Re: [PATCH v5 01/11] block: make generic_make_request handle arbitrarily sized bios

[Kent Overstreet <kent.overstreet@gmail.com>]

On Fri, Aug 07, 2015 at 04:40:06PM -0700, Ming Lin wrote:
> 
> On Fri, 2015-08-07 at 09:30 +0200, Christoph Hellwig wrote:
> > I'm for solution 3:
> > 
> >  - keep blk_bio_{discard,write_same}_split, but ensure we never built
> >    a > 4GB bio in blkdev_issue_{discard,write_same}.
> 
> This has problem as I mentioned in solution 1.
> We need to also make sure max discard size is of proper granularity.
> See below example.
> 
>       4G: 8388608 sectors
> UINT_MAX: 8388607 sectors
> 
> dm-thinp block size = default discard granularity = 128 sectors
> 
> blkdev_issue_discard(sector=0, nr_sectors=8388608)
> 
> 1. Only ensure bi_size not overflow
> 
> It doesn't work.
> 
> [start_sector, end_sector]
> [0, 8388607]
>     [0, 8388606], then dm-thinp splits it to 2 bios
>         [0, 8388479]
>         [8388480, 8388606] ---> this has problem in process_discard_bio(),
>                                 because the discard size(7 sectors) covers less than a block(128 sectors)
>     [8388607, 8388607] ---> same problem 
> 
> 2. Ensure bi_size not overflow and max discard size is of proper granularity

Ideally we'd get upper layers out of the business of knowing about the queue
limits at all - that was the point of the patch series, after all.

Instead of using UINT_MAX, would it work to just make the max 1 << 31 sectors?