Re: [PATCH 16/35] union-mount: Writable overlays/union mounts documentation

[J. R. Okajima]

Valerie Aurora:
> One of the major difficulties that arises even when doing unioning at
> the VFS level is keeping around the parent's path in order to do the
> copyup later on.  Take a look at the code pattern in the "union-mount:
> Implement union-aware syscall()" series of patches.  That's the
> prettiest and most efficient version I could come up with, after two
> other implementations, and it's in the VFS, at the vfs_foo_syscall()
> level.  I don't even know how I would start if I had to wait until the
> file system op is called.

I agree that is prettiest, and copup at open for write makes it easier.
But some applications issue mmap(MAP_PRIVATE) after open(O_RDWR), for
example modprobe(8). In this case, every kernel module will be copied-up
and it must be a waste of time and space. And I guess this is one reason
why other implementation took the approach of copyup at write.
At the same time, I guess this issue may be less important since other
parts are pretty enough.


J. R. Okajima
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