Re: [PATCH] block, nvme: Increase max segments parameter setting value

[Ming Lei <tom.leiming@gmail.com>]

On Sat, Mar 28, 2020 at 2:18 AM Keith Busch <kbusch@kernel.org> wrote:
>
> On Sat, Mar 28, 2020 at 02:50:43AM +0900, Tokunori Ikegami wrote:
> > On 2020/03/25 1:51, Tokunori Ikegami wrote:
> > > On 2020/03/24 9:02, Keith Busch wrote:
> > > > We didn't have 32-bit max segments before, though. Why was 16-bits
> > > > enough in older kernels? Which kernel did this stop working?
> > > Now I am asking the detail information to the reporter so let me
> > > update later.  That was able to use the same command script with the
> > > large data length in the past.
> >
> > I have just confirmed the detail so let me update below.
> >
> > The data length 20,531,712 (0x1394A00) is used on kernel 3.10.0 (CentOS
> > 64bit).
> > Also it is failed on kernel 10 4.10.0 (Ubuntu 32bit).
> > But just confirmed it as succeeded on both 4.15.0 (Ubuntu 32bit) and 4.15.1
> > (Ubuntu 64bit).
> > So the original 20,531,712 length failure issue seems already resolved.
> >
> > I tested the data length 0x10000000 (268,435,456) and it is failed
> > But now confirmed it as failed on all the above kernel versions.
> > Also the patch fixes only this 0x10000000 length failure issue.
>
> This is actually even more confusing. We do not support 256MB transfers
> within a single command in the pci nvme driver anymore. The max is 4MB,
> so I don't see how increasing the max segments will help: you should be
> hitting the 'max_sectors' limit if you don't hit the segment limit first.

That looks a bug for passthrough req, because 'max_sectors' limit is only
checked in bio_add_pc_page(), not done in blk_rq_append_bio(), something
like the following seems required:

diff --git a/block/blk-map.c b/block/blk-map.c
index b0790268ed9d..e120d80b75a5 100644
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -22,6 +22,10 @@ int blk_rq_append_bio(struct request *rq, struct bio **bio)
        struct bio_vec bv;
        unsigned int nr_segs = 0;

+       if (((rq->__data_len + (*bio)->bi_iter.bi_size) >> 9) >
+                       queue_max_hw_sectors(rq->q))
+               return -EINVAL;
+


Thanks,
Ming Lei