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@xxxxxxx> wrote: > >> > >> On 7/3/24 19:49, Joanne Koong wrote: > >>> On Wed, Jul 3, 2024 at 10:30 AM Josef Bacik <josef@xxxxxxxxxxxxxx> 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@xxxxxxx> > >>>>>>> > >>>>>>> --- > >>>>>>> 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