Re: [fuse-devel] mmap patch for FUSE? (fwd)

[Paul T. <twohey@CS.Stanford.EDU>]

> > However, we run into problems with mmap.  It turns out that FUSE doesn't
> > provide mmap.
>
> Memory mapping is pretty complex, and the kernel provides an
> implentation in the form of the page cache, so it makes no sense to
> export the mmap interface to the filesystem.  Most (all?) in-kernel
> filesystems also just use the page cache.
>
> > Do you guys have a mmap patch for FUSE on 2.6 or 2.4?  I'd really
> > appericate it.  I googled a bit and it seems that map on FUSE can
> > cause kernel mm system to deadlock.
> >
> That's a different problem.  The deadlock is only in case of writable
> shared mapping, which is _very_ rarely used (I haven't yet seen a real
> application using it only some testcases in LTP).  So all other kinds
> of mmap() are supported.

This strace of CVS seems to imply that fuse doesn't support mmap, even for 
private mappings. I was trying to run a cvs commit on a repository on the 
fusexmp file system. The cvs commit fails as a result of the failed mmap.

[pid  7777] chmod("/mnt/fuse/home/twohey/tmp/cvsroot/project/test-file,v", 0100444) = 0
[pid  7777] open("/mnt/fuse/home/twohey/tmp/cvsroot/project/test-file,v", O_RDONLY|O_LARGEFILE) = 4
[pid  7777] fstat64(4, {st_mode=S_IFREG|0444, st_size=79, ...}) = 0
[pid  7777] mmap2(NULL, 79, PROT_READ|PROT_WRITE, MAP_PRIVATE, 4, 0) = -1 ENODEV (No such device)

I'm mounting the file system as root with the following options:
./example/fusexmp /mnt/fuse -d -s -o allow_other,allow_root,direct_io,debug

As for wanting MAP_SHARED, berkeley db uses shared mappings. Thus,
applications like subversion with the berkeley db backend will want
support for MAP_SHARED.

Thanks for your help.

Paul Twohey
twohey@cs