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