linux-kernel - [RFC 08/26] mm, slub: restructure new page checks in ___slab

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Date:   Tue, 25 May 2021 01:39:28 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>, Vlastimil Babka <vbabka@...e.cz>
Subject: [RFC 08/26] mm, slub: restructure new page checks in ___slab_alloc()

When we allocate slab object from a newly acquired page (from node's partial
list or page allocator), we usually also retain the page as a new percpu slab.
There are two exceptions - when pfmemalloc status of the page doesn't match our
gfp flags, or when the cache has debugging enabled.

The current code for these decisions is not easy to follow, so restructure it
and add comments. No functional change.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 mm/slub.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index f240e424c861..06f30c9ad361 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2743,13 +2743,29 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 	c->page = page;
 
 check_new_page:
-	if (likely(!kmem_cache_debug(s) && pfmemalloc_match(page, gfpflags)))
-		goto load_freelist;
 
-	/* Only entered in the debug case */
-	if (kmem_cache_debug(s) &&
-			!alloc_debug_processing(s, page, freelist, addr))
-		goto new_slab;	/* Slab failed checks. Next slab needed */
+	if (kmem_cache_debug(s)) {
+		if (!alloc_debug_processing(s, page, freelist, addr))
+			/* Slab failed checks. Next slab needed */
+			goto new_slab;
+		else
+			/*
+			 * For debug case, we don't load freelist so that all
+			 * allocations go through alloc_debug_processing()
+			 */
+			goto return_single;
+	}
+
+	if (unlikely(!pfmemalloc_match(page, gfpflags)))
+		/*
+		 * For !pfmemalloc_match() case we don't load freelist so that
+		 * we don't make further mismatched allocations easier.
+		 */
+		goto return_single;
+
+	goto load_freelist;
+
+return_single:
 
 	deactivate_slab(s, page, get_freepointer(s, freelist), c);
 	return freelist;
-- 
2.31.1