[RFC PATCH 1/5] mm, slub: extend slub_debug syntax for multiple blocks

[Vlastimil Babka]

The slub_debug kernel boot parameter can either apply a single set of options
to all caches or a list of caches. There is a use case where debugging is
applied for all caches and then disabled at runtime for specific caches, for
performance and memory consumption reasons [1]. As runtime changes are
dangerous, extend the boot parameter syntax so that multiple blocks of either
global or slab-specific options can be specified, with blocks delimited by ';'.
This will also support the use case of [1] without runtime changes.

For details see the updated Documentation/vm/slub.rst

[1] https://lore.kernel.org/r/1383cd32-1ddc-4dac-b5f8-9c42282fa81c@xxxxxxxxxxxxxx

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |   2 +-
 Documentation/vm/slub.rst                     |  17 ++
 mm/slub.c                                     | 177 +++++++++++++-----
 3 files changed, 144 insertions(+), 52 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6253849afac2..6ea00c8dd635 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4613,7 +4613,7 @@
 			fragmentation.  Defaults to 1 for systems with
 			more than 32MB of RAM, 0 otherwise.
 
-	slub_debug[=options[,slabs]]	[MM, SLUB]
+	slub_debug[=options[,slabs][;[options[,slabs]]...]	[MM, SLUB]
 			Enabling slub_debug allows one to determine the
 			culprit if slab objects become corrupted. Enabling
 			slub_debug can create guard zones around objects and
diff --git a/Documentation/vm/slub.rst b/Documentation/vm/slub.rst
index 4eee598555c9..f1154f707e06 100644
--- a/Documentation/vm/slub.rst
+++ b/Documentation/vm/slub.rst
@@ -41,6 +41,11 @@ slub_debug=<Debug-Options>,<slab name1>,<slab name2>,...
 	Enable options only for select slabs (no spaces
 	after a comma)
 
+Multiple blocks of options for all slabs or selected slabs can be given, with
+blocks of options delimited by ';'. The last of "all slabs" blocks is applied
+to all slabs except those that match one of the "select slabs" block. Options
+of the first "select slabs" blocks that matches the slab's name are applied.
+
 Possible debug options are::
 
 	F		Sanity checks on (enables SLAB_DEBUG_CONSISTENCY_CHECKS
@@ -83,6 +88,18 @@ in low memory situations or if there's high fragmentation of memory.  To
 
 	slub_debug=O
 
+You can apply different options to different list of slab names, using blocks
+of options. This will enable red zoning for dentry and user tracking for
+kmalloc. All other slabs will not get any debugging enabled::
+
+	slub_debug=Z,dentry;U,kmalloc-*
+
+You can also enable options (e.g. sanity checks and poisoning) for all caches
+except some that are deemed too performance critical and don't need to be
+debugged::
+
+	slub_debug=FZ;-,zs_handle,zspage
+
 In case you forgot to enable debugging on the kernel command line: It is
 possible to enable debugging manually when the kernel is up. Look at the
 contents of::
diff --git a/mm/slub.c b/mm/slub.c
index 03e063cd979f..47430aad9a65 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -499,7 +499,7 @@ static slab_flags_t slub_debug = DEBUG_DEFAULT_FLAGS;
 static slab_flags_t slub_debug;
 #endif
 
-static char *slub_debug_slabs;
+static char *slub_debug_string;
 static int disable_higher_order_debug;
 
 /*
@@ -1262,68 +1262,132 @@ static noinline int free_debug_processing(
 	return ret;
 }
 
-static int __init setup_slub_debug(char *str)
+/*
+ * Parse a block of slub_debug options. Blocks are delimited by ';'
+ *
+ * @str:    start of block
+ * @flags:  returns parsed flags, or DEBUG_DEFAULT_FLAGS if none specified
+ * @slabs:  return start of list of slabs, or NULL when there's no list
+ * @init:   assume this is initial parsing and not per-kmem-create parsing
+ *
+ * returns the start of next block if there's any, or NULL
+ */
+char *
+parse_slub_debug_flags(char *str, slab_flags_t *flags, char **slabs, bool init)
 {
-	slub_debug = DEBUG_DEFAULT_FLAGS;
-	if (*str++ != '=' || !*str)
-		/*
-		 * No options specified. Switch on full debugging.
-		 */
-		goto out;
+	bool higher_order_disable = false;
 
-	if (*str == ',')
+	/* Skip any completely empty blocks */
+	while (*str && *str == ';')
+		str++;
+
+	if (*str == ',') {
 		/*
 		 * No options but restriction on slabs. This means full
 		 * debugging for slabs matching a pattern.
 		 */
+		*flags = DEBUG_DEFAULT_FLAGS;
 		goto check_slabs;
+	}
+	*flags = 0;
 
-	slub_debug = 0;
-	if (*str == '-')
-		/*
-		 * Switch off all debugging measures.
-		 */
-		goto out;
-
-	/*
-	 * Determine which debug features should be switched on
-	 */
-	for (; *str && *str != ','; str++) {
+	/* Determine which debug features should be switched on */
+	for (; *str && *str != ',' && *str != ';'; str++) {
 		switch (tolower(*str)) {
+		case '-':
+			*flags = 0;
+			break;
 		case 'f':
-			slub_debug |= SLAB_CONSISTENCY_CHECKS;
+			*flags |= SLAB_CONSISTENCY_CHECKS;
 			break;
 		case 'z':
-			slub_debug |= SLAB_RED_ZONE;
+			*flags |= SLAB_RED_ZONE;
 			break;
 		case 'p':
-			slub_debug |= SLAB_POISON;
+			*flags |= SLAB_POISON;
 			break;
 		case 'u':
-			slub_debug |= SLAB_STORE_USER;
+			*flags |= SLAB_STORE_USER;
 			break;
 		case 't':
-			slub_debug |= SLAB_TRACE;
+			*flags |= SLAB_TRACE;
 			break;
 		case 'a':
-			slub_debug |= SLAB_FAILSLAB;
+			*flags |= SLAB_FAILSLAB;
 			break;
 		case 'o':
 			/*
 			 * Avoid enabling debugging on caches if its minimum
 			 * order would increase as a result.
 			 */
-			disable_higher_order_debug = 1;
+			higher_order_disable = true;
 			break;
 		default:
-			pr_err("slub_debug option '%c' unknown. skipped\n",
-			       *str);
+			if (init)
+				pr_err("slub_debug option '%c' unknown. skipped\n", *str);
 		}
 	}
-
 check_slabs:
 	if (*str == ',')
-		slub_debug_slabs = str + 1;
+		*slabs = ++str;
+	else
+		*slabs = NULL;
+
+	/* Skip over the slab list */
+	while (*str && *str != ';')
+		str++;
+
+	/* Skip any completely empty blocks */
+	while (*str && *str == ';')
+		str++;
+
+	if (init && higher_order_disable)
+		disable_higher_order_debug = 1;
+
+	if (*str)
+		return str;
+	else
+		return NULL;
+}
+
+static int __init setup_slub_debug(char *str)
+{
+	slab_flags_t flags;
+	char *saved_str;
+	char *slab_list;
+	bool global_slub_debug_changed = false;
+	bool slab_list_specified = false;
+
+	slub_debug = DEBUG_DEFAULT_FLAGS;
+	if (*str++ != '=' || !*str)
+		/*
+		 * No options specified. Switch on full debugging.
+		 */
+		goto out;
+
+	saved_str = str;
+	while (str) {
+		str = parse_slub_debug_flags(str, &flags, &slab_list, true);
+
+		if (!slab_list) {
+			slub_debug = flags;
+			global_slub_debug_changed = true;
+		} else {
+			slab_list_specified = true;
+		}
+	}
+
+	/*
+	 * 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
+	 * long as there is no option specifying flags without a slab list.
+	 */
+	if (slab_list_specified) {
+		if (!global_slub_debug_changed)
+			slub_debug = 0;
+		slub_debug_string = saved_str;
+	}
 out:
 	if ((static_branch_unlikely(&init_on_alloc) ||
 	     static_branch_unlikely(&init_on_free)) &&
@@ -1352,36 +1416,47 @@ slab_flags_t kmem_cache_flags(unsigned int object_size,
 {
 	char *iter;
 	size_t len;
+	char *next_block;
+	slab_flags_t block_flags;
 
 	/* If slub_debug = 0, it folds into the if conditional. */
-	if (!slub_debug_slabs)
+	if (!slub_debug_string)
 		return flags | slub_debug;
 
 	len = strlen(name);
-	iter = slub_debug_slabs;
-	while (*iter) {
-		char *end, *glob;
-		size_t cmplen;
-
-		end = strchrnul(iter, ',');
+	next_block = slub_debug_string;
+	/* Go through all blocks of debug options, see if any matches our slab's name */
+	while (next_block) {
+		next_block = parse_slub_debug_flags(next_block, &block_flags, &iter, false);
+		if (!iter)
+			continue;
+		/* Found a block that has a slab list, search it */
+		while (*iter) {
+			char *end, *glob;
+			size_t cmplen;
+
+			end = strchrnul(iter, ',');
+			if (next_block && next_block < end)
+				end = next_block - 1;
+
+			glob = strnchr(iter, end - iter, '*');
+			if (glob)
+				cmplen = glob - iter;
+			else
+				cmplen = max_t(size_t, len, (end - iter));
 
-		glob = strnchr(iter, end - iter, '*');
-		if (glob)
-			cmplen = glob - iter;
-		else
-			cmplen = max_t(size_t, len, (end - iter));
+			if (!strncmp(name, iter, cmplen)) {
+				flags |= block_flags;
+				return flags;
+			}
 
-		if (!strncmp(name, iter, cmplen)) {
-			flags |= slub_debug;
-			break;
+			if (!*end || *end == ';')
+				break;
+			iter = end + 1;
 		}
-
-		if (!*end)
-			break;
-		iter = end + 1;
 	}
 
-	return flags;
+	return slub_debug;
 }
 #else /* !CONFIG_SLUB_DEBUG */
 static inline void setup_object_debug(struct kmem_cache *s,
-- 
2.26.2