Re: [PATCH V12 00/20] block: support multi-page bvec

[Ming Lei]

On Wed, Nov 28, 2018 at 06:44:00AM -0700, Jens Axboe wrote:
> On 11/25/18 7:17 PM, Ming Lei wrote:
> > Hi,
> > 
> > This patchset brings multi-page bvec into block layer:
> > 
> > 1) what is multi-page bvec?
> > 
> > Multipage bvecs means that one 'struct bio_bvec' can hold multiple pages
> > which are physically contiguous instead of one single page used in linux
> > kernel for long time.
> > 
> > 2) why is multi-page bvec introduced?
> > 
> > Kent proposed the idea[1] first. 
> > 
> > As system's RAM becomes much bigger than before, and huge page, transparent
> > huge page and memory compaction are widely used, it is a bit easy now
> > to see physically contiguous pages from fs in I/O. On the other hand, from
> > block layer's view, it isn't necessary to store intermediate pages into bvec,
> > and it is enough to just store the physicallly contiguous 'segment' in each
> > io vector.
> > 
> > Also huge pages are being brought to filesystem and swap [2][6], we can
> > do IO on a hugepage each time[3], which requires that one bio can transfer
> > at least one huge page one time. Turns out it isn't flexiable to change
> > BIO_MAX_PAGES simply[3][5]. Multipage bvec can fit in this case very well.
> > As we saw, if CONFIG_THP_SWAP is enabled, BIO_MAX_PAGES can be configured
> > as much bigger, such as 512, which requires at least two 4K pages for holding
> > the bvec table.
> 
> I'm pretty happy with this patchset at this point, looks like it just
> needs a respin to address the last comments. My only concern is whether

I will address the last comment from Omar on patch of '[PATCH V12 01/20] btrfs:
remove various bio_offset arguments', we may use the approach in V11 simply.

> it's a good idea to target this for 4.21, or if we should wait until
> 4.22. 4.21 has a fairly substantial amount of changes in terms of block
> already, it's not the best timing for something of this magnitude too.

Yeah, I understand.

> 
> I'm going back and forth on those one a bit. Any concerns with
> pushing this to 4.22?

My only one concern is about the warning of "blk_cloned_rq_check_limits:
over max segments limit" on dm multipath, and seems Ewan and Mike is waiting
for this fix.

thanks,
Ming