[f2fs-dev] [PATCH 2/3] f2fs: introduce max_write_bio_{folio, entry}_cnt module parameters

[Chao Yu via Linux-f2fs-devel <linux-f2fs-devel@lists.sourceforge.net>]

From: Chao Yu <chao@kernel.org>

Commit f32744a8b83d ("f2fs: introduce max_atc_write_bio_entry_cnt")
added a per-superblock sysfs entry, max_atc_write_bio_entry_cnt, to
offload the completion of a write bio to sbi->wq if it contains too
many entries and f2fs_write_end_io() is called in atomic context.

However, it has two issues:

1. The limit is used to bound the CPU latency of f2fs_write_end_io() in
   atomic context, which is a property of the platform (CPU and storage
   completion path) rather than of each f2fs instance, so a global knob
   set once at boot fits better than a per-superblock sysfs entry.

2. Pagecache folios and cached metadata blocks are completed in
   different loops with different per-entry costs, i.e.
   f2fs_write_end_bio() and f2fs_cache_write_end_bio(), so it's better
   to tune them separately rather than sharing a single threshold.

To address these issues, let's replace max_atc_write_bio_entry_cnt
sysfs entry with two module parameters:

- max_write_bio_folio_cnt: for write bios of pagecache folios, which
  are completed in f2fs_write_end_bio().
- max_write_bio_entry_cnt: for write bios of cached metadata blocks,
  which are completed in f2fs_cache_write_end_bio().

Both parameters are read-only at runtime, they can be set via
f2fs.max_write_bio_{folio,entry}_cnt=<n> in the kernel command line, or
via modprobe, and apply to all f2fs instances. The default value is
UINT_MAX, which means this functionality is disabled by default.

Suggested-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Chao Yu <chao@kernel.org>
---
 Documentation/ABI/testing/sysfs-fs-f2fs | 11 -----------
 fs/f2fs/data.c                          | 18 +++++++++++++++++-
 fs/f2fs/f2fs.h                          |  2 --
 fs/f2fs/super.c                         |  1 -
 fs/f2fs/sysfs.c                         |  2 --
 5 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs
index e7e968c6df92..65d45d8edc68 100644
--- a/Documentation/ABI/testing/sysfs-fs-f2fs
+++ b/Documentation/ABI/testing/sysfs-fs-f2fs
@@ -1014,17 +1014,6 @@ Description:	Every time a write operation completes f2fs_write_end_io() is
 		(atc) context. The default value for this attribute is UINT_MAX
 		which means that this functionality is disabled by default.
 
-What:		/sys/fs/f2fs/<disk>/max_atc_write_bio_entry_cnt
-Date:		October 2026
-Contact:	Chao Yu <chao@kernel.org>
-Description:	Every time a write operation completes f2fs_write_end_io() is
-		called. This function may be called from an atomic context,
-		e.g. from inside an interrupt handler. This attribute controls
-		the maximum number of entries (folios or cached blocks) in a
-		write bio that is completed in atomic (atc) context. The default
-		value for this attribute is UINT_MAX which means that this
-		functionality is disabled by default.
-
 What:		/sys/fs/f2fs/<disk>/pinned_area_max_secno
 Date:		August 2026
 Contact:	"Daeho Jeong" <daehojeong@google.com>
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 16ab2c693aa9..e2f5b95b5c6d 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -36,6 +36,16 @@ static struct kmem_cache *ffs_entry_slab;
 static mempool_t *bio_post_read_ctx_pool;
 static struct bio_set f2fs_bioset;
 
+static unsigned int max_write_bio_folio_cnt = UINT_MAX;
+module_param(max_write_bio_folio_cnt, uint, 0444);
+MODULE_PARM_DESC(max_write_bio_folio_cnt,
+		"Maximum number of folios in a write bio completed in atomic context");
+
+static unsigned int max_write_bio_entry_cnt = UINT_MAX;
+module_param(max_write_bio_entry_cnt, uint, 0444);
+MODULE_PARM_DESC(max_write_bio_entry_cnt,
+		"Maximum number of cache entries in a write bio completed in atomic context");
+
 struct f2fs_folio_state {
 	spinlock_t		state_lock;
 	unsigned int		read_pages_pending;
@@ -390,6 +400,12 @@ static void f2fs_write_end_io_work(struct work_struct *work)
 	f2fs_write_end_bio(bio);
 }
 
+static unsigned int f2fs_max_write_bio_cnt(struct bio *bio)
+{
+	return f2fs_is_cache_bio(bio) ? max_write_bio_entry_cnt :
+					max_write_bio_folio_cnt;
+}
+
 static void f2fs_write_end_io(struct bio *bio)
 {
 	struct f2fs_sb_info *sbi;
@@ -400,7 +416,7 @@ static void f2fs_write_end_io(struct bio *bio)
 
 	if (bio_in_atomic() &&
 	    (bio->bi_iter.bi_size > sbi->max_atc_write_bio_size ||
-	     F2FS_BIO(bio)->entry_cnt > sbi->max_atc_write_bio_entry_cnt)) {
+	     F2FS_BIO(bio)->entry_cnt > f2fs_max_write_bio_cnt(bio))) {
 		struct work_struct *w;
 
 		w = &container_of(bio, struct f2fs_bio, bio)->work;
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index d6b3955f9a6f..05a7e96febdf 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -1808,8 +1808,6 @@ struct f2fs_sb_info {
 	/* for bio operations */
 	/* Largest write bio size completed in atomic context (atc). */
 	u32 max_atc_write_bio_size;
-	/* Largest write bio entry count completed in atomic context (atc). */
-	u32 max_atc_write_bio_entry_cnt;
 	struct f2fs_bio_info *write_io[NR_PAGE_TYPE];	/* for write bios */
 	/* keep migration IO order for LFS mode */
 	struct f2fs_rwsem io_order_lock;
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 12a051ba51ac..bd79cbbeba20 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -5262,7 +5262,6 @@ static int f2fs_fill_super(struct super_block *sb, struct fs_context *fc)
 		goto free_sb_buf;
 	}
 	sbi->max_atc_write_bio_size = UINT_MAX;
-	sbi->max_atc_write_bio_entry_cnt = UINT_MAX;
 
 	INIT_WORK(&sbi->s_error_work, f2fs_record_error_work);
 	memcpy(sbi->errors, raw_super->s_errors, MAX_F2FS_ERRORS);
diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
index 430356e500af..3f50c291b355 100644
--- a/fs/f2fs/sysfs.c
+++ b/fs/f2fs/sysfs.c
@@ -1285,7 +1285,6 @@ F2FS_SBI_RW_ATTR(umount_discard_timeout, interval_time[UMOUNT_DISCARD_TIMEOUT]);
 F2FS_SBI_RW_ATTR(gc_pin_file_thresh, gc_pin_file_threshold);
 F2FS_SBI_RW_ATTR(gc_reclaimed_segments, gc_reclaimed_segs);
 F2FS_SBI_RW_ATTR(max_atc_write_bio_size, max_atc_write_bio_size);
-F2FS_SBI_RW_ATTR(max_atc_write_bio_entry_cnt, max_atc_write_bio_entry_cnt);
 F2FS_SBI_GENERAL_RW_ATTR(max_victim_search);
 F2FS_SBI_GENERAL_RW_ATTR(migration_granularity);
 F2FS_SBI_GENERAL_RW_ATTR(migration_window_granularity);
@@ -1538,7 +1537,6 @@ static struct attribute *f2fs_attrs[] = {
 	ATTR_LIST(gc_segment_mode),
 	ATTR_LIST(gc_reclaimed_segments),
 	ATTR_LIST(max_atc_write_bio_size),
-	ATTR_LIST(max_atc_write_bio_entry_cnt),
 	ATTR_LIST(max_fragment_chunk),
 	ATTR_LIST(max_fragment_hole),
 	ATTR_LIST(current_atomic_write),
-- 
2.49.0



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