Re: [PATCH] raid1: ensure bio doesn't have more than BIO_MAX_VECS sectors

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On Sat, Aug 14, 2021 at 04:57:06PM +0800, Ming Lei wrote:
> > +	if (bitmap)
> > +		max_sectors = min_t(int, max_sectors, BIO_MAX_VECS * PAGE_SIZE);
> 
> s/PAGE_SIZE/PAGE_SECTORS

Yeah, max_sectors is in size units, I messed that up.

> 
> > +
> >  	if (max_sectors < bio_sectors(bio)) {
> >  		struct bio *split = bio_split(bio, max_sectors,
> >  					      GFP_NOIO, &conf->bio_split);
> > 
> 
> Here the limit is max single-page vectors, and the above way may not work,
> such as:
> 
> 0 ~ 254: each bvec's length is 512
> 255: bvec's length is 8192
> 
> the total length is just 512*255 + 8192 = 138752 bytes = 271 sectors, but it
> still may need 257 bvecs, which can't be allocated via bio_alloc_bioset().

Yes, we still need the rounding magic that alloc_behind_master_bio uses
here.

> One solution is to add queue limit of max_single_page_bvec, and let
> blk_queue_split() handle it.

I'd rather not bloat the core with this.



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