Re: [LSF/MM/BPF TOPIC] block drivers in user space

[Gabriel Krisman Bertazi <krisman@collabora.com>]

Ming Lei <ming.lei@redhat.com> writes:

> IMO it needn't 'inverse io_uring', the normal io_uring SQE/CQE model
> does cover this case, the userspace part can submit SQEs beforehand
> for getting notification of each incoming io request from kernel driver,
> then after one io request is queued to the driver, the driver can
> queue a CQE for the previous submitted SQE. Recent posted patch of
> IORING_OP_URING_CMD[1] is perfect for such purpose.
>
> I have written one such userspace block driver recently, and [2] is the
> kernel part blk-mq driver(ubd driver), the userspace part is ubdsrv[3].
> Both the two parts look quite simple, but still in very early stage, so
> far only ubd-loop and ubd-null targets are implemented in [3]. Not only
> the io command communication channel is done via IORING_OP_URING_CMD, but
> also IO handling for ubd-loop is implemented via plain io_uring too.
>
> It is basically working, for ubd-loop, not see regression in 'xfstests -g auto'
> on the ubd block device compared with same xfstests on underlying disk, and
> my simple performance test on VM shows the result isn't worse than kernel loop
> driver with dio, or even much better on some test situations.

Thanks for sharing.  This is a very interesting implementation that
seems to cover quite well the original use case.  I'm giving it a try and
will report back.

> Wrt. this userspace block driver things, I am more interested in the following
> sub-topics:
>
> 1) zero copy
> - the ubd driver[2] needs one data copy: for WRITE request, copy pages
>   in io request to userspace buffer before handling the WRITE IO by ubdsrv;
>   for READ request, the reverse copy is done after READ request is
>   handled by ubdsrv
>
> - I tried to apply zero copy via remap_pfn_range() for avoiding this
>   data copy, but looks it can't work for ubd driver, since pages in the
>   remapped vm area can't be retrieved by get_user_pages_*() which is called in
>   direct io code path
>
> - recently Xiaoguang Wang posted one RFC patch[4] for support zero copy on
>   tcmu, and vm_insert_page(s)_mkspecial() is added for such purpose, but
>   it has same limit of remap_pfn_range; Also Xiaoguang mentioned that
>   vm_insert_pages may work, but anonymous pages can not be remapped by
>   vm_insert_pages.
>
> - here the requirement is to remap either anonymous pages or page cache
>   pages into userspace vm, and the mapping/unmapping can be done for
>   each IO runtime. Is this requirement reasonable? If yes, is there any
>   easy way to implement it in kernel?

I've run into the same issue with my fd implementation and haven't been
able to workaround it.

> 4) apply eBPF in userspace block driver
> - it is one open topic, still not have specific or exact idea yet,
>
> - is there chance to apply ebpf for mapping ubd io into its target handling
> for avoiding data copy and remapping cost for zero copy?

I was thinking of something like this, or having a way for the server to
only operate on the fds and do splice/sendfile.  But, I don't know if it
would be useful for many use cases.  We also want to be able to send the
data to userspace, for instance, for userspace networking.

-- 
Gabriel Krisman Bertazi