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).
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).
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.
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.
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.
Frankly, I think something like support for a copied-up file, page-by-page,
should probably be supported by a block layer virtual driver (this might be
easier in a BSD-like geom layer.)
BTW, I believe FSCache has page-wise caching, right? Caching is a
copy-on-read operation, and it doesn't take much to make it cache (read:
copy) on writes. So FScache might be a good starting point for such an
effort.
I think a union-tmpfs is a better solution than a cramfs-specific one, b/c
at least with union-tmpfs, many more users could use it. Even if you
restrict yourself to using tmpfs as the r-w layer, and read-only from just
one other source f/s, that still will cover a large portion of unioning
users.
Cheers,
Erez.
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