On Tue, 17 Mar 2026 16:46:21 +0800
Li Chen <[email protected]> wrote:
> Commit-time fast commit snapshots run under jbd2_journal_lock_updates(),
> so it is useful to quantify the time spent with updates locked and to
> understand why snapshotting can fail.
>
> Add a new tracepoint, ext4_fc_lock_updates, reporting the time spent in
> the updates-locked window along with the number of snapshotted inodes
> and ranges. Record the first snapshot failure reason in a stable snap_err
> field for tooling.
>
> Signed-off-by: Li Chen <[email protected]>
> ---
> fs/ext4/ext4.h | 15 ++++++++
> fs/ext4/fast_commit.c | 71 +++++++++++++++++++++++++++++--------
> include/trace/events/ext4.h | 61 +++++++++++++++++++++++++++++++
> 3 files changed, 132 insertions(+), 15 deletions(-)
>
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 68a64fa0be926..b9e146f3dd9e4 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1037,6 +1037,21 @@ enum {
>
> struct ext4_fc_inode_snap;
>
> +/*
> + * Snapshot failure reasons for ext4_fc_lock_updates tracepoint.
> + * Keep these stable for tooling.
> + */
> +enum ext4_fc_snap_err {
> + EXT4_FC_SNAP_ERR_NONE = 0,
> + EXT4_FC_SNAP_ERR_ES_MISS = 1,
> + EXT4_FC_SNAP_ERR_ES_DELAYED = 2,
> + EXT4_FC_SNAP_ERR_ES_OTHER = 3,
> + EXT4_FC_SNAP_ERR_INODES_CAP = 4,
> + EXT4_FC_SNAP_ERR_RANGES_CAP = 5,
> + EXT4_FC_SNAP_ERR_NOMEM = 6,
> + EXT4_FC_SNAP_ERR_INODE_LOC = 7,
You don't need to explicitly state the assignments, the enum will increment
them without them.
> +};
> +
> /*
> * fourth extended file system inode data in memory
> */
> diff --git a/fs/ext4/fast_commit.c b/fs/ext4/fast_commit.c
> index d1eefee609120..4929e2990b292 100644
> --- a/fs/ext4/fast_commit.c
> +++ b/fs/ext4/fast_commit.c
> @@ -193,6 +193,12 @@ static struct kmem_cache *ext4_fc_range_cachep;
> #define EXT4_FC_SNAPSHOT_MAX_INODES 1024
> #define EXT4_FC_SNAPSHOT_MAX_RANGES 2048
>
> +static inline void ext4_fc_set_snap_err(int *snap_err, int err)
> +{
> + if (snap_err && *snap_err == EXT4_FC_SNAP_ERR_NONE)
> + *snap_err = err;
> +}
> +
> static void ext4_end_buffer_io_sync(struct buffer_head *bh, int uptodate)
> {
> BUFFER_TRACE(bh, "");
> @@ -983,11 +989,12 @@ static void ext4_fc_free_inode_snap(struct inode *inode)
> static int ext4_fc_snapshot_inode_data(struct inode *inode,
> struct list_head *ranges,
> unsigned int nr_ranges_total,
> - unsigned int *nr_rangesp)
> + unsigned int *nr_rangesp,
> + int *snap_err)
> {
> struct ext4_inode_info *ei = EXT4_I(inode);
> - unsigned int nr_ranges = 0;
> ext4_lblk_t start_lblk, end_lblk, cur_lblk;
> + unsigned int nr_ranges = 0;
>
> spin_lock(&ei->i_fc_lock);
> if (ei->i_fc_lblk_len == 0) {
> @@ -1010,11 +1017,16 @@ static int ext4_fc_snapshot_inode_data(struct inode
> *inode,
> struct ext4_fc_range *range;
> ext4_lblk_t len;
>
> - if (!ext4_es_lookup_extent(inode, cur_lblk, NULL, &es, NULL))
> + if (!ext4_es_lookup_extent(inode, cur_lblk, NULL, &es, NULL)) {
> + ext4_fc_set_snap_err(snap_err,
> EXT4_FC_SNAP_ERR_ES_MISS);
> return -EAGAIN;
> + }
>
> - if (ext4_es_is_delayed(&es))
> + if (ext4_es_is_delayed(&es)) {
> + ext4_fc_set_snap_err(snap_err,
> + EXT4_FC_SNAP_ERR_ES_DELAYED);
> return -EAGAIN;
> + }
>
> len = es.es_len - (cur_lblk - es.es_lblk);
> if (len > end_lblk - cur_lblk + 1)
> @@ -1024,12 +1036,17 @@ static int ext4_fc_snapshot_inode_data(struct inode
> *inode,
> continue;
> }
>
> - if (nr_ranges_total + nr_ranges >= EXT4_FC_SNAPSHOT_MAX_RANGES)
> + if (nr_ranges_total + nr_ranges >= EXT4_FC_SNAPSHOT_MAX_RANGES)
> {
> + ext4_fc_set_snap_err(snap_err,
> + EXT4_FC_SNAP_ERR_RANGES_CAP);
> return -E2BIG;
> + }
>
> range = kmem_cache_alloc(ext4_fc_range_cachep, GFP_NOFS);
> - if (!range)
> + if (!range) {
> + ext4_fc_set_snap_err(snap_err, EXT4_FC_SNAP_ERR_NOMEM);
> return -ENOMEM;
> + }
> nr_ranges++;
>
> range->lblk = cur_lblk;
> @@ -1054,6 +1071,7 @@ static int ext4_fc_snapshot_inode_data(struct inode
> *inode,
> range->len = max;
> } else {
> kmem_cache_free(ext4_fc_range_cachep, range);
> + ext4_fc_set_snap_err(snap_err,
> EXT4_FC_SNAP_ERR_ES_OTHER);
> return -EAGAIN;
> }
>
> @@ -1070,7 +1088,7 @@ static int ext4_fc_snapshot_inode_data(struct inode
> *inode,
>
> static int ext4_fc_snapshot_inode(struct inode *inode,
> unsigned int nr_ranges_total,
> - unsigned int *nr_rangesp)
> + unsigned int *nr_rangesp, int *snap_err)
> {
> struct ext4_inode_info *ei = EXT4_I(inode);
> struct ext4_fc_inode_snap *snap;
> @@ -1082,8 +1100,10 @@ static int ext4_fc_snapshot_inode(struct inode *inode,
> int alloc_ctx;
>
> ret = ext4_get_inode_loc_noio(inode, &iloc);
> - if (ret)
> + if (ret) {
> + ext4_fc_set_snap_err(snap_err, EXT4_FC_SNAP_ERR_INODE_LOC);
> return ret;
> + }
>
> if (ext4_test_inode_flag(inode, EXT4_INODE_INLINE_DATA))
> inode_len = EXT4_INODE_SIZE(inode->i_sb);
> @@ -1092,6 +1112,7 @@ static int ext4_fc_snapshot_inode(struct inode *inode,
>
> snap = kmalloc(struct_size(snap, inode_buf, inode_len), GFP_NOFS);
> if (!snap) {
> + ext4_fc_set_snap_err(snap_err, EXT4_FC_SNAP_ERR_NOMEM);
> brelse(iloc.bh);
> return -ENOMEM;
> }
> @@ -1102,7 +1123,7 @@ static int ext4_fc_snapshot_inode(struct inode *inode,
> brelse(iloc.bh);
>
> ret = ext4_fc_snapshot_inode_data(inode, &ranges, nr_ranges_total,
> - &nr_ranges);
> + &nr_ranges, snap_err);
> if (ret) {
> kfree(snap);
> ext4_fc_free_ranges(&ranges);
> @@ -1203,7 +1224,10 @@ static int ext4_fc_alloc_snapshot_inodes(struct
> super_block *sb,
> unsigned int *nr_inodesp);
>
> static int ext4_fc_snapshot_inodes(journal_t *journal, struct inode **inodes,
> - unsigned int inodes_size)
> + unsigned int inodes_size,
> + unsigned int *nr_inodesp,
> + unsigned int *nr_rangesp,
> + int *snap_err)
> {
> struct super_block *sb = journal->j_private;
> struct ext4_sb_info *sbi = EXT4_SB(sb);
> @@ -1221,6 +1245,8 @@ static int ext4_fc_snapshot_inodes(journal_t *journal,
> struct inode **inodes,
> alloc_ctx = ext4_fc_lock(sb);
> list_for_each_entry(iter, &sbi->s_fc_q[FC_Q_MAIN], i_fc_list) {
> if (i >= inodes_size) {
> + ext4_fc_set_snap_err(snap_err,
> + EXT4_FC_SNAP_ERR_INODES_CAP);
> ret = -E2BIG;
> goto unlock;
> }
> @@ -1244,6 +1270,8 @@ static int ext4_fc_snapshot_inodes(journal_t *journal,
> struct inode **inodes,
> continue;
>
> if (i >= inodes_size) {
> + ext4_fc_set_snap_err(snap_err,
> + EXT4_FC_SNAP_ERR_INODES_CAP);
> ret = -E2BIG;
> goto unlock;
> }
> @@ -1268,16 +1296,20 @@ static int ext4_fc_snapshot_inodes(journal_t
> *journal, struct inode **inodes,
> unsigned int inode_ranges = 0;
>
> ret = ext4_fc_snapshot_inode(inodes[idx], nr_ranges,
> - &inode_ranges);
> + &inode_ranges, snap_err);
> if (ret)
> break;
> nr_ranges += inode_ranges;
> }
>
> + if (nr_inodesp)
> + *nr_inodesp = i;
> + if (nr_rangesp)
> + *nr_rangesp = nr_ranges;
> return ret;
> }
>
> -static int ext4_fc_perform_commit(journal_t *journal)
> +static int ext4_fc_perform_commit(journal_t *journal, tid_t commit_tid)
> {
> struct super_block *sb = journal->j_private;
> struct ext4_sb_info *sbi = EXT4_SB(sb);
> @@ -1286,10 +1318,15 @@ static int ext4_fc_perform_commit(journal_t *journal)
> struct inode *inode;
> struct inode **inodes;
> unsigned int inodes_size;
> + unsigned int snap_inodes = 0;
> + unsigned int snap_ranges = 0;
> + int snap_err = EXT4_FC_SNAP_ERR_NONE;
> struct blk_plug plug;
> int ret = 0;
> u32 crc = 0;
> int alloc_ctx;
> + ktime_t lock_start;
> + u64 locked_ns;
>
> /*
> * Step 1: Mark all inodes on s_fc_q[MAIN] with
> @@ -1337,13 +1374,13 @@ static int ext4_fc_perform_commit(journal_t *journal)
> if (ret)
> return ret;
>
> -
> ret = ext4_fc_alloc_snapshot_inodes(sb, &inodes, &inodes_size);
> if (ret)
> return ret;
>
> /* Step 4: Mark all inodes as being committed. */
> jbd2_journal_lock_updates(journal);
> + lock_start = ktime_get();
> /*
> * The journal is now locked. No more handles can start and all the
> * previous handles are now drained. Snapshotting happens in this
> @@ -1357,8 +1394,12 @@ static int ext4_fc_perform_commit(journal_t *journal)
> }
> ext4_fc_unlock(sb, alloc_ctx);
>
> - ret = ext4_fc_snapshot_inodes(journal, inodes, inodes_size);
> + ret = ext4_fc_snapshot_inodes(journal, inodes, inodes_size,
> + &snap_inodes, &snap_ranges, &snap_err);
> jbd2_journal_unlock_updates(journal);
> + locked_ns = ktime_to_ns(ktime_sub(ktime_get(), lock_start));
If locked_ns is only used for the tracepoint, it should either be
calculated in the tracepoint, or add:
if (trace_ext4_fc_lock_updates_enabled()) {
locked_ns = ktime_to_ns(ktime_sub(ktime_get(), lock_start));
> + trace_ext4_fc_lock_updates(sb, commit_tid, locked_ns, snap_inodes,
> + snap_ranges, ret, snap_err);
}
Note, we are going to also add a code to call the tracepoint directly, to
remove the double static_branch.
https://lore.kernel.org/all/[email protected]/
But that code is still being worked on so you don't need to worry about it
at the moment.
-- Steve
> kvfree(inodes);
> if (ret)
> return ret;
> @@ -1563,7 +1604,7 @@ int ext4_fc_commit(journal_t *journal, tid_t commit_tid)
> journal_ioprio = EXT4_DEF_JOURNAL_IOPRIO;
> set_task_ioprio(current, journal_ioprio);
> fc_bufs_before = (sbi->s_fc_bytes + bsize - 1) / bsize;
> - ret = ext4_fc_perform_commit(journal);
> + ret = ext4_fc_perform_commit(journal, commit_tid);
> if (ret < 0) {
> if (ret == -EAGAIN || ret == -E2BIG || ret == -ECANCELED)
> status = EXT4_FC_STATUS_INELIGIBLE;
> diff --git a/include/trace/events/ext4.h b/include/trace/events/ext4.h
> index fd76d14c2776e..dc084f39b74ad 100644
> --- a/include/trace/events/ext4.h
> +++ b/include/trace/events/ext4.h
> @@ -104,6 +104,26 @@ TRACE_DEFINE_ENUM(EXT4_FC_REASON_INODE_JOURNAL_DATA);
> TRACE_DEFINE_ENUM(EXT4_FC_REASON_ENCRYPTED_FILENAME);
> TRACE_DEFINE_ENUM(EXT4_FC_REASON_MAX);
>
> +#undef EM
> +#undef EMe
> +#define EM(a) TRACE_DEFINE_ENUM(EXT4_FC_SNAP_ERR_##a);
> +#define EMe(a) TRACE_DEFINE_ENUM(EXT4_FC_SNAP_ERR_##a);
> +
> +#define TRACE_SNAP_ERR \
> + EM(NONE) \
> + EM(ES_MISS) \
> + EM(ES_DELAYED) \
> + EM(ES_OTHER) \
> + EM(INODES_CAP) \
> + EM(RANGES_CAP) \
> + EM(NOMEM) \
> + EMe(INODE_LOC)
> +
> +TRACE_SNAP_ERR
> +
> +#undef EM
> +#undef EMe
> +
> #define show_fc_reason(reason)
> \
> __print_symbolic(reason, \
> { EXT4_FC_REASON_XATTR, "XATTR"}, \
> @@ -2812,6 +2832,47 @@ TRACE_EVENT(ext4_fc_commit_stop,
> __entry->num_fc_ineligible, __entry->nblks_agg, __entry->tid)
> );
>
> +#define EM(a) { EXT4_FC_SNAP_ERR_##a, #a },
> +#define EMe(a) { EXT4_FC_SNAP_ERR_##a, #a }
> +
> +TRACE_EVENT(ext4_fc_lock_updates,
> + TP_PROTO(struct super_block *sb, tid_t commit_tid, u64 locked_ns,
> + unsigned int nr_inodes, unsigned int nr_ranges, int err,
> + int snap_err),
> +
> + TP_ARGS(sb, commit_tid, locked_ns, nr_inodes, nr_ranges, err, snap_err),
> +
> + TP_STRUCT__entry(/* entry */
> + __field(dev_t, dev)
> + __field(tid_t, tid)
> + __field(u64, locked_ns)
> + __field(unsigned int, nr_inodes)
> + __field(unsigned int, nr_ranges)
> + __field(int, err)
> + __field(int, snap_err)
> + ),
> +
> + TP_fast_assign(/* assign */
> + __entry->dev = sb->s_dev;
> + __entry->tid = commit_tid;
> + __entry->locked_ns = locked_ns;
> + __entry->nr_inodes = nr_inodes;
> + __entry->nr_ranges = nr_ranges;
> + __entry->err = err;
> + __entry->snap_err = snap_err;
> + ),
> +
> + TP_printk("dev %d,%d tid %u locked_ns %llu nr_inodes %u nr_ranges %u
> err %d snap_err %s",
> + MAJOR(__entry->dev), MINOR(__entry->dev), __entry->tid,
> + __entry->locked_ns, __entry->nr_inodes, __entry->nr_ranges,
> + __entry->err, __print_symbolic(__entry->snap_err,
> + TRACE_SNAP_ERR))
> +);
> +
> +#undef EM
> +#undef EMe
> +#undef TRACE_SNAP_ERR
> +
> #define FC_REASON_NAME_STAT(reason) \
> show_fc_reason(reason), \
> __entry->fc_ineligible_rc[reason]
