Re: [PATCH] iov_iter: don't require contiguous pages in iov_iter_extract_bvec_pages

[Klara Modin <klarasmodin@gmail.com>]

On 2024-10-31 01:22, Ming Lei wrote:
> On Thu, Oct 31, 2024 at 08:14:49AM +0800, Ming Lei wrote:
>> On Wed, Oct 30, 2024 at 06:56:48PM +0100, Klara Modin wrote:
>>> Hi,
>>>
>>> On 2024-10-24 07:00, Christoph Hellwig wrote:
>>>> From: Ming Lei <ming.lei@redhat.com>
>>>>
>>>> The iov_iter_extract_pages interface allows to return physically
>>>> discontiguous pages, as long as all but the first and last page
>>>> in the array are page aligned and page size.  Rewrite
>>>> iov_iter_extract_bvec_pages to take advantage of that instead of only
>>>> returning ranges of physically contiguous pages.
>>>>
>>>> Signed-off-by: Ming Lei <ming.lei@redhat.com>
>>>> [hch: minor cleanups, new commit log]
>>>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>>>
>>> With this patch (e4e535bff2bc82bb49a633775f9834beeaa527db in next-20241030),
>>> I'm unable to connect via nvme-tcp with this in the log:
>>>
>>> nvme nvme1: failed to send request -5
>>> nvme nvme1: Connect command failed: host path error
>>> nvme nvme1: failed to connect queue: 0 ret=880
>>>
>>> With the patch reverted it works as expected:
>>>
>>> nvme nvme1: creating 24 I/O queues.
>>> nvme nvme1: mapped 24/0/0 default/read/poll queues.
>>> nvme nvme1: new ctrl: NQN
>>> "nqn.2018-06.eu.kasm.int:freenas:backup:parmesan.int.kasm.eu", addr
>>> [2001:0678:0a5c:1204:6245:cbff:fe9c:4f59]:4420, hostnqn:
>>> nqn.2018-06.eu.kasm.int:parmesan
>>
>> I can't reproduce it by running blktest 'nvme_trtype=tcp ./check nvme/'
>> on both next tree & for-6.13/block.
>>
>> Can you collect the following bpftrace log by running the script before
>> connecting to nvme-tcp?

I didn't seem to get any output from the bpftrace script (I confirmed 
that I had the config as you requested, but I'm not very familiar with 
bpftrace so I could have done something wrong). I could, however, 
reproduce the issue in qemu and added breakpoints on 
nvmf_connect_io_queue and iov_iter_extract_pages. The breakpoint on 
iov_iter_extract_pages got hit once when running nvme connect:

(gdb) break nvmf_connect_io_queue
Breakpoint 1 at 0xffffffff81a5d960: file 
/home/klara/git/linux/drivers/nvme/host/fabrics.c, line 525.
(gdb) break iov_iter_extract_pages
Breakpoint 2 at 0xffffffff817633b0: file 
/home/klara/git/linux/lib/iov_iter.c, line 1900.
(gdb) c
Continuing.
[Switching to Thread 1.1]

Thread 1 hit Breakpoint 2, iov_iter_extract_pages 
(i=i@entry=0xffffc900001ebd68,
     pages=pages@entry=0xffffc900001ebb08, maxsize=maxsize@entry=72, 
maxpages=8,
     extraction_flags=extraction_flags@entry=0, 
offset0=offset0@entry=0xffffc900001ebb10)
     at /home/klara/git/linux/lib/iov_iter.c:1900
1900	{
(gdb) print i->count
$5 = 72
(gdb) print i->iov_offset
$6 = 0
(gdb) print i->bvec->bv_offset
$7 = 3952
(gdb) print i->bvec->bv_len
$8 = 72
(gdb) c
Continuing.

I didn't hit the breakpoint in nvmf_connect_io_queue, but I instead hit 
it if I add it to nvmf_connect_admin_queue. I added this function to the 
bpftrace script but that didn't produce any output either.

> 
> And please try the following patch:
> 
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 9fc06f5fb748..c761f6db3cb4 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1699,6 +1699,7 @@ static ssize_t iov_iter_extract_bvec_pages(struct iov_iter *i,
>                  i->bvec++;
>                  skip = 0;
>          }
> +       bi.bi_idx = 0;
>          bi.bi_size = maxsize + skip;
>          bi.bi_bvec_done = skip;
> 
> 

Applying this seems to fix the problem.

Thanks,
Klara Modin

> 
> Thanks,
> Ming
>