From: Mike Hommey Subject: Re: [RFC/PATCH 0/2] ext4: Transparent Decompression Support Date: Tue, 30 Jul 2013 08:15:39 +0900 Message-ID: <20130729231539.GA28383@glandium.org> References: <1374699833.7083.2.camel@localhost> <20130724233628.GD3641@logfs.org> <51F14136.30409@mozilla.com> <51F1556A.20909@mozilla.com> <51FB2C87-854F-4250-9587-B5BBF4E85EE8@dubeyko.com> <51F17005.7030309@mozilla.com> <1374825683.3671.35.camel@slavad-ubuntu> <20130726132034.GB21977@logfs.org> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Vyacheslav Dubeyko , Dhaval Giani , Taras Glek , linux-kernel@vger.kernel.org, tytso@mit.edu, vdjeric@mozilla.com, glandium@mozilla.com, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org To: =?iso-8859-15?Q?J=F6rn?= Engel Return-path: Content-Disposition: inline In-Reply-To: <20130726132034.GB21977@logfs.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Fri, Jul 26, 2013 at 09:20:34AM -0400, J=F6rn Engel wrote: > On Fri, 26 July 2013 12:01:23 +0400, Vyacheslav Dubeyko wrote: > >=20 > > 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 patc= h > > 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= =2E >=20 > I don't think the e2compr patches are strictly necessary. They are a > good option, but not the only one. >=20 > One trick to simplify the problem is to make Dhaval's compressed file= s > strictly read-only. It will require some dance to load the compresse= d > content, flip the switch, then uncompress data on the fly and disallo= w > writes. Not the most pleasing of interfaces, but yet another option. >=20 > > 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. >=20 > The kernel controls the page cache. Once the page cache is filled > with uncompressed file content, you can do mmap, regular file io, etc= =2E > Putting uncompression code into the kernel makes sense to me. Whethe= r > a solution different from e2compr makes sense is yet to be seen. >=20 > 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 supporte= d > 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 i= s 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