Re: [PATCH v2 0/4] virtiofs,fuse: support per-file DAX

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jul 21, 2021 at 08:48:57AM -0400, Vivek Goyal wrote:
[..]
> > > So is "dax=inode" enough for your needs? What's your requirement,
> > > can you give little bit of more details.
> > 
> > In our use case, the backend fs is something like SquashFS on host. The
> > content of the file on host is downloaded *as needed*. When the file is
> > not completely ready (completely downloaded), the guest will follow the
> > normal IO routine, i.e., by FUSE_READ/FUSE_WRITE request. While the file
> > is completely ready, per-file DAX is enabled for this file. IOW the FUSE
> > server need to dynamically decide if per-file DAX shall be enabled,
> > depending on if the file is completely downloaded.
> 
> So you don't want to enable DAX yet because guest might fault on
> a section of file which has not been downloaded yet?
> 
> I am wondering if somehow user fault handling can help with this.
> If we could handle faults for this file in user space, then you
> should be able to download that particular page[s] and resolve
> the fault?

Stefan mentioned that can't we block when fuse mmap request comes
in and download corresponding section of file. Or do whatever you
are doing in FUSE_READ. 

IOW, even if you enable dax in your use case on all files,
FUSE_SETUPMAPPING request will give you control to make sure 
file section being mmaped has been downloaded.

Vivek