Can I make a question ?
On Wed, 23 Nov 2011 14:34:16 +0100
Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> + /*
> + * When allocating a page cache page for writing, we
> + * want to get it from a zone that is within its dirty
> + * limit, such that no single zone holds more than its
> + * proportional share of globally allowed dirty pages.
> + * The dirty limits take into account the zone's
> + * lowmem reserves and high watermark so that kswapd
> + * should be able to balance it without having to
> + * write pages from its LRU list.
> + *
> + * This may look like it could increase pressure on
> + * lower zones by failing allocations in higher zones
> + * before they are full. But the pages that do spill
> + * over are limited as the lower zones are protected
> + * by this very same mechanism. It should not become
> + * a practical burden to them.
> + *
> + * XXX: For now, allow allocations to potentially
> + * exceed the per-zone dirty limit in the slowpath
> + * (ALLOC_WMARK_LOW unset) before going into reclaim,
> + * which is important when on a NUMA setup the allowed
> + * zones are together not big enough to reach the
> + * global limit. The proper fix for these situations
> + * will require awareness of zones in the
> + * dirty-throttling and the flusher threads.
> + */
> + if ((alloc_flags & ALLOC_WMARK_LOW) &&
> + (gfp_mask & __GFP_WRITE) && !zone_dirty_ok(zone))
> + goto this_zone_full;
>
> BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
> if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
This wil call
if (NUMA_BUILD)
zlc_mark_zone_full(zonelist, z);
And this zone will be marked as full.
IIUC, zlc_clear_zones_full() is called only when direct reclaim ends.
So, if no one calls direct-reclaim, 'full' mark may never be cleared
even when number of dirty pages goes down to safe level ?
I'm sorry if this is alread discussed.
Thanks,
-Kame
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