Re: [patch v2] vmscan: protect zone rotation stats by lru lock

From: Rik van Riel
Date: Mon Dec 01 2008 - 16:47:26 EST

Andrew Morton wrote:
On Mon, 01 Dec 2008 03:00:35 +0100
Johannes Weiner <hannes@xxxxxxxxxxxx> wrote:

The zone's rotation statistics must not be accessed without the
corresponding LRU lock held. Fix an unprotected write in

I don't think it really matters. It's quite common in that code to do
unlocked, racy update to statistics such as this. Because on those
rare occasions where a race does happen, there's a small glitch in the
reclaim logic which nobody will notice anyway.

Of course, this does need to be done with some care, to ensure the
glitch _will_ be small.

Processing at most SWAP_CLUSTER_MAX pages at once probably
ensures that glitches will be small most of the time.

The only way this could be a big problem is if we end up
racing with the divide-by-two logic in get_scan_ratio,
leaving the rotated pages a factor two higher than they
should be.

Putting all the writes to the stats under the LRU lock
should ensure that never happens.

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