[PATCH v2] mm/page_alloc: fix defrag_mode for non-reclaimable allocations

[Dmitry Ilvokhin <d@ilvokhin.com>]

When defrag_mode is enabled, ALLOC_NOFRAGMENT is enforced to prevent
migratetype fallbacks and keep pageblocks clean. The allocator relies on
reclaim and compaction to free pages of the correct type before allowing
fallback as a last resort.

However, non-reclaimable allocations such as GFP_ATOMIC cannot invoke
direct reclaim or compaction. With defrag_mode=1, these allocations hit
the !can_direct_reclaim bailout in __alloc_pages_slowpath() with
ALLOC_NOFRAGMENT still set, and fail without ever attempting a fallback.

This causes a large number of SLUB allocation failures for
skbuff_head_cache under network-heavy workloads, despite free memory
being available in other migratetype freelists.

Clear ALLOC_NOFRAGMENT and retry for allocations that request kswapd
reclaim but cannot do direct reclaim themselves (GFP_ATOMIC).  Purely
speculative allocations like GFP_TRANSHUGE_LIGHT that don't set
__GFP_KSWAPD_RECLAIM are left to fail, since they have reasonable
fallbacks and should not cause fragmentation.

Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode")

Signed-off-by: Dmitry Ilvokhin <d@ilvokhin.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
---
Changes in v2:

- Add check for __GFP_KSWAPD_RECLAIM.
- Picked up Johannes acked-by tag.

v1: https://lore.kernel.org/all/20260518163736.173910-1-d@ilvokhin.com/

 mm/page_alloc.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 227d58dc3de6..c5a077de1be0 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4811,8 +4811,19 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	}
 
 	/* Caller is not willing to reclaim, we can't balance anything */
-	if (!can_direct_reclaim)
+	if (!can_direct_reclaim) {
+		/*
+		 * Reclaim/compaction cannot run, so defrag_mode's strategy
+		 * of enforcing ALLOC_NOFRAGMENT cannot be fulfilled. Allow
+		 * fallbacks rather than failing the allocation outright.
+		 */
+		if (defrag_mode && (alloc_flags & ALLOC_NOFRAGMENT) &&
+		    (gfp_mask & __GFP_KSWAPD_RECLAIM)) {
+			alloc_flags &= ~ALLOC_NOFRAGMENT;
+			goto retry;
+		}
 		goto nopage;
+	}
 
 	/* Avoid recursion of direct reclaim */
 	if (current->flags & PF_MEMALLOC)
-- 
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