Re: [RFC 0/7] [RFC] cramfs: fake write support

[Jamie Lokier <jamie@shareable.org>]

Erez Zadok wrote:
> 
> > Jamie Lokier wrote:
> > > Phillip Lougher wrote:
> > > If I read the patches correctly, when a file page is written to, only 
> > > that page gets copied into the page cache and locked, the other pages 
> > > continue to be read off disk from cramfs?  With Unionfs a page write 
> > > causes the entire file to be copied up to the r/w tmpfs and locked into 
> > > the page cache causing unnecessary RAM overhead.
> 
> Yes, unionfs does copyup whole files, but it doesn't lock the entire file
> into the page cache.  But I agree, that copying up large files to a tmpfs
> partition adds more memory pressure, at least temporarily (until pdflush
> kicks in).

1: I'm thinking systems which have union-over-cramfs probably don't have
swap at all...

2: It's a problem when you modify a very large file, even on a fast PC
with plenty of RAM.  LVM snapshots might be better for this sort of
thing.

> > Ok, so why not fix that in unionfs?  An option so that holes in the
> > overlay file let through data from the underlying file sounds like it
> > would be generally useful, and quite easy to implement.
> 
> If I understand you right, you want to copyup one page at a time, right?
> That's not nearly as easy as one might imagine.  First, you can't do it on
> file systems which don't support holes.  Second, holes is a file-systems
> specific implementation issue, and the knowledge of holes AFAIC, is hidden
> from the VFS (IIRC, FreeBSD has a specific "zfod" page flag, which is turned
> on when the VM has a page that came out of a f/s hole).

True, although the new FIEMAP ioctl is supposed to make holes more
filesystem independent, when they are supported.

> You'll need a way to tell if a given page was copied up or not, and
> distinguish b/t pages which are naturally filled with zeros vs. those which
> came from f/s holes.

Metadata.  Don't you have other metadata anyway, like whiteouts? :-)

> Copyup is also providing persistency: you can copyup to a persistent f/s
> such as ext2.  So you'll need a bitmap or some sort of record that will
> survive file system remount and system reboot; such a bitmap will have to
> tell which pages of a file have been copied up or not.

Yes.

> I'm not saying it's not possible, but it's to do this page-wise caching at a
> stackable layer than inside a native f/s such as ext2.  Now, if there was a
> generic VFS op that allowed me to query a file system whether a page it a
> given file is a hole or not, then unionfs would be able to do page-wise
> copyup easily.

See FIEMAP.  Is it any use?

-- Jamie