Re: [PATCH 10/10] mm, page_alloc: Only enforce watermarks for order-0 allocations

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Sep 29, 2015 at 02:05:07PM -0700, Andrew Morton wrote:
> >  static bool __zone_watermark_ok(struct zone *z, unsigned int order,
> >  			unsigned long mark, int classzone_idx, int alloc_flags,
> > @@ -2317,7 +2319,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
> >  {
> >  	long min = mark;
> >  	int o;
> > -	long free_cma = 0;
> > +	const bool alloc_harder = (alloc_flags & ALLOC_HARDER);
> 
> hmpf.  Setting a bool to 0x10 is a bit grubby.
>   

Should be safe, but I see your point. For any other type it would be
truncated and look like a bug.

> >  	/* free_pages may go negative - that's OK */
> >  	free_pages -= (1 << order) - 1;
> > @@ -2330,7 +2332,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
> >  	 * the high-atomic reserves. This will over-estimate the size of the
> >  	 * atomic reserve but it avoids a search.
> >  	 */
> > -	if (likely(!(alloc_flags & ALLOC_HARDER)))
> > +	if (likely(!alloc_harder))
> >  		free_pages -= z->nr_reserved_highatomic;
> >  	else
> >  		min -= min / 4;
> > @@ -2338,22 +2340,43 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
> >  #ifdef CONFIG_CMA
> >  	/* If allocation can't use CMA areas don't use free CMA pages */
> >  	if (!(alloc_flags & ALLOC_CMA))
> > -		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
> > +		free_pages -= zone_page_state(z, NR_FREE_CMA_PAGES);
> >  #endif
> >  
> > -	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
> > +	if (free_pages <= min + z->lowmem_reserve[classzone_idx])
> >  		return false;
> > -	for (o = 0; o < order; o++) {
> > -		/* At the next order, this order's pages become unavailable */
> > -		free_pages -= z->free_area[o].nr_free << o;
> >  
> > -		/* Require fewer higher order pages to be free */
> > -		min >>= 1;
> > +	/* order-0 watermarks are ok */
> 
> because?
> 

The wizard of oz because because!

This should fix it up better than clicking my shoes three times.

---8<---
From: Mel Gorman <mgorman@techsingularity.net>
Subject: [PATCH] mm, page_alloc: only enforce watermarks for order-0
 allocations -fix

This patch is updating comments for clarity and converts a bool to an
int. The code as-is is ok as the compiler is meant to cast it correctly
but it looks odd to people who know the value would be truncated and
lost for other types.

This is a fix to the mmotm patch
mm-page_alloc-only-enforce-watermarks-for-order-0-allocations.patch

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 mm/page_alloc.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 25731624d734..fedec98aafca 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2332,7 +2332,7 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
 {
 	long min = mark;
 	int o;
-	const bool alloc_harder = (alloc_flags & ALLOC_HARDER);
+	const int alloc_harder = (alloc_flags & ALLOC_HARDER);
 
 	/* free_pages may go negative - that's OK */
 	free_pages -= (1 << order) - 1;
@@ -2356,14 +2356,19 @@ static bool __zone_watermark_ok(struct zone *z, unsigned int order,
 		free_pages -= zone_page_state(z, NR_FREE_CMA_PAGES);
 #endif
 
+	/*
+	 * Check watermarks for an order-0 allocation request. If these
+	 * are not met, then a high-order request also cannot go ahead
+	 * even if a suitable page happened to be free.
+	 */
 	if (free_pages <= min + z->lowmem_reserve[classzone_idx])
 		return false;
 
-	/* order-0 watermarks are ok */
+	/* If this is an order-0 request then the watermark is fine */
 	if (!order)
 		return true;
 
-	/* Check at least one high-order page is free */
+	/* For a high-order request, check at least one suitable page is free */
 	for (o = order; o < MAX_ORDER; o++) {
 		struct free_area *area = &z->free_area[o];
 		int mt;

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