Re: [PATCH v14 00/12] FUSE passthrough for file io

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On Wed, 29 Nov 2023 at 16:52, Amir Goldstein <amir73il@xxxxxxxxx> wrote:

> direct I/O read()/write() is never a problem.
>
> The question is whether mmap() on a file opened with FOPEN_DIRECT_IO
> when the inode is in passthrough mode, also uses fuse_passthrough_mmap()?

I think it should.

> or denied, similar to how mmap with ff->open_flags & FOPEN_DIRECT_IO &&
> vma->vm_flags & VM_MAYSHARE) && !fc->direct_io_relax
> is denied?

What would be the use case for FOPEN_DIRECT_IO with passthrough mmap?

> A bit more challenging, because we will need to track unmounts, or at
> least track
> "was_cached_mmaped" state per file, but doable.

Tracking unmaps via fuse_vma_close() should not be difficult.


>
> > >
> > > Therefore, my proposal is than when filesystem is FUSE_PASSTHROUGH capable,
> > > only passthrough file and cached file may be mmaped, but never allow to
> > > mmap a FOPEN_DIRECT_IO file.
> > >
> > > Does that make sense?
> >
> > I'm not sure I understand how this is supposed to work.   Disallowing
> > mmap will break applications.
>
> How is this different from existing -ENODEV in fuse_file_mmap() for
> !fc->direct_io_relax? What am I missing?
> Is it because VM_MAYSHARE is far less common for applications?

Yes, MAP_SHARE is far less common than MAP_PRIVATE.

Thanks,
Miklos




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