Re: [RFC/PATCH 0/2] ext4: Transparent Decompression Support

[Mike Hommey]

On Fri, Jul 26, 2013 at 09:20:34AM -0400, Jörn Engel wrote:
> On Fri, 26 July 2013 12:01:23 +0400, Vyacheslav Dubeyko wrote:
> > 
> > We are discussing not about good or bad idea. We need to elaborate a
> > right solution. I think that suggested idea is not clear. Do you
> > want to support compression in ext4? Or do you want to add some new
> > compression feature (likewise file-oriented compression)? If we are
> > talking about compression in ext4 then it needs to use e2compr patch
> > set. Otherwise, if we are talking about file compression then it is
> > not question of concrete filesystem. And we need to make
> > implementation on VFS level. It is only architectural point of view.
> 
> I don't think the e2compr patches are strictly necessary.  They are a
> good option, but not the only one.
> 
> One trick to simplify the problem is to make Dhaval's compressed files
> strictly read-only.  It will require some dance to load the compressed
> content, flip the switch, then uncompress data on the fly and disallow
> writes.  Not the most pleasing of interfaces, but yet another option.
> 
> > Why do you try to implement likewise concept on kernel level? It
> > looks like you try to move some user-space concept in kernel-space.
> 
> The kernel controls the page cache.  Once the page cache is filled
> with uncompressed file content, you can do mmap, regular file io, etc.
> Putting uncompression code into the kernel makes sense to me.  Whether
> a solution different from e2compr makes sense is yet to be seen.
> 
> Whatever you do, it will require support from the on-disk format and
> the userspace ABI.  Setting the compression bit on a file has the
> clear advantage that it is an established interface and also supported
> by other filesystems.  Introducing yet another interface requires a
> fairly strong case to be made.  But who knows, maybe Dhaval can pull
> it off.

Come to think of it, the whole thing could be handled entirely in user
space through fuse. While this is probably a workable solution on
desktop/server environments, it doesn't pan out on Android: /dev/fuse is
rarely available, and even if it were, fusermount needs to be there and
be a setuid program (or have the right capabilities). So, another angle
could be to allow some things to happen without privileges, such as
mounting filesystems in a private namespace. That wouldn't solve the
lack of /dev/fuse, though.

Mike
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