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

[Salvatore Dipietro]

On Sat, 05 Sep 2026 17:42:39 -0700 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> Is there anything particularly unusual about this test case?

It is a stock pgbench simple-update PostgreSQL workload on a large
instance (96 vCPUs), using standard PostgreSQL settings and with no huge
pages assigned to the database. We deliberately overprovision the
pgbench clients: 1024 clients over 96 threads. That keeps enough writers
in the buffered write path concurrently to hit the costly-order
allocation failure path continuously. The memory fragmentation comes from 
page tables: PostgreSQL spawns a new process per client, and those page
tables consume ~40% of memory, which significantly limits the page cache
and the free memory available.


> > Results (average of 3 runs, TPS):
> >
> >   Config                   Avg TPS      % vs Baseline
> >   baseline (no patch)       59,408       -
> >   With this patch          155,409      +161.6%
>
> Is this back to pre-5d8edfb900d5 performance?

Yes - fully recovered. Here is the summary, same host and workload
throughout, average of 3 runs:

  Config                                        Avg TPS   % vs baseline
  AL2023 stock 6.1 kernel                       136,942            n/a
  v7.3-rc1 baseline (no patch)                   59,408             -
  v7.3-rc1, 5d8edfb900d5 behaviour reverted     151,184        +154.5%
  v7.3-rc1 + v4                                 155,409        +161.6%

The 6.1 row predates 5d8edfb900d5 entirely. A different kernel version,
so not directly comparable, but it shows the performance this workload
used to get on this host.

A literal "git revert 5d8edfb900d5" does not apply to v7.3-rc1 -
iomap_write_iter() has been rewritten since - so the reverted row is a
one-line behavioural revert, forcing the write loop back to copying at
most PAGE_SIZE per iteration:

  -	size_t chunk = mapping_max_folio_size(mapping);
  +	size_t chunk = PAGE_SIZE;

which is what the pre-5d8edfb900d5 loop computed. That makes
iomap_get_folio() pass no order hint, so the path issues only order-0
allocations.


> AI review asked a few serious-looking questions:
> 	https://sashiko.dev/#/patchset/20260904115629.3993331-1-dipiets@xxxxxxxxx

Thanks for pointing that out. To address them, we can have something
like the patch below. Performance results are still similar to v4. Happy
to submit a formal v5 patch with it if you would like.

  Config                                        Avg TPS   % vs baseline
  v7.3-rc1 baseline (no patch)                   59,408             -
  v7.3-rc1 + v4                                 155,409        +161.6%
  v7.3-rc1 + proposed patch                     161,994        +172.7%

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;


Thanks,
Salvatore



AMAZON DEVELOPMENT CENTER ITALY SRL, viale Monte Grappa 3/5, 20124 Milano, Italia, Registro delle Imprese di Milano Monza Brianza Lodi REA n. 2504859, Capitale Sociale: 10.000 EUR i.v., Cod. Fisc. e P.IVA 10100050961, Societa con Socio Unico