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Message-ID: <20250410014945.2140781-9-mcgrof@kernel.org>
Date: Wed, 9 Apr 2025 18:49:45 -0700
From: Luis Chamberlain <mcgrof@...nel.org>
To: brauner@...nel.org,
jack@...e.cz,
tytso@....edu,
adilger.kernel@...ger.ca,
linux-ext4@...r.kernel.org,
riel@...riel.com
Cc: dave@...olabs.net,
willy@...radead.org,
hannes@...xchg.org,
oliver.sang@...el.com,
david@...hat.com,
axboe@...nel.dk,
hare@...e.de,
david@...morbit.com,
djwong@...nel.org,
ritesh.list@...il.com,
linux-fsdevel@...r.kernel.org,
linux-block@...r.kernel.org,
linux-mm@...ck.org,
gost.dev@...sung.com,
p.raghav@...sung.com,
da.gomez@...sung.com,
mcgrof@...nel.org
Subject: [PATCH v2 8/8] mm: add migration buffer-head debugfs interface
If you are working on enhancing folio migration it is easy to not
be certain on improvements. This debugfs interface enables you to
evaluate gains on improvements on buffer-head folio migration.
This can easily tell you *why* folio migration might fail, for example,
here is the output of a generic/750 run for 18 hours:
root@...ext4-2k ~ # cat /sys/kernel/debug/mm/migrate/bh/stats
[buffer_migrate_folio]
calls 50160811
success 50047572
fails 113239
[buffer_migrate_folio_norefs]
calls 23577082468
success 2939858
fails 23574142610
jbd-meta 23425956714
no-head-success 102
no-head-fails 0
invalid 147919982
valid 2939881
valid-success 2939756
valid-fails 125
Success ratios:
buffer_migrate_folio: 99% success (50047572/50160811)
buffer_migrate_folio_norefs: 0% success (2939858/23577082468)
Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
---
mm/migrate.c | 184 +++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 178 insertions(+), 6 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index 8fed2655f2e8..c478e8218cb0 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -44,6 +44,7 @@
#include <linux/sched/sysctl.h>
#include <linux/memory-tiers.h>
#include <linux/pagewalk.h>
+#include <linux/debugfs.h>
#include <asm/tlbflush.h>
@@ -791,6 +792,126 @@ int migrate_folio(struct address_space *mapping, struct folio *dst,
EXPORT_SYMBOL(migrate_folio);
#ifdef CONFIG_BUFFER_HEAD
+
+static const char * const bh_routine_names[] = {
+ "buffer_migrate_folio",
+ "buffer_migrate_folio_norefs",
+};
+
+#define BH_STATS(X) \
+ X(bh_migrate_folio, 0, "calls") \
+ X(bh_migrate_folio_success, 0, "success") \
+ X(bh_migrate_folio_fails, 0, "fails") \
+ X(bh_migrate_folio_norefs, 1, "calls") \
+ X(bh_migrate_folio_norefs_success, 1, "success") \
+ X(bh_migrate_folio_norefs_fails, 1, "fails") \
+ X(bh_migrate_folio_norefs_meta, 1, "jbd-meta") \
+ X(bh_migrate_folio_norefs_nohead_success, 1, "no-head-success") \
+ X(bh_migrate_folio_norefs_nohead_fails, 1, "no-head-fails") \
+ X(bh_migrate_folio_norefs_invalid, 1, "invalid") \
+ X(bh_migrate_folio_norefs_valid, 1, "valid") \
+ X(bh_migrate_folio_norefs_valid_success, 1, "valid-success") \
+ X(bh_migrate_folio_norefs_valid_fails, 1, "valid-fails")
+
+
+#define DECLARE_STAT(name, routine_idx, meaning) static atomic_long_t name;
+BH_STATS(DECLARE_STAT)
+
+#define BH_STAT_PTR(name, routine_idx, meaning) &name,
+static atomic_long_t * const bh_stat_array[] = {
+ BH_STATS(BH_STAT_PTR)
+};
+
+#define BH_STAT_ROUTINE_IDX(name, routine_idx, meaning) routine_idx,
+static const int bh_stat_routine_index[] = {
+ BH_STATS(BH_STAT_ROUTINE_IDX)
+};
+
+#define BH_STAT_MEANING(name, routine_idx, meaning) meaning,
+static const char * const bh_stat_meanings[] = {
+ BH_STATS(BH_STAT_MEANING)
+};
+
+#define NUM_BH_STATS ARRAY_SIZE(bh_stat_array)
+
+static ssize_t read_file_bh_migrate_stats(struct file *file,
+ char __user *user_buf,
+ size_t count, loff_t *ppos)
+{
+ char *buf;
+ unsigned int i, len = 0, size = NUM_BH_STATS * 128;
+ int ret, last_routine = -1;
+ unsigned long total, success, rate;
+
+ BUILD_BUG_ON(ARRAY_SIZE(bh_stat_array) != ARRAY_SIZE(bh_stat_meanings));
+
+ buf = kzalloc(size, GFP_KERNEL);
+ if (buf == NULL)
+ return -ENOMEM;
+
+ for (i = 0; i < NUM_BH_STATS; i++) {
+ int routine_idx = bh_stat_routine_index[i];
+
+ if (routine_idx != last_routine) {
+ len += scnprintf(buf + len, size - len, "\n[%s]\n",
+ bh_routine_names[routine_idx]);
+ last_routine = routine_idx;
+ }
+
+ len += scnprintf(buf + len, size - len, "%25s\t%lu\n",
+ bh_stat_meanings[i],
+ atomic_long_read(bh_stat_array[i]));
+
+ }
+
+ len += scnprintf(buf + len, size - len, "\nSuccess ratios:\n");
+
+ total = atomic_long_read(&bh_migrate_folio);
+ success = atomic_long_read(&bh_migrate_folio_success);
+ rate = total ? (success * 100) / total : 0;
+ len += scnprintf(buf + len, size - len,
+ "%s: %lu%% success (%lu/%lu)\n",
+ "buffer_migrate_folio", rate, success, total);
+
+ total = atomic_long_read(&bh_migrate_folio_norefs);
+ success = atomic_long_read(&bh_migrate_folio_norefs_success);
+ rate = total ? (success * 100) / total : 0;
+ len += scnprintf(buf + len, size - len,
+ "%s: %lu%% success (%lu/%lu)\n",
+ "buffer_migrate_folio_norefs", rate, success, total);
+
+ ret = simple_read_from_buffer(user_buf, count, ppos, buf, len);
+ kfree(buf);
+ return ret;
+}
+
+static const struct file_operations fops_bh_migrate_stats = {
+ .read = read_file_bh_migrate_stats,
+ .open = simple_open,
+ .owner = THIS_MODULE,
+ .llseek = default_llseek,
+};
+
+static void mm_migrate_bh_init(struct dentry *migrate_debug_root)
+{
+ struct dentry *parent_dirs[ARRAY_SIZE(bh_routine_names)] = { NULL };
+ struct dentry *root = debugfs_create_dir("bh", migrate_debug_root);
+ int i;
+
+ for (i = 0; i < ARRAY_SIZE(bh_routine_names); i++)
+ parent_dirs[i] = debugfs_create_dir(bh_routine_names[i], root);
+
+ for (i = 0; i < NUM_BH_STATS; i++) {
+ int routine = bh_stat_routine_index[i];
+ debugfs_create_ulong(bh_stat_meanings[i], 0400,
+ parent_dirs[routine],
+ (unsigned long *)
+ &bh_stat_array[i]->counter);
+ }
+
+ debugfs_create_file("stats", 0400, root, root, &fops_bh_migrate_stats);
+}
+
/* Returns true if all buffers are successfully locked */
static bool buffer_migrate_lock_buffers(struct buffer_head *head,
enum migrate_mode mode)
@@ -833,16 +954,26 @@ static int __buffer_migrate_folio(struct address_space *mapping,
int expected_count;
head = folio_buffers(src);
- if (!head)
- return migrate_folio(mapping, dst, src, mode);
+ if (!head) {
+ rc = migrate_folio(mapping, dst, src, mode);
+ if (check_refs) {
+ if (rc == 0)
+ atomic_long_inc(&bh_migrate_folio_norefs_nohead_success);
+ else
+ atomic_long_inc(&bh_migrate_folio_norefs_nohead_fails);
+ }
+ return rc;
+ }
/* Check whether page does not have extra refs before we do more work */
expected_count = folio_expected_refs(mapping, src);
if (folio_ref_count(src) != expected_count)
return -EAGAIN;
- if (buffer_meta(head))
+ if (buffer_meta(head)) {
+ atomic_long_inc(&bh_migrate_folio_norefs_meta);
return -EAGAIN;
+ }
if (!buffer_migrate_lock_buffers(head, mode))
return -EAGAIN;
@@ -868,17 +999,23 @@ static int __buffer_migrate_folio(struct address_space *mapping,
if (busy) {
if (invalidated) {
rc = -EAGAIN;
+ atomic_long_inc(&bh_migrate_folio_norefs_invalid);
goto unlock_buffers;
}
invalidate_bh_lrus();
invalidated = true;
goto recheck_buffers;
}
+ atomic_long_inc(&bh_migrate_folio_norefs_valid);
}
rc = filemap_migrate_folio(mapping, dst, src, mode);
- if (rc != MIGRATEPAGE_SUCCESS)
+ if (rc != MIGRATEPAGE_SUCCESS) {
+ if (check_refs)
+ atomic_long_inc(&bh_migrate_folio_norefs_valid_fails);
goto unlock_buffers;
+ } else if (check_refs)
+ atomic_long_inc(&bh_migrate_folio_norefs_valid_success);
bh = head;
do {
@@ -915,7 +1052,16 @@ static int __buffer_migrate_folio(struct address_space *mapping,
int buffer_migrate_folio(struct address_space *mapping,
struct folio *dst, struct folio *src, enum migrate_mode mode)
{
- return __buffer_migrate_folio(mapping, dst, src, mode, false);
+ int ret;
+ atomic_long_inc(&bh_migrate_folio);
+
+ ret = __buffer_migrate_folio(mapping, dst, src, mode, false);
+ if (ret == 0)
+ atomic_long_inc(&bh_migrate_folio_success);
+ else
+ atomic_long_inc(&bh_migrate_folio_fails);
+
+ return ret;
}
EXPORT_SYMBOL(buffer_migrate_folio);
@@ -936,9 +1082,21 @@ EXPORT_SYMBOL(buffer_migrate_folio);
int buffer_migrate_folio_norefs(struct address_space *mapping,
struct folio *dst, struct folio *src, enum migrate_mode mode)
{
- return __buffer_migrate_folio(mapping, dst, src, mode, true);
+ int ret;
+
+ atomic_long_inc(&bh_migrate_folio_norefs);
+
+ ret = __buffer_migrate_folio(mapping, dst, src, mode, true);
+ if (ret == 0)
+ atomic_long_inc(&bh_migrate_folio_norefs_success);
+ else
+ atomic_long_inc(&bh_migrate_folio_norefs_fails);
+
+ return ret;
}
EXPORT_SYMBOL_GPL(buffer_migrate_folio_norefs);
+#else
+static inline void mm_migrate_bh_init(struct dentry *migrate_debug_root) { }
#endif /* CONFIG_BUFFER_HEAD */
int filemap_migrate_folio(struct address_space *mapping,
@@ -2737,3 +2895,17 @@ int migrate_misplaced_folio(struct folio *folio, int node)
}
#endif /* CONFIG_NUMA_BALANCING */
#endif /* CONFIG_NUMA */
+
+static __init int mm_migrate_debugfs_init(void)
+{
+ struct dentry *mm_debug_root;
+ struct dentry *migrate_debug_root;
+
+ mm_debug_root = debugfs_create_dir("mm", NULL);
+ migrate_debug_root = debugfs_create_dir("migrate", mm_debug_root);
+
+ mm_migrate_bh_init(migrate_debug_root);
+
+ return 0;
+}
+fs_initcall(mm_migrate_debugfs_init);
--
2.47.2
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