linux-kernel - Re: [RFC PATCH 0/5] Union Mount: A Directory listing approach with lseek support

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Date:	Fri, 07 Dec 2007 10:48:07 +0900
From:	sfjro@...rs.sourceforge.net
To:	bharata@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Jan Blunck <jblunck@...e.de>, Erez Zadok <ezk@...sunysb.edu>,
	viro@...iv.linux.org.uk, Christoph Hellwig <hch@....de>,
	Dave Hansen <haveblue@...ibm.com>
Subject: Re: [RFC PATCH 0/5] Union Mount: A Directory listing approach with lseek support 


Bharata B Rao:
> - The cache can grow arbitrarily large in size for big directories thereby
> consuming lots of memory. Pruning individual cache entries is out of question
> as entire cache is needed for subsequent readdirs for duplicate elimination.

Additionally, the memory usage may be a problem too since your
implementation calls kmalloc() for every names.


> - Whenever _any_ directory that is part of the union gets
> modified (addition/deletion of entries), the dirent cache of all the unions
> which this directory is part of, needs to be purged and rebuilt. This is
> expensive not only due to re-reads of dirents but also because
> readdir(2)/getdents(2) needs to be synchronized with other operations
> like mkdir/mknod/link/unlink etc.

The cache in struct file doesn't need to be refreshed unless rewinddir()
is issued. Also you can maintain the cache in every add/del entries,
instead of discarding the cache entirely.


> After all this, I am beginning to think if it would be better to delegate
> this readdir and whiteout processing to userspace. Can this be better handled

Yes, I had such idea once. And copy-up too. They can be done in
userspace (while you need to be careful about the privilege).

Anyway I agree with you. As I wrote before, this approach consumes a lot
of memory and cpu (for comparing whiteouted names).


Junjiro Okajima
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