linux-kernel - Re: [Q] missing unused dentry in prune

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Message-ID: <4541BDE2.6050703@sw.ru>
Date:	Fri, 27 Oct 2006 12:05:54 +0400
From:	Vasily Averin <vvs@...ru>
To:	David Howells <dhowells@...hat.com>
CC:	Neil Brown <neilb@...e.de>, Jan Blunck <jblunck@...e.de>,
	Olaf Hering <olh@...e.de>, Balbir Singh <balbir@...ibm.com>,
	Kirill Korotaev <dev@...nvz.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...nvz.org, Andrew Morton <akpm@...l.org>
Subject: Re: [Q] missing unused dentry in prune_dcache()?

David Howells wrote:
> Vasily Averin <vvs@...ru> wrote:
> 
>> Therefore I've removed break of cycle and insert this dentry to head of the
>> list. Theoretically it can lead to the second using of the same dentry,
>> however I do not think that it is a big problem.
> 
> Hmmm...  Or maybe it could be a problem.  If whenever we find the dentry we
> stick it back on the head of the list, this could be a problem as we're
> traversing the list from tail to head.

I still do not think that it could be a problem:
count argument of prune_dcache is 128 if prune_dcache is called from
shrink_dcache_memory() function and even lesser if prune_dcache is called from
shrink_dcache_parent() function. I think usually the size of unused_dentry list
(nr_usnused) is much greater and may be compared with these values only in case
of very hard memory shortage. From my point of view it is very rare situation
and I even not sure that it can really happen at all.

However even if this situation will happen I do not see here any seriously
troubles: Yes, we will try to free the same dentries, much probably without
success again, but why it is bad in case of hard memory shortage?

If my arguments are not convincing, I can protect second use of the same dentry:
we can compare counter of the skipped dentries with nr_unused value.

And what the alternatives we have?
1) We can move this dentry to end of list? But it is bad because it will prevent
shrink_dcache_memory().
2) we can move these dentries into some temporal list, and insert it back to
unused_list later? But it is bad because of we can be rescheduled and drop
dcache_lock and may confuse someone who will assume that these dentries are in
unused_list.

Therefore I believe that my patch is optimal solution.

Thank you,
	Vasily Averin
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