[PATCH 00/10] shrinker debugging, .to_text() report

[PATCH 00/10] shrinker debugging, .to_text() report

[Kent Overstreet]

recently new OOMs have been cropping up, and reclaim is implicated, so
I've had to dust off these patches.

nothing significant has changed since the last time I posted, and they
have been valuable - Dave, think we can get them in?

Kent Overstreet (10):
  seq_buf: seq_buf_human_readable_u64()
  mm: shrinker: Add a .to_text() method for shrinkers
  mm: shrinker: Add new stats for .to_text()
  mm: Centralize & improve oom reporting in show_mem.c
  mm: shrinker: Add shrinker_to_text() to debugfs interface
  bcachefs: shrinker.to_text() methods
  percpu: per_cpu_sum()
  fs: Add super_block->s_inodes_nr
  fs/dcache: Add per-sb accounting for nr dentries
  fs: super_cache_to_text()

 fs/bcachefs/btree_cache.c     | 13 +++++
 fs/bcachefs/btree_key_cache.c | 14 ++++++
 fs/bcachefs/util.h            | 10 ----
 fs/dcache.c                   | 18 ++-----
 fs/inode.c                    |  2 +
 fs/super.c                    | 25 ++++++++++
 include/linux/fs.h            |  2 +
 include/linux/percpu.h        | 10 ++++
 include/linux/seq_buf.h       |  4 ++
 include/linux/shrinker.h      | 13 ++++-
 lib/seq_buf.c                 | 10 ++++
 mm/oom_kill.c                 | 23 ---------
 mm/show_mem.c                 | 43 ++++++++++++++++
 mm/shrinker.c                 | 94 ++++++++++++++++++++++++++++++++++-
 mm/shrinker_debug.c           | 18 +++++++
 mm/slab.h                     |  6 ++-
 mm/slab_common.c              | 52 +++++++++++++++----
 17 files changed, 298 insertions(+), 59 deletions(-)

-- 
2.45.2

[PATCH 01/10] seq_buf: seq_buf_human_readable_u64()

[Kent Overstreet]

This adds a seq_buf wrapper for string_get_size().

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 include/linux/seq_buf.h |  4 ++++
 lib/seq_buf.c           | 10 ++++++++++
 2 files changed, 14 insertions(+)

diff --git a/include/linux/seq_buf.h b/include/linux/seq_buf.h
index fe41da005970..1cba369e1821 100644
--- a/include/linux/seq_buf.h
+++ b/include/linux/seq_buf.h
@@ -173,4 +173,8 @@ seq_buf_bprintf(struct seq_buf *s, const char *fmt, const u32 *binary);
 
 void seq_buf_do_printk(struct seq_buf *s, const char *lvl);
 
+enum string_size_units;
+void seq_buf_human_readable_u64(struct seq_buf *s, u64 v,
+				const enum string_size_units units);
+
 #endif /* _LINUX_SEQ_BUF_H */
diff --git a/lib/seq_buf.c b/lib/seq_buf.c
index f3f3436d60a9..3c41ca83a0c3 100644
--- a/lib/seq_buf.c
+++ b/lib/seq_buf.c
@@ -436,3 +436,13 @@ int seq_buf_hex_dump(struct seq_buf *s, const char *prefix_str, int prefix_type,
 	}
 	return 0;
 }
+
+void seq_buf_human_readable_u64(struct seq_buf *s, u64 v, const enum string_size_units units)
+{
+	char *buf;
+	size_t size = seq_buf_get_buf(s, &buf);
+	int wrote = string_get_size(v, 1, units, buf, size);
+
+	seq_buf_commit(s, wrote);
+}
+EXPORT_SYMBOL(seq_buf_human_readable_u64);
-- 
2.45.2

[PATCH 02/10] mm: shrinker: Add a .to_text() method for shrinkers

[Kent Overstreet]

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 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.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: linux-mm@kvack.org
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 include/linux/shrinker.h |  7 +++-
 mm/shrinker.c            | 73 +++++++++++++++++++++++++++++++++++++++-
 2 files changed, 78 insertions(+), 2 deletions(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 1a00be90d93a..6193612617a1 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -24,6 +24,8 @@ struct shrinker_info {
 	struct shrinker_info_unit *unit[];
 };
 
+struct seq_buf;
+
 /*
  * This struct is used to pass information from page reclaim to the shrinkers.
  * We consolidate the values for easier extension later.
@@ -80,10 +82,12 @@ 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 */
@@ -110,7 +114,6 @@ struct shrinker {
 #endif
 #ifdef CONFIG_SHRINKER_DEBUG
 	int debugfs_id;
-	const char *name;
 	struct dentry *debugfs_entry;
 #endif
 	/* objs pending delete, per node */
@@ -135,6 +138,8 @@ __printf(2, 3)
 struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...);
 void shrinker_register(struct shrinker *shrinker);
 void shrinker_free(struct shrinker *shrinker);
+void shrinker_to_text(struct seq_buf *, struct shrinker *);
+void shrinkers_to_text(struct seq_buf *);
 
 static inline bool shrinker_try_get(struct shrinker *shrinker)
 {
diff --git a/mm/shrinker.c b/mm/shrinker.c
index dc5d2a6fcfc4..ad52c269bb48 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -1,8 +1,9 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/memcontrol.h>
+#include <linux/rculist.h>
 #include <linux/rwsem.h>
+#include <linux/seq_buf.h>
 #include <linux/shrinker.h>
-#include <linux/rculist.h>
 #include <trace/events/vmscan.h>
 
 #include "internal.h"
@@ -807,3 +808,73 @@ void shrinker_free(struct shrinker *shrinker)
 	call_rcu(&shrinker->rcu, shrinker_free_rcu_cb);
 }
 EXPORT_SYMBOL_GPL(shrinker_free);
+
+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 (!mutex_trylock(&shrinker_mutex)) {
+		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);
+
+	mutex_unlock(&shrinker_mutex);
+}
-- 
2.45.2

[PATCH 03/10] mm: shrinker: Add new stats for .to_text()

[Kent Overstreet]

Add a few new shrinker stats.

number of objects requested to free, number of objects freed:

Shrinkers won't necessarily free all objects requested for a variety of
reasons, but if the two counts are wildly different something is likely
amiss.

.scan_objects runtime:

If one shrinker is taking an excessive amount of time to free
objects that will block kswapd from running other shrinkers.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: linux-mm@kvack.org
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 include/linux/shrinker.h |  6 ++++++
 mm/shrinker.c            | 23 ++++++++++++++++++++++-
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 6193612617a1..106622ddac77 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -118,6 +118,12 @@ struct shrinker {
 #endif
 	/* objs pending delete, per node */
 	atomic_long_t *nr_deferred;
+
+	atomic_long_t	objects_requested_to_free;
+	atomic_long_t	objects_freed;
+	unsigned long	last_freed;	/* timestamp, in jiffies */
+	unsigned long	last_scanned;	/* timestamp, in jiffies */
+	atomic64_t	ns_run;
 };
 #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
 
diff --git a/mm/shrinker.c b/mm/shrinker.c
index ad52c269bb48..feaa8122afc9 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -430,13 +430,24 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
 	       total_scan >= freeable) {
 		unsigned long ret;
 		unsigned long nr_to_scan = min(batch_size, total_scan);
+		u64 start_time = ktime_get_ns();
+
+		atomic_long_add(nr_to_scan, &shrinker->objects_requested_to_free);
 
 		shrinkctl->nr_to_scan = nr_to_scan;
 		shrinkctl->nr_scanned = nr_to_scan;
 		ret = shrinker->scan_objects(shrinker, shrinkctl);
+
+		atomic64_add(ktime_get_ns() - start_time, &shrinker->ns_run);
 		if (ret == SHRINK_STOP)
 			break;
 		freed += ret;
+		unsigned long now = jiffies;
+		if (ret) {
+			atomic_long_add(ret, &shrinker->objects_freed);
+			shrinker->last_freed = now;
+		}
+		shrinker->last_scanned = now;
 
 		count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned);
 		total_scan -= shrinkctl->nr_scanned;
@@ -812,9 +823,19 @@ EXPORT_SYMBOL_GPL(shrinker_free);
 void shrinker_to_text(struct seq_buf *out, struct shrinker *shrinker)
 {
 	struct shrink_control sc = { .gfp_mask = GFP_KERNEL, };
+	unsigned long nr_freed = atomic_long_read(&shrinker->objects_freed);
 
 	seq_buf_puts(out, shrinker->name);
-	seq_buf_printf(out, " objects: %lu\n", shrinker->count_objects(shrinker, &sc));
+	seq_buf_putc(out, '\n');
+
+	seq_buf_printf(out, "objects:             %lu\n", shrinker->count_objects(shrinker, &sc));
+	seq_buf_printf(out, "requested to free:   %lu\n", atomic_long_read(&shrinker->objects_requested_to_free));
+	seq_buf_printf(out, "objects freed:       %lu\n", nr_freed);
+	seq_buf_printf(out, "last scanned:        %li sec ago\n", (jiffies - shrinker->last_scanned) / HZ);
+	seq_buf_printf(out, "last freed:          %li sec ago\n", (jiffies - shrinker->last_freed) / HZ);
+	seq_buf_printf(out, "ns per object freed: %llu\n", nr_freed
+		       ? div64_ul(atomic64_read(&shrinker->ns_run), nr_freed)
+		       : 0);
 
 	if (shrinker->to_text) {
 		shrinker->to_text(out, shrinker);
-- 
2.45.2

[PATCH 04/10] mm: Centralize & improve oom reporting in show_mem.c

[Kent Overstreet]

This patch:
 - Changes show_mem() to always report on slab usage
 - Instead of reporting on all slabs, we only report on top 10 slabs,
   and in sorted order
 - Also reports on shrinkers, with the new shrinkers_to_text().
   Shrinkers need to be included in OOM/allocation failure reporting
   because they're responsible for memory reclaim - if a shrinker isn't
   giving up its memory, we need to know which one and why.

More OOM reporting can be moved to show_mem.c and improved, this patch
is only a start.

New example output on OOM/memory allocation failure:

00177 Mem-Info:
00177 active_anon:13706 inactive_anon:32266 isolated_anon:16
00177  active_file:1653 inactive_file:1822 isolated_file:0
00177  unevictable:0 dirty:0 writeback:0
00177  slab_reclaimable:6242 slab_unreclaimable:11168
00177  mapped:3824 shmem:3 pagetables:1266 bounce:0
00177  kernel_misc_reclaimable:0
00177  free:4362 free_pcp:35 free_cma:0
00177 Node 0 active_anon:54824kB inactive_anon:129064kB active_file:6612kB inactive_file:7288kB unevictable:0kB isolated(anon):64kB isolated(file):0kB mapped:15296kB dirty:0kB writeback:0kB shmem:12kB writeback_tmp:0kB kernel_stack:3392kB pagetables:5064kB all_unreclaimable? no
00177 DMA free:2232kB boost:0kB min:88kB low:108kB high:128kB reserved_highatomic:0KB active_anon:2924kB inactive_anon:6596kB active_file:428kB inactive_file:384kB unevictable:0kB writepending:0kB present:15992kB managed:15360kB mlocked:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
00177 lowmem_reserve[]: 0 426 426 426
00177 DMA32 free:15092kB boost:5836kB min:8432kB low:9080kB high:9728kB reserved_highatomic:0KB active_anon:52196kB inactive_anon:122392kB active_file:6176kB inactive_file:7068kB unevictable:0kB writepending:0kB present:507760kB managed:441816kB mlocked:0kB bounce:0kB free_pcp:72kB local_pcp:0kB free_cma:0kB
00177 lowmem_reserve[]: 0 0 0 0
00177 DMA: 284*4kB (UM) 53*8kB (UM) 21*16kB (U) 11*32kB (U) 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 2248kB
00177 DMA32: 2765*4kB (UME) 375*8kB (UME) 57*16kB (UM) 5*32kB (U) 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 15132kB
00177 4656 total pagecache pages
00177 1031 pages in swap cache
00177 Swap cache stats: add 6572399, delete 6572173, find 488603/3286476
00177 Free swap  = 509112kB
00177 Total swap = 2097148kB
00177 130938 pages RAM
00177 0 pages HighMem/MovableOnly
00177 16644 pages reserved
00177 Unreclaimable slab info:
00177 9p-fcall-cache    total: 8.25 MiB active: 8.25 MiB
00177 kernfs_node_cache total: 2.15 MiB active: 2.15 MiB
00177 kmalloc-64        total: 2.08 MiB active: 2.07 MiB
00177 task_struct       total: 1.95 MiB active: 1.95 MiB
00177 kmalloc-4k        total: 1.50 MiB active: 1.50 MiB
00177 signal_cache      total: 1.34 MiB active: 1.34 MiB
00177 kmalloc-2k        total: 1.16 MiB active: 1.16 MiB
00177 bch_inode_info    total: 1.02 MiB active: 922 KiB
00177 perf_event        total: 1.02 MiB active: 1.02 MiB
00177 biovec-max        total: 992 KiB active: 960 KiB
00177 Shrinkers:
00177 super_cache_scan: objects: 127
00177 super_cache_scan: objects: 106
00177 jbd2_journal_shrink_scan: objects: 32
00177 ext4_es_scan: objects: 32
00177 bch2_btree_cache_scan: objects: 8
00177   nr nodes:          24
00177   nr dirty:          0
00177   cannibalize lock:  0000000000000000
00177
00177 super_cache_scan: objects: 8
00177 super_cache_scan: objects: 1

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: linux-mm@kvack.org
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 mm/oom_kill.c    | 23 ---------------------
 mm/show_mem.c    | 43 +++++++++++++++++++++++++++++++++++++++
 mm/slab.h        |  6 ++++--
 mm/slab_common.c | 52 +++++++++++++++++++++++++++++++++++++++---------
 4 files changed, 90 insertions(+), 34 deletions(-)

diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 4d7a0004df2c..dc56239ff057 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -169,27 +169,6 @@ static bool oom_unkillable_task(struct task_struct *p)
 	return false;
 }
 
-/*
- * Check whether unreclaimable slab amount is greater than
- * all user memory(LRU pages).
- * dump_unreclaimable_slab() could help in the case that
- * oom due to too much unreclaimable slab used by kernel.
-*/
-static bool should_dump_unreclaim_slab(void)
-{
-	unsigned long nr_lru;
-
-	nr_lru = global_node_page_state(NR_ACTIVE_ANON) +
-		 global_node_page_state(NR_INACTIVE_ANON) +
-		 global_node_page_state(NR_ACTIVE_FILE) +
-		 global_node_page_state(NR_INACTIVE_FILE) +
-		 global_node_page_state(NR_ISOLATED_ANON) +
-		 global_node_page_state(NR_ISOLATED_FILE) +
-		 global_node_page_state(NR_UNEVICTABLE);
-
-	return (global_node_page_state_pages(NR_SLAB_UNRECLAIMABLE_B) > nr_lru);
-}
-
 /**
  * oom_badness - heuristic function to determine which candidate task to kill
  * @p: task struct of which task we should calculate
@@ -464,8 +443,6 @@ static void dump_header(struct oom_control *oc)
 		mem_cgroup_print_oom_meminfo(oc->memcg);
 	else {
 		__show_mem(SHOW_MEM_FILTER_NODES, oc->nodemask, gfp_zone(oc->gfp_mask));
-		if (should_dump_unreclaim_slab())
-			dump_unreclaimable_slab();
 	}
 	if (sysctl_oom_dump_tasks)
 		dump_tasks(oc);
diff --git a/mm/show_mem.c b/mm/show_mem.c
index bdb439551eef..a8ea4c41ced5 100644
--- a/mm/show_mem.c
+++ b/mm/show_mem.c
@@ -7,15 +7,18 @@
 
 #include <linux/blkdev.h>
 #include <linux/cma.h>
+#include <linux/console.h>
 #include <linux/cpuset.h>
 #include <linux/highmem.h>
 #include <linux/hugetlb.h>
 #include <linux/mm.h>
 #include <linux/mmzone.h>
+#include <linux/seq_buf.h>
 #include <linux/swap.h>
 #include <linux/vmstat.h>
 
 #include "internal.h"
+#include "slab.h"
 #include "swap.h"
 
 atomic_long_t _totalram_pages __read_mostly;
@@ -397,10 +400,31 @@ static void show_free_areas(unsigned int filter, nodemask_t *nodemask, int max_z
 	show_swap_cache_info();
 }
 
+static void print_string_as_lines(const char *prefix, const char *lines)
+{
+	if (!lines) {
+		printk("%s (null)\n", prefix);
+		return;
+	}
+
+	bool locked = console_trylock();
+
+	while (1) {
+		const char *p = strchrnul(lines, '\n');
+		printk("%s%.*s\n", prefix, (int) (p - lines), lines);
+		if (!*p)
+			break;
+		lines = p + 1;
+	}
+	if (locked)
+		console_unlock();
+}
+
 void __show_mem(unsigned int filter, nodemask_t *nodemask, int max_zone_idx)
 {
 	unsigned long total = 0, reserved = 0, highmem = 0;
 	struct zone *zone;
+	char *buf;
 
 	printk("Mem-Info:\n");
 	show_free_areas(filter, nodemask, max_zone_idx);
@@ -449,4 +473,23 @@ void __show_mem(unsigned int filter, nodemask_t *nodemask, int max_zone_idx)
 		}
 	}
 #endif
+
+	const unsigned buf_size = 8192;
+	buf = kmalloc(buf_size, GFP_ATOMIC);
+	if (buf) {
+		struct seq_buf s;
+
+		printk("Unreclaimable slab info:\n");
+		seq_buf_init(&s, buf, buf_size);
+		dump_unreclaimable_slab(&s);
+		print_string_as_lines(KERN_NOTICE, seq_buf_str(&s));
+
+		printk("Shrinkers:\n");
+		seq_buf_init(&s, buf, buf_size);
+		shrinkers_to_text(&s);
+		print_string_as_lines(KERN_NOTICE, seq_buf_str(&s));
+		/* previous output doesn't get flushed without this - why? */
+
+		kfree(buf);
+	}
 }
diff --git a/mm/slab.h b/mm/slab.h
index dcdb56b8e7f5..b523b3e3d9d3 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -611,10 +611,12 @@ static inline size_t slab_ksize(const struct kmem_cache *s)
 	return s->size;
 }
 
+struct seq_buf;
+
 #ifdef CONFIG_SLUB_DEBUG
-void dump_unreclaimable_slab(void);
+void dump_unreclaimable_slab(struct seq_buf *);
 #else
-static inline void dump_unreclaimable_slab(void)
+static inline void dump_unreclaimable_slab(struct seq_buf *out)
 {
 }
 #endif
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 40b582a014b8..bd50a57161cf 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -27,6 +27,7 @@
 #include <asm/tlbflush.h>
 #include <asm/page.h>
 #include <linux/memcontrol.h>
+#include <linux/seq_buf.h>
 #include <linux/stackdepot.h>
 
 #include "internal.h"
@@ -1181,10 +1182,15 @@ static int slab_show(struct seq_file *m, void *p)
 	return 0;
 }
 
-void dump_unreclaimable_slab(void)
+void dump_unreclaimable_slab(struct seq_buf *out)
 {
 	struct kmem_cache *s;
 	struct slabinfo sinfo;
+	struct slab_by_mem {
+		struct kmem_cache *s;
+		size_t total, active;
+	} slabs_by_mem[10], n;
+	int i, nr = 0;
 
 	/*
 	 * Here acquiring slab_mutex is risky since we don't prefer to get
@@ -1194,24 +1200,52 @@ void dump_unreclaimable_slab(void)
 	 * without acquiring the mutex.
 	 */
 	if (!mutex_trylock(&slab_mutex)) {
-		pr_warn("excessive unreclaimable slab but cannot dump stats\n");
+		seq_buf_puts(out, "excessive unreclaimable slab but cannot dump stats\n");
 		return;
 	}
 
-	pr_info("Unreclaimable slab info:\n");
-	pr_info("Name                      Used          Total\n");
-
 	list_for_each_entry(s, &slab_caches, list) {
 		if (s->flags & SLAB_RECLAIM_ACCOUNT)
 			continue;
 
 		get_slabinfo(s, &sinfo);
 
-		if (sinfo.num_objs > 0)
-			pr_info("%-17s %10luKB %10luKB\n", s->name,
-				(sinfo.active_objs * s->size) / 1024,
-				(sinfo.num_objs * s->size) / 1024);
+		if (!sinfo.num_objs)
+			continue;
+
+		n.s = s;
+		n.total = sinfo.num_objs * s->size;
+		n.active = sinfo.active_objs * s->size;
+
+		for (i = 0; i < nr; i++)
+			if (n.total < slabs_by_mem[i].total)
+				break;
+
+		if (nr < ARRAY_SIZE(slabs_by_mem)) {
+			memmove(&slabs_by_mem[i + 1],
+				&slabs_by_mem[i],
+				sizeof(slabs_by_mem[0]) * (nr - i));
+			nr++;
+		} else if (i) {
+			i--;
+			memmove(&slabs_by_mem[0],
+				&slabs_by_mem[1],
+				sizeof(slabs_by_mem[0]) * i);
+		} else {
+			continue;
+		}
+
+		slabs_by_mem[i] = n;
 	}
+
+	for (i = nr - 1; i >= 0; --i) {
+		seq_buf_printf(out, "%-17s total: ", slabs_by_mem[i].s->name);
+		seq_buf_human_readable_u64(out, slabs_by_mem[i].total, STRING_UNITS_2);
+		seq_buf_printf(out, " active: ");
+		seq_buf_human_readable_u64(out, slabs_by_mem[i].active, STRING_UNITS_2);
+		seq_buf_putc(out, '\n');
+	}
+
 	mutex_unlock(&slab_mutex);
 }
 
-- 
2.45.2

[PATCH 05/10] mm: shrinker: Add shrinker_to_text() to debugfs interface

[Kent Overstreet]

Previously, we added shrinker_to_text() and hooked it up to the OOM
report - now, the same report is available via debugfs.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 mm/shrinker_debug.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index 12ea5486a3e9..39342aa9f4ca 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -2,6 +2,7 @@
 #include <linux/idr.h>
 #include <linux/slab.h>
 #include <linux/debugfs.h>
+#include <linux/seq_buf.h>
 #include <linux/seq_file.h>
 #include <linux/shrinker.h>
 #include <linux/memcontrol.h>
@@ -159,6 +160,21 @@ static const struct file_operations shrinker_debugfs_scan_fops = {
 	.write	 = shrinker_debugfs_scan_write,
 };
 
+static int shrinker_debugfs_report_show(struct seq_file *m, void *v)
+{
+	struct shrinker *shrinker = m->private;
+	char *bufp;
+	size_t buflen = seq_get_buf(m, &bufp);
+	struct seq_buf out;
+
+	seq_buf_init(&out, bufp, buflen);
+	shrinker_to_text(&out, shrinker);
+	seq_commit(m, seq_buf_used(&out));
+
+	return 0;
+}
+DEFINE_SHOW_ATTRIBUTE(shrinker_debugfs_report);
+
 int shrinker_debugfs_add(struct shrinker *shrinker)
 {
 	struct dentry *entry;
@@ -190,6 +206,8 @@ int shrinker_debugfs_add(struct shrinker *shrinker)
 			    &shrinker_debugfs_count_fops);
 	debugfs_create_file("scan", 0220, entry, shrinker,
 			    &shrinker_debugfs_scan_fops);
+	debugfs_create_file("report", 0440, entry, shrinker,
+			    &shrinker_debugfs_report_fops);
 	return 0;
 }
 
-- 
2.45.2

[PATCH 06/10] bcachefs: shrinker.to_text() methods

[Kent Overstreet]

This adds shrinker.to_text() methods for our shrinkers and hooks them up
to our existing to_text() functions.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 fs/bcachefs/btree_cache.c     | 13 +++++++++++++
 fs/bcachefs/btree_key_cache.c | 14 ++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/fs/bcachefs/btree_cache.c b/fs/bcachefs/btree_cache.c
index 662f0f79b7af..9f096fdcaf9a 100644
--- a/fs/bcachefs/btree_cache.c
+++ b/fs/bcachefs/btree_cache.c
@@ -15,6 +15,7 @@
 
 #include <linux/prefetch.h>
 #include <linux/sched/mm.h>
+#include <linux/seq_buf.h>
 
 #define BTREE_CACHE_NOT_FREED_INCREMENT(counter) \
 do {						 \
@@ -487,6 +488,17 @@ static unsigned long bch2_btree_cache_count(struct shrinker *shrink,
 	return btree_cache_can_free(bc);
 }
 
+static void bch2_btree_cache_shrinker_to_text(struct seq_buf *s, struct shrinker *shrink)
+{
+	struct bch_fs *c = shrink->private_data;
+	char *cbuf;
+	size_t buflen = seq_buf_get_buf(s, &cbuf);
+	struct printbuf out = PRINTBUF_EXTERN(cbuf, buflen);
+
+	bch2_btree_cache_to_text(&out, &c->btree_cache);
+	seq_buf_commit(s, out.pos);
+}
+
 void bch2_fs_btree_cache_exit(struct bch_fs *c)
 {
 	struct btree_cache *bc = &c->btree_cache;
@@ -570,6 +582,7 @@ int bch2_fs_btree_cache_init(struct bch_fs *c)
 	bc->shrink = shrink;
 	shrink->count_objects	= bch2_btree_cache_count;
 	shrink->scan_objects	= bch2_btree_cache_scan;
+	shrink->to_text		= bch2_btree_cache_shrinker_to_text;
 	shrink->seeks		= 4;
 	shrink->private_data	= c;
 	shrinker_register(shrink);
diff --git a/fs/bcachefs/btree_key_cache.c b/fs/bcachefs/btree_key_cache.c
index 2e49ca71194f..af84516fb607 100644
--- a/fs/bcachefs/btree_key_cache.c
+++ b/fs/bcachefs/btree_key_cache.c
@@ -13,6 +13,7 @@
 #include "trace.h"
 
 #include <linux/sched/mm.h>
+#include <linux/seq_buf.h>
 
 static inline bool btree_uses_pcpu_readers(enum btree_id id)
 {
@@ -746,6 +747,18 @@ void bch2_fs_btree_key_cache_init_early(struct btree_key_cache *c)
 {
 }
 
+static void bch2_btree_key_cache_shrinker_to_text(struct seq_buf *s, struct shrinker *shrink)
+{
+	struct bch_fs *c = shrink->private_data;
+	struct btree_key_cache *bc = &c->btree_key_cache;
+	char *cbuf;
+	size_t buflen = seq_buf_get_buf(s, &cbuf);
+	struct printbuf out = PRINTBUF_EXTERN(cbuf, buflen);
+
+	bch2_btree_key_cache_to_text(&out, bc);
+	seq_buf_commit(s, out.pos);
+}
+
 int bch2_fs_btree_key_cache_init(struct btree_key_cache *bc)
 {
 	struct bch_fs *c = container_of(bc, struct bch_fs, btree_key_cache);
@@ -770,6 +783,7 @@ int bch2_fs_btree_key_cache_init(struct btree_key_cache *bc)
 	bc->shrink = shrink;
 	shrink->count_objects	= bch2_btree_key_cache_count;
 	shrink->scan_objects	= bch2_btree_key_cache_scan;
+	shrink->to_text		= bch2_btree_key_cache_shrinker_to_text;
 	shrink->batch		= 1 << 14;
 	shrink->seeks		= 0;
 	shrink->private_data	= c;
-- 
2.45.2

[PATCH 07/10] percpu: per_cpu_sum()

[Kent Overstreet]

Add a little helper to replace open coded versions.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.krenel.org
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 fs/bcachefs/util.h     | 10 ----------
 fs/dcache.c            | 16 +++-------------
 include/linux/percpu.h | 10 ++++++++++
 3 files changed, 13 insertions(+), 23 deletions(-)

diff --git a/fs/bcachefs/util.h b/fs/bcachefs/util.h
index fb02c1c36004..e90c2f546007 100644
--- a/fs/bcachefs/util.h
+++ b/fs/bcachefs/util.h
@@ -584,16 +584,6 @@ do {									\
 	}								\
 } while (0)
 
-#define per_cpu_sum(_p)							\
-({									\
-	typeof(*_p) _ret = 0;						\
-									\
-	int cpu;							\
-	for_each_possible_cpu(cpu)					\
-		_ret += *per_cpu_ptr(_p, cpu);				\
-	_ret;								\
-})
-
 static inline u64 percpu_u64_get(u64 __percpu *src)
 {
 	return per_cpu_sum(src);
diff --git a/fs/dcache.c b/fs/dcache.c
index 3d8daaecb6d1..64108cbd52f6 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -151,29 +151,19 @@ static struct dentry_stat_t dentry_stat = {
  */
 static long get_nr_dentry(void)
 {
-	int i;
-	long sum = 0;
-	for_each_possible_cpu(i)
-		sum += per_cpu(nr_dentry, i);
+	long sum = per_cpu_sum(&nr_dentry);
 	return sum < 0 ? 0 : sum;
 }
 
 static long get_nr_dentry_unused(void)
 {
-	int i;
-	long sum = 0;
-	for_each_possible_cpu(i)
-		sum += per_cpu(nr_dentry_unused, i);
+	long sum = per_cpu_sum(&nr_dentry_unused);
 	return sum < 0 ? 0 : sum;
 }
 
 static long get_nr_dentry_negative(void)
 {
-	int i;
-	long sum = 0;
-
-	for_each_possible_cpu(i)
-		sum += per_cpu(nr_dentry_negative, i);
+	long sum = per_cpu_sum(&nr_dentry_negative);
 	return sum < 0 ? 0 : sum;
 }
 
diff --git a/include/linux/percpu.h b/include/linux/percpu.h
index 4b2047b78b67..0df28ff54f66 100644
--- a/include/linux/percpu.h
+++ b/include/linux/percpu.h
@@ -162,4 +162,14 @@ extern phys_addr_t per_cpu_ptr_to_phys(void *addr);
 
 extern unsigned long pcpu_nr_pages(void);
 
+#define per_cpu_sum(_p)							\
+({									\
+	typeof(*(_p)) sum = 0;						\
+	int cpu;							\
+									\
+	for_each_possible_cpu(cpu)					\
+		sum += *per_cpu_ptr(_p, cpu);				\
+	sum;								\
+})
+
 #endif /* __LINUX_PERCPU_H */
-- 
2.45.2

[PATCH 08/10] fs: Add super_block->s_inodes_nr

[Kent Overstreet]

Upcoming shrinker debugging patchset is going to give us a callback for
reporting on all memory owned by a shrinker.

This adds a counter for total number of inodes allocated for a given
superblock, so we can compare with the number of reclaimable inodes we
already have.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 fs/inode.c         | 2 ++
 fs/super.c         | 7 +++++++
 include/linux/fs.h | 1 +
 3 files changed, 10 insertions(+)

diff --git a/fs/inode.c b/fs/inode.c
index 5e7dcdeedd4d..2650c5ce74e1 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -489,12 +489,14 @@ void inode_sb_list_add(struct inode *inode)
 	spin_lock(&inode->i_sb->s_inode_list_lock);
 	list_add(&inode->i_sb_list, &inode->i_sb->s_inodes);
 	spin_unlock(&inode->i_sb->s_inode_list_lock);
+	this_cpu_inc(*inode->i_sb->s_inodes_nr);
 }
 EXPORT_SYMBOL_GPL(inode_sb_list_add);
 
 static inline void inode_sb_list_del(struct inode *inode)
 {
 	if (!list_empty(&inode->i_sb_list)) {
+		this_cpu_dec(*inode->i_sb->s_inodes_nr);
 		spin_lock(&inode->i_sb->s_inode_list_lock);
 		list_del_init(&inode->i_sb_list);
 		spin_unlock(&inode->i_sb->s_inode_list_lock);
diff --git a/fs/super.c b/fs/super.c
index b7913b55debc..b1b6ae491b6c 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -278,6 +278,7 @@ static void destroy_super_work(struct work_struct *work)
 	security_sb_free(s);
 	put_user_ns(s->s_user_ns);
 	kfree(s->s_subtype);
+	free_percpu(s->s_inodes_nr);
 	for (int i = 0; i < SB_FREEZE_LEVELS; i++)
 		percpu_free_rwsem(&s->s_writers.rw_sem[i]);
 	kfree(s);
@@ -298,6 +299,7 @@ static void destroy_unused_super(struct super_block *s)
 	super_unlock_excl(s);
 	list_lru_destroy(&s->s_dentry_lru);
 	list_lru_destroy(&s->s_inode_lru);
+	free_percpu(s->s_inodes_nr);
 	shrinker_free(s->s_shrink);
 	/* no delays needed */
 	destroy_super_work(&s->destroy_work);
@@ -375,6 +377,10 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 	s->s_time_min = TIME64_MIN;
 	s->s_time_max = TIME64_MAX;
 
+	s->s_inodes_nr = alloc_percpu(size_t);
+	if (!s->s_inodes_nr)
+		goto fail;
+
 	s->s_shrink = shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
 				     "sb-%s", type->name);
 	if (!s->s_shrink)
@@ -408,6 +414,7 @@ static void __put_super(struct super_block *s)
 		WARN_ON(s->s_dentry_lru.node);
 		WARN_ON(s->s_inode_lru.node);
 		WARN_ON(!list_empty(&s->s_mounts));
+		WARN_ON(per_cpu_sum(s->s_inodes_nr));
 		call_rcu(&s->rcu, destroy_super_rcu);
 	}
 }
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8fc4bad3b6ae..86636831b9d0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1346,6 +1346,7 @@ struct super_block {
 	/* s_inode_list_lock protects s_inodes */
 	spinlock_t		s_inode_list_lock ____cacheline_aligned_in_smp;
 	struct list_head	s_inodes;	/* all inodes */
+	size_t __percpu		*s_inodes_nr;
 
 	spinlock_t		s_inode_wblist_lock;
 	struct list_head	s_inodes_wb;	/* writeback inodes */
-- 
2.45.2

[PATCH 09/10] fs/dcache: Add per-sb accounting for nr dentries

[Kent Overstreet]

Like the previous patch, add a counter for total dentries, so we can
print total vs. reclaimable.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 fs/dcache.c        | 2 ++
 fs/super.c         | 6 ++++++
 include/linux/fs.h | 1 +
 3 files changed, 9 insertions(+)

diff --git a/fs/dcache.c b/fs/dcache.c
index 64108cbd52f6..4bbb2c87f824 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -601,6 +601,7 @@ static struct dentry *__dentry_kill(struct dentry *dentry)
 	else
 		spin_unlock(&dentry->d_lock);
 	this_cpu_dec(nr_dentry);
+	this_cpu_dec(*dentry->d_sb->s_dentry_nr);
 	if (dentry->d_op && dentry->d_op->d_release)
 		dentry->d_op->d_release(dentry);
 
@@ -1683,6 +1684,7 @@ static struct dentry *__d_alloc(struct super_block *sb, const struct qstr *name)
 	}
 
 	this_cpu_inc(nr_dentry);
+	this_cpu_inc(*sb->s_dentry_nr);
 
 	return dentry;
 }
diff --git a/fs/super.c b/fs/super.c
index b1b6ae491b6c..5b0fea6ff1cd 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -278,6 +278,7 @@ static void destroy_super_work(struct work_struct *work)
 	security_sb_free(s);
 	put_user_ns(s->s_user_ns);
 	kfree(s->s_subtype);
+	free_percpu(s->s_dentry_nr);
 	free_percpu(s->s_inodes_nr);
 	for (int i = 0; i < SB_FREEZE_LEVELS; i++)
 		percpu_free_rwsem(&s->s_writers.rw_sem[i]);
@@ -299,6 +300,7 @@ static void destroy_unused_super(struct super_block *s)
 	super_unlock_excl(s);
 	list_lru_destroy(&s->s_dentry_lru);
 	list_lru_destroy(&s->s_inode_lru);
+	free_percpu(s->s_dentry_nr);
 	free_percpu(s->s_inodes_nr);
 	shrinker_free(s->s_shrink);
 	/* no delays needed */
@@ -381,6 +383,10 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 	if (!s->s_inodes_nr)
 		goto fail;
 
+	s->s_dentry_nr = alloc_percpu(size_t);
+	if (!s->s_dentry_nr)
+		goto fail;
+
 	s->s_shrink = shrinker_alloc(SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE,
 				     "sb-%s", type->name);
 	if (!s->s_shrink)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 86636831b9d0..493fb8e72bf0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1332,6 +1332,7 @@ struct super_block {
 	 * There is no need to put them into separate cachelines.
 	 */
 	struct list_lru		s_dentry_lru;
+	size_t __percpu		*s_dentry_nr;
 	struct list_lru		s_inode_lru;
 	struct rcu_head		rcu;
 	struct work_struct	destroy_work;
-- 
2.45.2

[PATCH 10/10] fs: super_cache_to_text()

[Kent Overstreet]

Implement shrinker.to_text() for the superblock shrinker: print out nr
of dentries and inodes, total and shrinkable.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 fs/super.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/super.c b/fs/super.c
index 5b0fea6ff1cd..d3e43127e311 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -36,6 +36,7 @@
 #include <linux/lockdep.h>
 #include <linux/user_namespace.h>
 #include <linux/fs_context.h>
+#include <linux/seq_buf.h>
 #include <uapi/linux/mount.h>
 #include "internal.h"
 
@@ -270,6 +271,16 @@ static unsigned long super_cache_count(struct shrinker *shrink,
 	return total_objects;
 }
 
+static void super_cache_to_text(struct seq_buf *out, struct shrinker *shrink)
+{
+	struct super_block *sb = shrink->private_data;
+
+	seq_buf_printf(out, "inodes:   total %zu shrinkable %lu\n",
+		       per_cpu_sum(sb->s_inodes_nr), list_lru_count(&sb->s_inode_lru));
+	seq_buf_printf(out, "dentries: toal %zu shrinkbale %lu\n",
+		       per_cpu_sum(sb->s_dentry_nr), list_lru_count(&sb->s_dentry_lru));
+}
+
 static void destroy_super_work(struct work_struct *work)
 {
 	struct super_block *s = container_of(work, struct super_block,
@@ -394,6 +405,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 
 	s->s_shrink->scan_objects = super_cache_scan;
 	s->s_shrink->count_objects = super_cache_count;
+	s->s_shrink->to_text = super_cache_to_text;
 	s->s_shrink->batch = 1024;
 	s->s_shrink->private_data = s;
 
-- 
2.45.2

[PATCH 10/10] fs: super_cache_to_text()

[Kent Overstreet]

Implement shrinker.to_text() for the superblock shrinker: print out nr
of dentries and inodes, total and shrinkable.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 fs/super.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/fs/super.c b/fs/super.c
index 5b0fea6ff1cd..d3e43127e311 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -36,6 +36,7 @@
 #include <linux/lockdep.h>
 #include <linux/user_namespace.h>
 #include <linux/fs_context.h>
+#include <linux/seq_buf.h>
 #include <uapi/linux/mount.h>
 #include "internal.h"
 
@@ -270,6 +271,16 @@ static unsigned long super_cache_count(struct shrinker *shrink,
 	return total_objects;
 }
 
+static void super_cache_to_text(struct seq_buf *out, struct shrinker *shrink)
+{
+	struct super_block *sb = shrink->private_data;
+
+	seq_buf_printf(out, "inodes:   total %zu shrinkable %lu\n",
+		       per_cpu_sum(sb->s_inodes_nr), list_lru_count(&sb->s_inode_lru));
+	seq_buf_printf(out, "dentries: toal %zu shrinkbale %lu\n",
+		       per_cpu_sum(sb->s_dentry_nr), list_lru_count(&sb->s_dentry_lru));
+}
+
 static void destroy_super_work(struct work_struct *work)
 {
 	struct super_block *s = container_of(work, struct super_block,
@@ -394,6 +405,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 
 	s->s_shrink->scan_objects = super_cache_scan;
 	s->s_shrink->count_objects = super_cache_count;
+	s->s_shrink->to_text = super_cache_to_text;
 	s->s_shrink->batch = 1024;
 	s->s_shrink->private_data = s;
 
-- 
2.45.2

Re: [PATCH 10/10] fs: super_cache_to_text()

[Dave Chinner]

On Sat, Aug 24, 2024 at 03:10:17PM -0400, Kent Overstreet wrote:
> Implement shrinker.to_text() for the superblock shrinker: print out nr
> of dentries and inodes, total and shrinkable.
> 
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Christian Brauner <brauner@kernel.org>
> Cc: Dave Chinner <david@fromorbit.com>
> Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> ---
>  fs/super.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 5b0fea6ff1cd..d3e43127e311 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -36,6 +36,7 @@
>  #include <linux/lockdep.h>
>  #include <linux/user_namespace.h>
>  #include <linux/fs_context.h>
> +#include <linux/seq_buf.h>
>  #include <uapi/linux/mount.h>
>  #include "internal.h"
>  
> @@ -270,6 +271,16 @@ static unsigned long super_cache_count(struct shrinker *shrink,
>  	return total_objects;
>  }
>  
> +static void super_cache_to_text(struct seq_buf *out, struct shrinker *shrink)
> +{
> +	struct super_block *sb = shrink->private_data;
> +
> +	seq_buf_printf(out, "inodes:   total %zu shrinkable %lu\n",
> +		       per_cpu_sum(sb->s_inodes_nr), list_lru_count(&sb->s_inode_lru));
> +	seq_buf_printf(out, "dentries: toal %zu shrinkbale %lu\n",
> +		       per_cpu_sum(sb->s_dentry_nr), list_lru_count(&sb->s_dentry_lru));

There's no superblock identification in this output - how are we
supposed to take this information and relate it to the filesystems
that are mounted on the system?

Also, list_lru_count() only counts root memcg objects, so any inodes
and dentries accounted to memcgs and are freeable will not be
included in this output. For systems with lots of memcgs, that will
result in the superblock reporting lots of inodes and dentries, but
almost nothing being freeable. hence to do this correctly, there
needs to be per-node, per-memcg list_lru_count_one() iteration here...

-Dave.
-- 
Dave Chinner
david@fromorbit.com