linux-kernel - Re: [PATCH 2/2] [RFC] fadvise: Add _VOLATILE,_ISVOLATILE, and

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Date:	Tue, 14 Feb 2012 16:29:10 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 2/2] [RFC] fadvise: Add _VOLATILE,_ISVOLATILE, and
 _NONVOLATILE flags

On Wed, 2012-02-15 at 10:51 +1100, Dave Chinner wrote:
> Given that it is a single state bit per page (volatile/non volatile)
> you could just use a radix tree tag for keeping the state. Changing
> the state isn't a performance critical operation, and tagging large
> ranges isn't that expensive (e.g. we do that in the writeback code),
> so I'm not sure the overhead of a separate tree is necessary here....

Hrm. I'll look into this.

> That doesn't help with the reclaim side of things, but I would have
> thought that such functioanlity would be better integrated into the
> VM page cache/lru scanning code than adding a shrinker to shrink the
> page cache additionally on top of what the VM has already done
> before calling the shrinkers. I'm not sure what is best here,
> though...

Yea. My previous version did eviction from shmem_writepage(), I believe
much as you suggest here, but the concern with that is that you could
have a larger volatile range that was for the most part recently in use,
but one idle page causes the entire thing to be evicted first. Using
least-recently-marked-volatile order seems more natural for the use case
(although Dimitry and others have already pointed out that the
inheritance from the coalescing of neighboring ranges results in a
similar issue).

But I'm open to other ideas and arguments.

Thanks again for the feedback!
-john

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