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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