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

[Ming Lei <ming.lei@redhat.com>]

On Mon, Aug 16, 2021 at 10:37:54AM +0100, Christoph Hellwig wrote:
> 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.

But it is wrong to use max sectors to limit number of bvecs(segments), isn't it?


Thanks,
Ming