[PATCH v4] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations

[Salvatore Dipietro]

Commit 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
introduced high-order folio allocations in the iomap buffered write
path.  When memory is fragmented, each failed costly-order allocation
enters __alloc_pages_slowpath() which runs direct compaction and
drain_all_pages(), causing a 0.38x throughput drop on PostgreSQL
pgbench (simple-update) with 1024 clients on a 96-vCPU arm64 system.

The root issue is that direct compaction is too expensive for hot
allocation paths that have fallbacks to smaller allocations.
__filemap_get_folio_mpol() already marks higher-order allocations with
__GFP_NORETRY | __GFP_NOWARN, signalling that the caller can handle
failure.  However, the page allocator still attempts full direct
compaction for costly orders with __GFP_NORETRY, which is unnecessarily
aggressive when the caller will simply retry at a lower order.

For costly-order allocations with __GFP_NORETRY, clear
__GFP_DIRECT_RECLAIM at the very start of the slowpath, before
can_direct_reclaim, can_compact and the nofail checks are evaluated.
This makes the entire slowpath treat the request as non-blocking: no
direct reclaim, no direct compaction and no drain_all_pages() IPI
across every CPU. kswapd (and in turn kcompactd) is still woken further
down for background defragmentation, so compaction keeps working for
long-term system health while being removed from the latency-critical
direct allocation path.

Allocations that also request __GFP_THISNODE are exempted.  That flag
pairing identifies the local-node-first THP attempt issued by
alloc_pages_mpol() (mempolicy.c), which relies on direct compaction to
form transparent huge pages.

Test environment:
  Hardware:  AWS EC2 m8g.24xlarge (96 vCPU, arm64)
             12x 1TB IO2 32000 IOPS RAID0 XFS
  OS:        AL2023
  Kernel:    v7.3-rc1
  Database:  PostgreSQL 18.4
  Workload:  pgbench simple-update, 1024 clients, 96 threads, 1200s

Results (average of 3 runs, TPS):

  Config                   Avg TPS      % vs Baseline
  baseline (no patch)       59,408       -
  With this patch          155,409      +161.6%

Link: https://lore.kernel.org/all/20260403193535.9970-1-dipiets@xxxxxxxxx/T/#t [v1]
Link: https://lore.kernel.org/linux-mm/20260420161404.642-1-dipiets@xxxxxxxxx/T/#u [v2]
Link: https://lore.kernel.org/all/20260710143437.12379-1-dipiets@xxxxxxxxx/T/#u [v3]
Fixes: 5d8edfb900d5 ("iomap: Copy larger chunks from userspace")
Cc: stable@xxxxxxxxxxxxxxx
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Dave Chinner <dgc@xxxxxxxxxx>
Cc: Ritesh Harjani <ritesh.list@xxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-xfs@xxxxxxxxxxxxxxx
Signed-off-by: Salvatore Dipietro <dipiets@xxxxxxxxx>
---
v4: Clear __GFP_DIRECT_RECLAIM early in the slowpath and exempt
    __GFP_THISNODE so THP attempt keeps using direct compaction
v3: Move to mm/page_alloc.c, wake kcompactd instead of avoiding it
v2: Move from fs/iomap/buffered-io.c to mm/filemap.c
v1: Avoid compaction in iomap folio allocation

 mm/page_alloc.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 12fac9084c48..542c2ec31061 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4784,10 +4784,10 @@ static inline struct page *
 __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 						struct alloc_context *ac)
 {
-	bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
-	bool can_compact = can_direct_reclaim && gfp_compaction_allowed(gfp_mask);
-	bool nofail = gfp_mask & __GFP_NOFAIL;
 	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
+	bool can_direct_reclaim;
+	bool can_compact;
+	bool nofail;
 	struct page *page = NULL;
 	unsigned int alloc_flags;
 	unsigned long did_some_progress;
@@ -4802,6 +4802,18 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	bool can_retry_reserves = true;
 	unsigned long alloc_start_time = jiffies;
 
+	/*
+	 * Costly __GFP_NORETRY callers have a cheap fallback, so don't stall
+	 * them in reclaim or compaction. __GFP_THISNODE callers are exempt.
+	 */
+	if (costly_order && (gfp_mask & __GFP_NORETRY) &&
+	    !(gfp_mask & __GFP_THISNODE))
+		gfp_mask &= ~__GFP_DIRECT_RECLAIM;
+
+	can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM;
+	can_compact = can_direct_reclaim && gfp_compaction_allowed(gfp_mask);
+	nofail = gfp_mask & __GFP_NOFAIL;
+
 	if (unlikely(nofail)) {
 		/*
 		 * Also we don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM,
-- 
2.50.1




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