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

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Date:	Fri, 30 Apr 2010 01:10:16 +0900
From:	"J. R. Okajima" <hooanon05@...oo.co.jp>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 16/35] union-mount: Writable overlays/union mounts documentation


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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