linux-kernel - [PATCH 1/4] slab: separate struct freelist_tid from kmem_cache

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Message-Id: <20251107-slab-fms-cleanup-v1-1-650b1491ac9e@suse.cz>
Date: Fri, 07 Nov 2025 14:51:23 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
 Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>, 
 Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>, 
 linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH 1/4] slab: separate struct freelist_tid from kmem_cache_cpu

In kmem_cache_cpu we currently have a union of the freelist+tid pair
with freelist_aba_t, relying implicitly on the type compatibility with the
freelist+counters pair used in freelist_aba_t.

To allow further changes to freelist_aba_t, we can instead define a
separate struct freelist_tid (instead of a typedef, per the coding
style) for kmem_cache_cpu, as that affects only a single helper
__update_cpu_freelist_fast().

We can add the resulting struct freelist_tid to kmem_cache_cpu as
unnamed field thanks to -fms-extensions, so that freelist and tid fields
can still be accessed directly.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 mm/slub.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 074abe8e79f8..5f6408c9e0fd 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -411,18 +411,22 @@ enum stat_item {
 };
 
 #ifndef CONFIG_SLUB_TINY
-/*
- * When changing the layout, make sure freelist and tid are still compatible
- * with this_cpu_cmpxchg_double() alignment requirements.
- */
-struct kmem_cache_cpu {
+struct freelist_tid {
 	union {
 		struct {
-			void **freelist;	/* Pointer to next available object */
+			void *freelist;		/* Pointer to next available object */
 			unsigned long tid;	/* Globally unique transaction id */
 		};
-		freelist_aba_t freelist_tid;
+		freelist_full_t freelist_tid;
 	};
+};
+
+/*
+ * When changing the layout, make sure freelist and tid are still compatible
+ * with this_cpu_cmpxchg_double() alignment requirements.
+ */
+struct kmem_cache_cpu {
+	struct freelist_tid;
 	struct slab *slab;	/* The slab from which we are allocating */
 #ifdef CONFIG_SLUB_CPU_PARTIAL
 	struct slab *partial;	/* Partially allocated slabs */
@@ -4367,11 +4371,11 @@ __update_cpu_freelist_fast(struct kmem_cache *s,
 			   void *freelist_old, void *freelist_new,
 			   unsigned long tid)
 {
-	freelist_aba_t old = { .freelist = freelist_old, .counter = tid };
-	freelist_aba_t new = { .freelist = freelist_new, .counter = next_tid(tid) };
+	struct freelist_tid old = { .freelist = freelist_old, .tid = tid };
+	struct freelist_tid new = { .freelist = freelist_new, .tid = next_tid(tid) };
 
-	return this_cpu_try_cmpxchg_freelist(s->cpu_slab->freelist_tid.full,
-					     &old.full, new.full);
+	return this_cpu_try_cmpxchg_freelist(s->cpu_slab->freelist_tid,
+					     &old.freelist_tid, new.freelist_tid);
 }
 
 /*

-- 
2.51.1