linux-ext4 - [RFC 09/11] ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs

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Message-Id: <7540e4069b22fce42dbef34ee0796d5cf5d82fe3.1674822311.git.ojaswin@linux.ibm.com>
Date:   Fri, 27 Jan 2023 18:07:36 +0530
From:   Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To:     linux-ext4@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>
Cc:     Ritesh Harjani <riteshh@...ux.ibm.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jan Kara <jack@...e.cz>, Ritesh Harjani <ritesh.list@...il.com>
Subject: [RFC 09/11] ext4: Ensure ext4_mb_prefetch_fini() is called for all prefetched BGs

Before this patch, the call stack in ext4_run_li_request is as follows:

  /*
   * nr = no. of BGs we want to fetch (=s_mb_prefetch)
   * prefetch_ios = no. of BGs not uptodate after
   * 		    ext4_read_block_bitmap_nowait()
   */
  next_group = ext4_mb_prefetch(sb, group, nr, prefetch_ios);
  ext4_mb_prefetch_fini(sb, next_group prefetch_ios);

ext4_mb_prefetch_fini() will only try to initialize buddies for BGs in
range [next_group - prefetch_ios, next_group). This is incorrect since
sometimes (prefetch_ios < nr), which causes ext4_mb_prefetch_fini() to
incorrectly ignore some of the BGs that might need initialization. This
issue is more notable now with the previous patch enabling "fetching" of
BLOCK_UNINIT BGs which are marked buffer_uptodate by default.

Fix this by passing nr to ext4_mb_prefetch_fini() instead of
prefetch_ios so that it considers the right range of groups.

Similarly, make sure we don't pass nr=0 to ext4_mb_prefetch_fini() in
ext4_mb_regular_allocator() since we might have prefetched BLOCK_UNINIT
groups that would need buddy initialization.

Signed-off-by: Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
---
 fs/ext4/mballoc.c |  4 ----
 fs/ext4/super.c   | 11 ++++-------
 2 files changed, 4 insertions(+), 11 deletions(-)

diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index 48726a831264..410c9636907b 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -2702,8 +2702,6 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 			if ((prefetch_grp == group) &&
 			    (cr > CR1 ||
 			     prefetch_ios < sbi->s_mb_prefetch_limit)) {
-				unsigned int curr_ios = prefetch_ios;
-
 				nr = sbi->s_mb_prefetch;
 				if (ext4_has_feature_flex_bg(sb)) {
 					nr = 1 << sbi->s_log_groups_per_flex;
@@ -2712,8 +2710,6 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 				}
 				prefetch_grp = ext4_mb_prefetch(sb, group,
 							nr, &prefetch_ios);
-				if (prefetch_ios == curr_ios)
-					nr = 0;
 			}
 
 			/* This now checks without needing the buddy page */
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 72ead3b56706..9dbb09cfc8f7 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3636,16 +3636,13 @@ static int ext4_run_li_request(struct ext4_li_request *elr)
 	ext4_group_t group = elr->lr_next_group;
 	unsigned int prefetch_ios = 0;
 	int ret = 0;
+	int nr = EXT4_SB(sb)->s_mb_prefetch;
 	u64 start_time;
 
 	if (elr->lr_mode == EXT4_LI_MODE_PREFETCH_BBITMAP) {
-		elr->lr_next_group = ext4_mb_prefetch(sb, group,
-				EXT4_SB(sb)->s_mb_prefetch, &prefetch_ios);
-		if (prefetch_ios)
-			ext4_mb_prefetch_fini(sb, elr->lr_next_group,
-					      prefetch_ios);
-		trace_ext4_prefetch_bitmaps(sb, group, elr->lr_next_group,
-					    prefetch_ios);
+		elr->lr_next_group = ext4_mb_prefetch(sb, group, nr, &prefetch_ios);
+		ext4_mb_prefetch_fini(sb, elr->lr_next_group, nr);
+		trace_ext4_prefetch_bitmaps(sb, group, elr->lr_next_group, nr);
 		if (group >= elr->lr_next_group) {
 			ret = 1;
 			if (elr->lr_first_not_zeroed != ngroups &&
-- 
2.31.1