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

Josef Bacik
2024-07-04T15:10:49Z

On Wed, Jul 03, 2024 at 10:44:28PM +0200, Bernd Schubert wrote:
>
>
> 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.
>

Your original alternative I think is better, leave it in fuse_in_arg and take it
out of the write arg. Thanks,

Josef