[PATCH RFC 1/2] mm, page_alloc, thp: prevent reclaim for __GFP_THISNODE THP allocations

[Vlastimil Babka <vbabka@suse.cz>]

Since commit cc638f329ef6 ("mm, thp: tweak reclaim/compaction effort of
local-only and all-node allocations"), THP page fault allocations have
settled on the following scheme (from the commit log):

1. local node only THP allocation with no reclaim, just compaction.
2. for madvised VMA's or when synchronous compaction is enabled always - THP
   allocation from any node with effort determined by global defrag setting
   and VMA madvise
3. fallback to base pages on any node

Recent customer reports however revealed we have a gap in step 1 above.
What we have seen is excessive reclaim due to THP page faults on a NUMA
node that's close to its high watermark, while other nodes have plenty
of free memory.

The problem with step 1 is that it promises no reclaim after the
compaction attempt, however reclaim is only avoided for certain
compaction outcomes (deferred, or skipped due to insufficient free base
pages), and not e.g. when compaction is actually performed but fails (we
did see compact_fail vmstat counter increasing).

THP page faults can therefore exhibit a zone_reclaim_mode-like behavior,
which is not the intention.

Thus add a check for __GFP_THISNODE that corresponds to this exact
situation and prevents continuing with reclaim/compaction once the
initial compaction attempt isn't successful in allocating the page.

Note that commit cc638f329ef6 has not introduced this over-reclaim
possibility; it appears to exist in some form since commit 2f0799a0ffc0
("mm, thp: restore node-local hugepage allocations"). Followup commits
b39d0ee2632d ("mm, page_alloc: avoid expensive reclaim when compaction
may not succeed") and cc638f329ef6 have moved in the right direction,
but left the abovementioned gap.

Fixes: 2f0799a0ffc0 ("mm, thp: restore node-local hugepage allocations")
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/page_alloc.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 822e05f1a964..e6fd1213328b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4788,6 +4788,20 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 			    compact_result == COMPACT_DEFERRED)
 				goto nopage;
 
+			/*
+			 * THP page faults may attempt local node only first,
+			 * but are then allowed to only compact, not reclaim,
+			 * see alloc_pages_mpol()
+			 *
+			 * compaction can fail for other reasons than those
+			 * checked above and we don't want such THP allocations
+			 * to put reclaim pressure on a single node in a
+			 * situation where other nodes might have plenty of
+			 * available memory
+			 */
+			if (gfp_mask & __GFP_THISNODE)
+				goto nopage;
+
 			/*
 			 * Looks like reclaim/compaction is worth trying, but
 			 * sync compaction could be very expensive, so keep

-- 
2.52.0