mm, page_alloc: skip ->waternark_boost for atomic order-0 allocations

When boosting is enabled, it is observed that rate of atomic order-0
allocation failures are high due to the fact that free levels in the
system are checked with ->watermark_boost offset.  This is not a problem
for sleepable allocations but for atomic allocations which looks like
regression.

This problem is seen frequently on system setup of Android kernel running
on Snapdragon hardware with 4GB RAM size.  When no extfrag event occurred
in the system, ->watermark_boost factor is zero, thus the watermark
configurations in the system are:

   _watermark = (
          [WMARK_MIN] = 1272, --> ~5MB
          [WMARK_LOW] = 9067, --> ~36MB
          [WMARK_HIGH] = 9385), --> ~38MB
   watermark_boost = 0

After launching some memory hungry applications in Android which can cause
extfrag events in the system to an extent that ->watermark_boost can be
set to max i.e.  default boost factor makes it to 150% of high watermark.

   _watermark = (
          [WMARK_MIN] = 1272, --> ~5MB
          [WMARK_LOW] = 9067, --> ~36MB
          [WMARK_HIGH] = 9385), --> ~38MB
   watermark_boost = 14077, -->~57MB

With default system configuration, for an atomic order-0 allocation to
succeed, having free memory of ~2MB will suffice.  But boosting makes the
min_wmark to ~61MB thus for an atomic order-0 allocation to be successful
system should have minimum of ~23MB of free memory(from calculations of
zone_watermark_ok(), min = 3/4(min/2)).  But failures are observed despite
system is having ~20MB of free memory.  In the testing, this is
reproducible as early as first 300secs since boot and with furtherlowram
configurations(<2GB) it is observed as early as first 150secs since boot.

These failures can be avoided by excluding the ->watermark_boost in
watermark caluculations for atomic order-0 allocations.

[[email protected]: fix comment grammar, reflow comment]
[[email protected]: fix suggested by Mel Gorman]
  Link: http://lkml.kernel.org/r/[email protected]

Signed-off-by: Charan Teja Reddy <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Vinayak Menon <[email protected]>
Cc: Mel Gorman <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/page_alloc.c

@@ -3588,7 +3588,7 @@ bool zone_watermark_ok(struct zone *z, unsigned int order, unsigned long mark,
 
 static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
 				unsigned long mark, int highest_zoneidx,
-				unsigned int alloc_flags)
+				unsigned int alloc_flags, gfp_t gfp_mask)
 {
 	long free_pages;
 
@@ -3607,8 +3607,23 @@ static inline bool zone_watermark_fast(struct zone *z, unsigned int order,
 			return true;
 	}
 
-	return __zone_watermark_ok(z, order, mark, highest_zoneidx, alloc_flags,
-					free_pages);
+	if (__zone_watermark_ok(z, order, mark, highest_zoneidx, alloc_flags,
+					free_pages))
+		return true;
+	/*
+	 * Ignore watermark boosting for GFP_ATOMIC order-0 allocations
+	 * when checking the min watermark. The min watermark is the
+	 * point where boosting is ignored so that kswapd is woken up
+	 * when below the low watermark.
+	 */
+	if (unlikely(!order && (gfp_mask & __GFP_ATOMIC) && z->watermark_boost
+		&& ((alloc_flags & ALLOC_WMARK_MASK) == WMARK_MIN))) {
+		mark = z->_watermark[WMARK_MIN];
+		return __zone_watermark_ok(z, order, mark, highest_zoneidx,
+					alloc_flags, free_pages);
+	}
+
+	return false;
 }
 
 bool zone_watermark_ok_safe(struct zone *z, unsigned int order,
@@ -3752,7 +3767,8 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
 
 		mark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);
 		if (!zone_watermark_fast(zone, order, mark,
-				       ac->highest_zoneidx, alloc_flags)) {
+				       ac->highest_zoneidx, alloc_flags,
+				       gfp_mask)) {
 			int ret;
 
 #ifdef CONFIG_DEFERRED_STRUCT_PAGE_INIT