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Subject: Re: [PATCH] mm: slub: Fix slub_debug disablement for list of slabs
To:     vjitta@codeaurora.org, cl@linux.com, penberg@kernel.org,
        rientjes@google.com, iamjoonsoo.kim@lge.com,
        akpm@linux-foundation.org
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        vinmenon@codeaurora.org
References: <1626176750-13099-1-git-send-email-vjitta@codeaurora.org>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <bf2a8571-325c-6d94-0d5a-f6df71ae0c4f@suse.cz>
Date:   Tue, 27 Jul 2021 00:32:09 +0200
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On 7/13/21 1:45 PM, vjitta@codeaurora.org wrote:
> From: Vijayanand Jitta <vjitta@codeaurora.org>
> 
> Consider the scenario where CONFIG_SLUB_DEBUG_ON is set
> and we would want to disable slub_debug for few slabs.
> Using boot parameter with slub_debug=-,slab_name syntax
> doesn't work as expected i.e; only disabling debugging for
> the specified list of slabs, instead it disables debugging
> for all slabs. Fix this.
> 
> Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>

Would the following work too, and perhaps be easier to follow?
----8<----
diff --git a/mm/slub.c b/mm/slub.c
index 090fa14628f9..024f49706386 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1400,12 +1400,13 @@ parse_slub_debug_flags(char *str, slab_flags_t *flags, char **slabs, bool init)
 static int __init setup_slub_debug(char *str)
 {
 	slab_flags_t flags;
+	slab_flags_t global_flags;
 	char *saved_str;
 	char *slab_list;
 	bool global_slub_debug_changed = false;
 	bool slab_list_specified = false;
 
-	slub_debug = DEBUG_DEFAULT_FLAGS;
+	global_flags = DEBUG_DEFAULT_FLAGS;
 	if (*str++ != '=' || !*str)
 		/*
 		 * No options specified. Switch on full debugging.
@@ -1417,7 +1418,7 @@ static int __init setup_slub_debug(char *str)
 		str = parse_slub_debug_flags(str, &flags, &slab_list, true);
 
 		if (!slab_list) {
-			slub_debug = flags;
+			global_flags = flags;
 			global_slub_debug_changed = true;
 		} else {
 			slab_list_specified = true;
@@ -1426,16 +1427,18 @@ static int __init setup_slub_debug(char *str)
 
 	/*
 	 * For backwards compatibility, a single list of flags with list of
-	 * slabs means debugging is only enabled for those slabs, so the global
-	 * slub_debug should be 0. We can extended that to multiple lists as
+	 * slabs means debugging is only changed for those slabs, so the global
+	 * slub_debug should be unchanged (0 or DEBUG_DEFAULT_FLAGS, depending
+	 * on CONFIG_SLUB_DEBUG_ON). We can extended that to multiple lists as
 	 * long as there is no option specifying flags without a slab list.
 	 */
 	if (slab_list_specified) {
 		if (!global_slub_debug_changed)
-			slub_debug = 0;
+			global_flags = slub_debug;
 		slub_debug_string = saved_str;
 	}
 out:
+	slub_debug = global_flags;
 	if (slub_debug != 0 || slub_debug_string)
 		static_branch_enable(&slub_debug_enabled);
 	else
-- 
2.32.0