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

On Tue, Aug 17, 2021 at 06:06:12AM +0100, Christoph Hellwig wrote:

On Mon, Aug 16, 2021 at 07:40:48PM +0800, Ming Lei wrote:

quoted

quoted

quoted

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?

The raid1 write behind code cares about the size ofa bio it can reach by
adding order 0 pages to it.  The bvecs are part of that and I think the
calculation in the patch documents that a well.

Thinking of further, your and Guoqing's patch are correct & enough since
bio_copy_data() just copies bytes(sectors) stream from fs bio to the
write behind bio.


Thanks,
Ming