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

On Fri, Aug 13, 2021 at 02:05:10PM +0800, Guoqing Jiang wrote:

From: Guoqing Jiang <redacted>

We can't split bio with more than BIO_MAX_VECS sectors, otherwise the
below call trace was triggered because we could allocate oversized
write behind bio later.

[ 8.097936] bvec_alloc+0x90/0xc0
[ 8.098934] bio_alloc_bioset+0x1b3/0x260
[ 8.099959] raid1_make_request+0x9ce/0xc50 [raid1]

Which bio_alloc_bioset is this?  The one in alloc_behind_master_bio?

In which case I think you want to limit the reduction of max_sectors
to just the write behind case, and clearly document what is going on.

In general the size of a bio only depends on the number of vectors, not
the total I/O size.  But alloc_behind_master_bio allocates new backing
pages using order 0 allocations, so in this exceptional case the total
size oes actually matter.

While we're at it: this huge memory allocation looks really deadlock
prone.