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

[Lee Schermerhorn]

On Mon, 2008-12-01 at 16:46 -0500, Rik van Riel wrote:
> 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
> >> shrink_active_list().
> >>
> > 
> > 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.

And he's not actually adding a lock.  Just moving the exiting one up to
include the stats update.  The intervening pagevec, pgmoved and lru
initializations don't need to be under the lock, but that's probably not
a big deal?

Lee

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