Re: [PATCH 1/2] block: integrity: enable multi-page bvec for bio integrity

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

On Wed, Apr 24, 2019 at 09:14:36AM -0400, Martin K. Petersen wrote:
> 
> Ming,
> 
> I'm traveling today and probably won't be able to take a closer look
> until tomorrow. But from a quick glance this looks OK.
> 
> > The integrity buffer can't be very big, for example, the max sectors
> > for one bio is 2560, one sector may take at most 8bytes for integrity
> > info, so the max size of integrity buffer is just 20k(<=5 pages).
> 
> Just a comment on your rationale about 5 pages.
> 
> buffer_head submissions have traditionally been small, and depending on
> your choice of allocator, new allocations would grow backwards in
> memory. So there were several common I/O patterns that produced a
> single, non-mergeable 8 byte integrity metadata allocation for every 512
> bytes of data in the I/O. It's a pathological corner case. Just make
> sure it's something you handle when you muck with this. ext[23] and dd
> to the block device used to be able to reproduce this scenario easily.

Yeah, you are right.

I have realized that 5 pages aren't correct, and it should be one
protection data segment for each bio usually.

Thanks,
Ming