said. I'll set t->bio_max_bytes to UINT_MAX in
blk_stack_limits() as you mentioned.

> 
> > 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.


Thanks,

Changheun Lee.