[PATCH v5] 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, suppress direct reclaim
for the whole slowpath by computing can_direct_reclaim (and hence
can_compact) as false.  The !can_direct_reclaim check near the top of
the slowpath then short-circuits to nopage: 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.  __GFP_NOFAIL is exempted as well, so a
must-not-fail allocation is never made to fail.

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
  AL2023 stock 6.1 kernel (pre-5d8edfb900d5)   136,942           n/a
  v7.3-rc1 baseline (no patch)                  59,408             -
  v7.3-rc1 + this patch                        161,994       +172.7%

The patch fully recovers the pre-5d8edfb900d5 performance, bringing
throughput back above the pre-regression level and well clear of the
~59k baseline.  The AL2023 6.1 row runs a kernel that predates commit
5d8edfb900d5 ("iomap: Copy larger chunks from userspace"), so it is not
directly comparable, but it shows the pre-regression level and confirms
that the ~59k baseline is the anomaly and not the norm.


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]
Link: https://lore.kernel.org/all/20260904115629.3993331-1-dipiets@xxxxxxxxx/T/#u [v4]
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: Zi Yan <ziy@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-xfs@xxxxxxxxxxxxxxx
Signed-off-by: Salvatore Dipietro <dipiets@xxxxxxxxx>
---
v5: Derive the decision into a local flag instead of mutating gfp_mask
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 | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 12fac9084c48..be8b0d72a2db 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4784,10 +4784,22 @@ 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;
+	const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER;
+	/*
+	 * Costly __GFP_NORETRY callers have a cheap fallback to a lower order,
+	 * so don't stall them in direct reclaim or direct compaction.  Exempt
+	 * __GFP_THISNODE (the THP attempt from alloc_pages_mpol() needs direct
+	 * compaction) and __GFP_NOFAIL (must not be made to fail).  Don't
+	 * clear __GFP_DIRECT_RECLAIM from gfp_mask instead: that would also
+	 * change the alloc_flags derived by alloc_flags_slowpath().
+	 */
+	const bool costly_noretry = costly_order &&
+		(gfp_mask & __GFP_NORETRY) &&
+		!(gfp_mask & (__GFP_THISNODE | __GFP_NOFAIL));
+	bool can_direct_reclaim = !costly_noretry &&
+		(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;
 	struct page *page = NULL;
 	unsigned int alloc_flags;
 	unsigned long did_some_progress;
-- 
2.50.1




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