Re: [PATCH v3] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations

[Salvatore Dipietro]

On Wed, Jul 22, 2026 at 02:35:00PM +0200, Vlastimil Babka (SUSE) wrote:
> AFAICS both your v3 and the implemented Johannes' suggestion effectively
> make all the compact_first logic dead code - at least for the intended THP
> use case based on costly_order. It will only continue to do something for
> the non-movable order>0  case. So that's a significant change which will not
> show up in your results, but might affect other workloads (I'd expect mainly
> by having fewer THPs).

Right - the change above modifies THP allocation requests as well. So instead
of applying it to every costly __GFP_NORETRY allocation, I restrict it to
callers that do not ask for __GFP_THISNODE. That leaves the local-node THP
attempt untouched: the same bit pair is already tested further down the function 
to bail out of the compact_first block (page_alloc.c:4899), and alloc_pages_mpol()
sets both flags on the local-node-first attempt (mempolicy.c:2476-2478).

I tested this with my PostgreSQL workload on kernel v7.2-rc7 and results look good:

Baseline:       70,735 tps
This version:  144,670 tps  (145,280 / 144,037 / 144,694)  +104.5%


diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ee902a468c2f..1e4b0c1e7296 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4725,10 +4725,10 @@ 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;
@@ -4743,6 +4743,18 @@ __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,



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