mm: Add a .to_text() method for shrinkers

This adds a new callback method to shrinkers which they can use to
describe anything relevant to memory reclaim about their internal state,
for example object dirtyness.

This uses the new printbufs to output to heap allocated strings, so that
the .to_text() methods can be used both for messages logged to the
console, and also sysfs/debugfs.

This patch also adds shrinkers_to_text(), which reports on the top 10
shrinkers - by object count - in sorted order, to be used in OOM
reporting.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

2 files changed, 77 insertions, 17 deletions

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 224293b2dd06..ce7e9a3addf6 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -5,6 +5,8 @@
 #include <linux/atomic.h>
 #include <linux/types.h>
 
+struct seq_buf;
+
 /*
  * This struct is used to pass information from page reclaim to the shrinkers.
  * We consolidate the values for easier extension later.
@@ -61,10 +63,12 @@ struct shrink_control {
  * @flags determine the shrinker abilities, like numa awareness
  */
 struct shrinker {
+	const char *name;
 	unsigned long (*count_objects)(struct shrinker *,
 				       struct shrink_control *sc);
 	unsigned long (*scan_objects)(struct shrinker *,
 				      struct shrink_control *sc);
+	void (*to_text)(struct seq_buf *, struct shrinker *);
 
 	long batch;	/* reclaim batch size, 0 = default */
 	int seeks;	/* seeks to recreate an obj */
@@ -78,7 +82,6 @@ struct shrinker {
 #endif
 #ifdef CONFIG_SHRINKER_DEBUG
 	int debugfs_id;
-	const char *name;
 	struct dentry *debugfs_entry;
 #endif
 	/* objs pending delete, per node */
@@ -104,6 +107,7 @@ extern int __printf(2, 3) register_shrinker(struct shrinker *shrinker,
 extern void unregister_shrinker(struct shrinker *shrinker);
 extern void free_prealloced_shrinker(struct shrinker *shrinker);
 extern void synchronize_shrinkers(void);
+void shrinkers_to_text(struct seq_buf *);
 
 #ifdef CONFIG_SHRINKER_DEBUG
 extern int shrinker_debugfs_add(struct shrinker *shrinker);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6f13394b112e..c87848b2d329 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -57,6 +57,7 @@
 #include <linux/khugepaged.h>
 #include <linux/rculist_nulls.h>
 #include <linux/random.h>
+#include <linux/seq_buf.h>
 
 #include <asm/tlbflush.h>
 #include <asm/div64.h>
@@ -698,7 +699,6 @@ static int __prealloc_shrinker(struct shrinker *shrinker)
 	return 0;
 }
 
-#ifdef CONFIG_SHRINKER_DEBUG
 int prealloc_shrinker(struct shrinker *shrinker, const char *fmt, ...)
 {
 	va_list ap;
@@ -718,19 +718,12 @@ int prealloc_shrinker(struct shrinker *shrinker, const char *fmt, ...)
 
 	return err;
 }
-#else
-int prealloc_shrinker(struct shrinker *shrinker, const char *fmt, ...)
-{
-	return __prealloc_shrinker(shrinker);
-}
-#endif
 
 void free_prealloced_shrinker(struct shrinker *shrinker)
 {
-#ifdef CONFIG_SHRINKER_DEBUG
 	kfree_const(shrinker->name);
 	shrinker->name = NULL;
-#endif
+
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
 		down_write(&shrinker_rwsem);
 		unregister_memcg_shrinker(shrinker);
@@ -761,7 +754,6 @@ static int __register_shrinker(struct shrinker *shrinker)
 	return 0;
 }
 
-#ifdef CONFIG_SHRINKER_DEBUG
 int register_shrinker(struct shrinker *shrinker, const char *fmt, ...)
 {
 	va_list ap;
@@ -780,12 +772,6 @@ int register_shrinker(struct shrinker *shrinker, const char *fmt, ...)
 	}
 	return err;
 }
-#else
-int register_shrinker(struct shrinker *shrinker, const char *fmt, ...)
-{
-	return __register_shrinker(shrinker);
-}
-#endif
 EXPORT_SYMBOL(register_shrinker);
 
 /*
@@ -829,6 +815,76 @@ void synchronize_shrinkers(void)
 }
 EXPORT_SYMBOL(synchronize_shrinkers);
 
+void shrinker_to_text(struct seq_buf *out, struct shrinker *shrinker)
+{
+	struct shrink_control sc = { .gfp_mask = GFP_KERNEL, };
+
+	seq_buf_puts(out, shrinker->name);
+	seq_buf_printf(out, " objects: %lu\n", shrinker->count_objects(shrinker, &sc));
+
+	if (shrinker->to_text) {
+		shrinker->to_text(out, shrinker);
+		seq_buf_puts(out, "\n");
+	}
+}
+
+/**
+ * shrinkers_to_text - Report on shrinkers with highest usage
+ *
+ * This reports on the top 10 shrinkers, by object counts, in sorted order:
+ * intended to be used for OOM reporting.
+ */
+void shrinkers_to_text(struct seq_buf *out)
+{
+	struct shrinker *shrinker;
+	struct shrinker_by_mem {
+		struct shrinker	*shrinker;
+		unsigned long	mem;
+	} shrinkers_by_mem[10];
+	int i, nr = 0;
+
+	if (!down_read_trylock(&shrinker_rwsem)) {
+		seq_buf_puts(out, "(couldn't take shrinker lock)");
+		return;
+	}
+
+	list_for_each_entry(shrinker, &shrinker_list, list) {
+		struct shrink_control sc = { .gfp_mask = GFP_KERNEL, };
+		unsigned long mem = shrinker->count_objects(shrinker, &sc);
+
+		if (!mem || mem == SHRINK_STOP || mem == SHRINK_EMPTY)
+			continue;
+
+		for (i = 0; i < nr; i++)
+			if (mem < shrinkers_by_mem[i].mem)
+				break;
+
+		if (nr < ARRAY_SIZE(shrinkers_by_mem)) {
+			memmove(&shrinkers_by_mem[i + 1],
+				&shrinkers_by_mem[i],
+				sizeof(shrinkers_by_mem[0]) * (nr - i));
+			nr++;
+		} else if (i) {
+			i--;
+			memmove(&shrinkers_by_mem[0],
+				&shrinkers_by_mem[1],
+				sizeof(shrinkers_by_mem[0]) * i);
+		} else {
+			continue;
+		}
+
+		shrinkers_by_mem[i] = (struct shrinker_by_mem) {
+			.shrinker = shrinker,
+			.mem = mem,
+		};
+	}
+
+	for (i = nr - 1; i >= 0; --i)
+		shrinker_to_text(out, shrinkers_by_mem[i].shrinker);
+
+	up_read(&shrinker_rwsem);
+}
+
 #define SHRINK_BATCH 128
 
 static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,