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

[Lee Schermerhorn]

On Tue, 2008-12-02 at 13:34 +0100, Johannes Weiner wrote:
> On Mon, Dec 01, 2008 at 05:09:45PM -0500, Lee Schermerhorn wrote:
> > 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?
> 
> I did it like this to keep the diff as simple as possible and to not
> change existing code flow.
> 
> Here is an alternate version that moves the safe stuff out of the
> locked region.
> 
> tbh, I think it's worse.

As I said, I didn't think it was a big deal.  I'm fine with the prior
version.

Lee
> 
> 	Hannes
> 
> ---
> 
> The zone's rotation statistics must not be modified without the
> corresponding LRU lock held.  Fix an unprotected write in
> shrink_active_list().
> 
> ---
>  mm/vmscan.c |   16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1249,21 +1249,21 @@ static void shrink_active_list(unsigned 
>  	}
>  
>  	/*
> +	 * Move the pages to the [file or anon] inactive list.
> +	 */
> +
> +	pagevec_init(&pvec, 1);
> +	lru = LRU_BASE + file * LRU_FILE;
> +
> +	spin_lock_irq(&zone->lru_lock);
> +	/*
>  	 * Count referenced pages from currently used mappings as
>  	 * rotated, even though they are moved to the inactive list.
>  	 * This helps balance scan pressure between file and anonymous
>  	 * pages in get_scan_ratio.
>  	 */
>  	zone->recent_rotated[!!file] += pgmoved;
> -
> -	/*
> -	 * Move the pages to the [file or anon] inactive list.
> -	 */
> -	pagevec_init(&pvec, 1);
> -
>  	pgmoved = 0;
> -	lru = LRU_BASE + file * LRU_FILE;
> -	spin_lock_irq(&zone->lru_lock);
>  	while (!list_empty(&l_inactive)) {
>  		page = lru_to_page(&l_inactive);
>  		prefetchw_prev_lru_page(page, &l_inactive, flags);

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