linux-kernel - Re: [UNIONFS] 00/42 Unionfs and related patches review

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Date:	Mon, 10 Dec 2007 12:20:20 +0900
From:	hooanon05@...oo.co.jp
To:	Erez Zadok <ezk@...sunysb.edu>
Cc:	hch@...radead.org, viro@....linux.org.uk,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [UNIONFS] 00/42 Unionfs and related patches review 

Erez Zadok:
> (1) Cache coherency: by far, the biggest concern had been around cache
	:::
> unionfs.  The solution we have implemented is to compare the mtime/ctime of
> upper/lower objects during revalidation (esp. of dentries); and if the lower
> times are newer, we reconstruct the union object (drop the older objects,
> and re-lookup them).  This time-based cache-coherency works well and is
	:::

The resolution of mtime/ctime may be too low since some filesystems sets
them in unit of a second, which means you cannot detect the changes made
within a second.
I think it is better to use inotify for every directory while it
consumes a little more resources.
Additionally, if you implement vm_operations instead of
struggling along address_space_operations or VFS patches, in order to
share the mmap-ed memory pages between lower inode and unionfs inode,
then most of issues will be gone.
You can see this approach and how it is working in http://aufs.sf.net
(and get the source file from CVS).

But I am afraid the approach sharing memory pages will not be avaiable
for ecryptfs.

Junjiro Okajima
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