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

Joanne Koong
2024-07-03T20:28:25Z

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).

Thanks,
Joanne

>
> Thanks,
> Bernd
>