linux-kernel - [PATCH v2 12/16] ext4: factor out ext4_mb_might

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Message-ID: <20250623073304.3275702-13-libaokun1@huawei.com>
Date: Mon, 23 Jun 2025 15:33:00 +0800
From: Baokun Li <libaokun1@...wei.com>
To: <linux-ext4@...r.kernel.org>
CC: <tytso@....edu>, <jack@...e.cz>, <adilger.kernel@...ger.ca>,
	<ojaswin@...ux.ibm.com>, <linux-kernel@...r.kernel.org>,
	<yi.zhang@...wei.com>, <yangerkun@...wei.com>, <libaokun1@...wei.com>
Subject: [PATCH v2 12/16] ext4: factor out ext4_mb_might_prefetch()

Extract ext4_mb_might_prefetch() to make the code clearer and to
prepare for the later conversion of 'choose group' to 'scan groups'.
No functional changes.

Signed-off-by: Baokun Li <libaokun1@...wei.com>
---
 fs/ext4/mballoc.c | 62 +++++++++++++++++++++++++++++------------------
 fs/ext4/mballoc.h |  4 +++
 2 files changed, 42 insertions(+), 24 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index db5d8b1e5cce..683e7f8faab6 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2782,6 +2782,37 @@ ext4_group_t ext4_mb_prefetch(struct super_block *sb, ext4_group_t group,
 	return group;
 }
 
+/*
+ * Batch reads of the block allocation bitmaps to get
+ * multiple READs in flight; limit prefetching at inexpensive
+ * CR, otherwise mballoc can spend a lot of time loading
+ * imperfect groups
+ */
+static void ext4_mb_might_prefetch(struct ext4_allocation_context *ac,
+				   ext4_group_t group)
+{
+	struct ext4_sb_info *sbi;
+
+	if (ac->ac_prefetch_grp != group)
+		return;
+
+	sbi = EXT4_SB(ac->ac_sb);
+	if (ext4_mb_cr_expensive(ac->ac_criteria) ||
+	    ac->ac_prefetch_ios < sbi->s_mb_prefetch_limit) {
+		unsigned int nr = sbi->s_mb_prefetch;
+
+		if (ext4_has_feature_flex_bg(ac->ac_sb)) {
+			nr = 1 << sbi->s_log_groups_per_flex;
+			nr -= group & (nr - 1);
+			nr = min(nr, sbi->s_mb_prefetch);
+		}
+
+		ac->ac_prefetch_nr = nr;
+		ac->ac_prefetch_grp = ext4_mb_prefetch(ac->ac_sb, group, nr,
+						       &ac->ac_prefetch_ios);
+	}
+}
+
 /*
  * Prefetching reads the block bitmap into the buffer cache; but we
  * need to make sure that the buddy bitmap in the page cache has been
@@ -2818,10 +2849,9 @@ void ext4_mb_prefetch_fini(struct super_block *sb, ext4_group_t group,
 static noinline_for_stack int
 ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 {
-	ext4_group_t prefetch_grp = 0, ngroups, group, i;
+	ext4_group_t ngroups, group, i;
 	enum criteria new_cr, cr = CR_GOAL_LEN_FAST;
 	int err = 0, first_err = 0;
-	unsigned int nr = 0, prefetch_ios = 0;
 	struct ext4_sb_info *sbi;
 	struct super_block *sb;
 	struct ext4_buddy e4b;
@@ -2881,6 +2911,7 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 		cr = CR_POWER2_ALIGNED;
 
 	ac->ac_e4b = &e4b;
+	ac->ac_prefetch_ios = 0;
 repeat:
 	for (; cr < EXT4_MB_NUM_CRS && ac->ac_status == AC_STATUS_CONTINUE; cr++) {
 		ac->ac_criteria = cr;
@@ -2890,8 +2921,8 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 		 */
 		group = ac->ac_g_ex.fe_group;
 		ac->ac_groups_linear_remaining = sbi->s_mb_max_linear_groups;
-		prefetch_grp = group;
-		nr = 0;
+		ac->ac_prefetch_grp = group;
+		ac->ac_prefetch_nr = 0;
 
 		for (i = 0, new_cr = cr; i < ngroups; i++,
 		     ext4_mb_choose_next_group(ac, &new_cr, &group, ngroups)) {
@@ -2903,24 +2934,7 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 				goto repeat;
 			}
 
-			/*
-			 * Batch reads of the block allocation bitmaps
-			 * to get multiple READs in flight; limit
-			 * prefetching at inexpensive CR, otherwise mballoc
-			 * can spend a lot of time loading imperfect groups
-			 */
-			if ((prefetch_grp == group) &&
-			    (ext4_mb_cr_expensive(cr) ||
-			     prefetch_ios < sbi->s_mb_prefetch_limit)) {
-				nr = sbi->s_mb_prefetch;
-				if (ext4_has_feature_flex_bg(sb)) {
-					nr = 1 << sbi->s_log_groups_per_flex;
-					nr -= group & (nr - 1);
-					nr = min(nr, sbi->s_mb_prefetch);
-				}
-				prefetch_grp = ext4_mb_prefetch(sb, group,
-							nr, &prefetch_ios);
-			}
+			ext4_mb_might_prefetch(ac, group);
 
 			/* prevent unnecessary buddy loading. */
 			if (cr < CR_ANY_FREE &&
@@ -3014,8 +3028,8 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 		 ac->ac_b_ex.fe_len, ac->ac_o_ex.fe_len, ac->ac_status,
 		 ac->ac_flags, cr, err);
 
-	if (nr)
-		ext4_mb_prefetch_fini(sb, prefetch_grp, nr);
+	if (ac->ac_prefetch_nr)
+		ext4_mb_prefetch_fini(sb, ac->ac_prefetch_grp, ac->ac_prefetch_nr);
 
 	return err;
 }
diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
index d61d690d237c..772ee0264d33 100644
--- a/fs/ext4/mballoc.h
+++ b/fs/ext4/mballoc.h
@@ -201,6 +201,10 @@ struct ext4_allocation_context {
 	 */
 	ext4_grpblk_t	ac_orig_goal_len;
 
+	ext4_group_t ac_prefetch_grp;
+	unsigned int ac_prefetch_ios;
+	unsigned int ac_prefetch_nr;
+
 	__u32 ac_flags;		/* allocation hints */
 	__u32 ac_groups_linear_remaining;
 	__u16 ac_groups_scanned;
-- 
2.46.1