Re: [PATCH] fuse: Allow to align reads/writes

Bernd Schubert
2024-07-03T20:44:32Z

On 7/3/24 22:28, Joanne Koong wrote:
> On Wed, Jul 3, 2024 at 11:08 AM Bernd Schubert <bschubert@ddn.com> wrote:
>>
>> On 7/3/24 19:49, Joanne Koong wrote:
>>> On Wed, Jul 3, 2024 at 10:30 AM Josef Bacik <josef@toxicpanda.com> wrote:
>>>>
>>>> On Wed, Jul 03, 2024 at 05:58:20PM +0200, Bernd Schubert wrote:
>>>>>
>>>>>
>>>>> On 7/3/24 17:15, Josef Bacik wrote:
>>>>>> On Tue, Jul 02, 2024 at 06:31:08PM +0200, Bernd Schubert wrote:
>>>>>>> Read/writes IOs should be page aligned as fuse server
>>>>>>> might need to copy data to another buffer otherwise in
>>>>>>> order to fulfill network or device storage requirements.
>>>>>>>
>>>>>>> Simple reproducer is with libfuse, example/passthrough*
>>>>>>> and opening a file with O_DIRECT - without this change
>>>>>>> writing to that file failed with -EINVAL if the underlying
>>>>>>> file system was using ext4 (for passthrough_hp the
>>>>>>> 'passthrough' feature has to be disabled).
>>>>>>>
>>>>>>> Given this needs server side changes as new feature flag is
>>>>>>> introduced.
>>>>>>>
>>>>>>> Disadvantage of aligned writes is that server side needs
>>>>>>> needs another splice syscall (when splice is used) to seek
>>>>>>> over the unaligned area - i.e. syscall and memory copy overhead.
>>>>>>>
>>>>>>> Signed-off-by: Bernd Schubert <bschubert@ddn.com>
>>>>>>>
>>>>>>> ---
>>>>>>> From implementation point of view 'struct fuse_in_arg' /
>>>>>>> 'struct fuse_arg' gets another parameter 'align_size', which has to
>>>>>>> be set by fuse_write_args_fill. For all other fuse operations this
>>>>>>> parameter has to be 0, which is guranteed by the existing
>>>>>>> initialization via FUSE_ARGS and C99 style
>>>>>>> initialization { .size = 0, .value = NULL }, i.e. other members are
>>>>>>> zero.
>>>>>>> Another choice would have been to extend fuse_write_in to
>>>>>>> PAGE_SIZE - sizeof(fuse_in_header), but then would be an
>>>>>>> arch/PAGE_SIZE depending struct size and would also require
>>>>>>> lots of stack usage.
>>>>>>
>>>>>> Can I see the libfuse side of this? I'm confused why we need the align_size at
>>>>>> all? Is it enough to just say that this connection is aligned, negotiate what
>>>>>> the alignment is up front, and then avoid sending it along on every write?
>>>>>
>>>>> Sure, I had forgotten to post it
>>>>> https://github.com/bsbernd/libfuse/commit/89049d066efade047a72bcd1af8ad68061b11e7c
>>>>>
>>>>> We could also just act on fc->align_writes / FUSE_ALIGN_WRITES and always use
>>>>> sizeof(struct fuse_in_header) + sizeof(struct fuse_write_in) in libfuse and would
>>>>> avoid to send it inside of fuse_write_in. We still need to add it to struct fuse_in_arg,
>>>>> unless you want to check the request type within fuse_copy_args().
>>>>
>>>> I think I like this approach better, at the very least it allows us to use the
>>>> padding for other silly things in the future.
>>>>
>>>
>>> This approach seems cleaner to me as well.
>>> I also like the idea of having callers pass in whether alignment
>>> should be done or not to fuse_copy_args() instead of adding
>>> "align_writes" to struct fuse_in_arg.
>>
>> There is no caller for FUSE_WRITE for fuse_copy_args(), but it is called
>> from fuse_dev_do_read for all request types. I'm going to add in request
>> parsing within fuse_copy_args, I can't decide myself which of both
>> versions I like less.
>
> Sorry I should have clarified better :) By callers, I meant callers to
> fuse_copy_args(). I'm still getting up to speed with the fuse code but
> it looks like it gets called by both fuse_dev_do_read and
> fuse_dev_do_write (through copy_out_args() -> fuse_copy_args()). The
> cleanest solution to me seems like to pass in from those callers
> whether the request should be page-aligned after the headers or not,
> instead of doing the request parsing within fuse_copy_args() itself. I
> think if we do the request parsing within fuse_copy_args() then we
> would also need to have some way to differentiate between FUSE_WRITE
> requests from the dev_do_read vs dev_do_write side (since, as I
> understand it, writes only needs to be aligned for dev_do_read write
> requests).

fuse_dev_do_write() is used to submit results from fuse server
(userspace), i.e. not interesting here. If we don't parse in
fuse_copy_args(), we would have to do that in fuse_dev_do_read() - it
doesn't have knowledge about the request it handles either - it just
takes from lists what is there. So if we don't want to have it encoded
in fuse_in_arg, there has to request type checking. Given the existing
number of conditions in fuse_dev_do_read, I would like to avoid adding
in even more there.

Thanks,
Bernd