[RFC] mm, thp: tweak reclaim/compaction effort of local-only and all-node allocations

[Vlastimil Babka]

THP page faults now attempt a __GFP_THISNODE allocation first, which should
only compact existing free memory, followed by another attempt that can
allocate from any node using reclaim/compaction effort specified by global
defrag setting and madvise.

This patch makes the following changes to the scheme:

- before the patch, the first allocation relies on a check for pageblock order
  and __GFP_IO. This however affects also the second attempt, and also hugetlb
  allocations and other allocations of whole pageblock. Instead of that, reuse
  the existing check for costly order __GFP_NORETRY allocations, and make sure
  the first THP attempt uses __GFP_NORETRY. As a side-effect, all costly order
  __GFP_NORETRY allocations will bail out if compaction needs reclaim, while
  previously they only bailed out when compaction was deferred due to previous
  failures. This should be still acceptable within the __GFP_NORETRY semantics.

- before the patch, the second allocation attempt (on all nodes) was passing
  __GFP_NORETRY. This is redundant as the check for pageblock order (discussed
  above) was stronger. It's also contrary to madvise(MADV_HUGEPAGE) which means
  some effort to allocate THP is requested. After this patch, the second
  attempt doesn't pass __GFP_THISNODE nor __GFP_NORETRY.

To sum up, THP page faults now try the following attempt:

1. local node only THP allocation with no reclaim, just compaction.
2. THP allocation from any node with effort determined by global defrag setting
   and VMA madvise
3. fallback to base pages on any node

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/mempolicy.c  | 16 +++++++++-------
 mm/page_alloc.c | 23 +++++------------------
 2 files changed, 14 insertions(+), 25 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 4ae967bcf954..2c48146f3ee2 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -2129,18 +2129,20 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
 		nmask = policy_nodemask(gfp, pol);
 		if (!nmask || node_isset(hpage_node, *nmask)) {
 			mpol_cond_put(pol);
+			/*
+			 * First, try to allocate THP only on local node, but
+			 * don't reclaim unnecessarily, just compact.
+			 */
 			page = __alloc_pages_node(hpage_node,
-						gfp | __GFP_THISNODE, order);
+				gfp | __GFP_THISNODE | __GFP_NORETRY, order);
 
 			/*
-			 * If hugepage allocations are configured to always
-			 * synchronous compact or the vma has been madvised
-			 * to prefer hugepage backing, retry allowing remote
-			 * memory as well.
+			 * If that fails, allow both compaction and reclaim,
+			 * but on all nodes.
 			 */
-			if (!page && (gfp & __GFP_DIRECT_RECLAIM))
+			if (!page)
 				page = __alloc_pages_node(hpage_node,
-						gfp | __GFP_NORETRY, order);
+								gfp, order);
 
 			goto out;
 		}
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 15c2050c629b..da9075d4cdf6 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4467,7 +4467,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		if (page)
 			goto got_pg;
 
-		 if (order >= pageblock_order && (gfp_mask & __GFP_IO)) {
+		/*
+		 * Checks for costly allocations with __GFP_NORETRY, which
+		 * includes some THP page fault allocations
+		 */
+		if (costly_order && (gfp_mask & __GFP_NORETRY)) {
 			/*
 			 * If allocating entire pageblock(s) and compaction
 			 * failed because all zones are below low watermarks
@@ -4487,23 +4491,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 			if (compact_result == COMPACT_SKIPPED ||
 			    compact_result == COMPACT_DEFERRED)
 				goto nopage;
-		}
-
-		/*
-		 * Checks for costly allocations with __GFP_NORETRY, which
-		 * includes THP page fault allocations
-		 */
-		if (costly_order && (gfp_mask & __GFP_NORETRY)) {
-			/*
-			 * If compaction is deferred for high-order allocations,
-			 * it is because sync compaction recently failed. If
-			 * this is the case and the caller requested a THP
-			 * allocation, we do not want to heavily disrupt the
-			 * system, so we fail the allocation instead of entering
-			 * direct reclaim.
-			 */
-			if (compact_result == COMPACT_DEFERRED)
-				goto nopage;
 
 			/*
 			 * Looks like reclaim/compaction is worth trying, but
-- 
2.23.0