linux-kernel - [PATCH v2] f2fs: fix deadloop in foreground GC

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Date:   Wed,  4 May 2022 00:24:37 +0800
From:   Chao Yu <chao@...nel.org>
To:     jaegeuk@...nel.org
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, Chao Yu <chao@...nel.org>,
        stable@...r.kernel.org, Ming Yan <yanming@....edu.cn>,
        Chao Yu <chao.yu@...o.com>
Subject: [PATCH v2] f2fs: fix deadloop in foreground GC

As Yanming reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=215914

The root cause is: in a very small sized image, it's very easy to
exceed threshold of foreground GC, if we calculate free space and
dirty data based on section granularity, in corner case,
has_not_enough_free_secs() will always return true, result in
deadloop in f2fs_gc().

So this patch refactors has_not_enough_free_secs() as below to fix
this issue:
1. calculate needed space based on block granularity, and separate
all blocks to two parts, section part, and block part, comparing
section part to free section, and comparing block part to free space
in openned log.
2. account F2FS_DIRTY_NODES, F2FS_DIRTY_IMETA and F2FS_DIRTY_DENTS
as node block consumer;
3. account F2FS_DIRTY_DENTS as data block consumer;

Cc: stable@...r.kernel.org
Reported-by: Ming Yan <yanming@....edu.cn>
Signed-off-by: Chao Yu <chao.yu@...o.com>
---
v2:
- fix performance regression
 fs/f2fs/segment.h | 32 ++++++++++++++++++++------------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/fs/f2fs/segment.h b/fs/f2fs/segment.h
index 8a591455d796..c38263dbc5ca 100644
--- a/fs/f2fs/segment.h
+++ b/fs/f2fs/segment.h
@@ -575,11 +575,10 @@ static inline int reserved_sections(struct f2fs_sb_info *sbi)
 	return GET_SEC_FROM_SEG(sbi, reserved_segments(sbi));
 }
 
-static inline bool has_curseg_enough_space(struct f2fs_sb_info *sbi)
+static inline bool has_curseg_enough_space(struct f2fs_sb_info *sbi,
+			unsigned int node_blocks, unsigned int dent_blocks)
 {
-	unsigned int node_blocks = get_pages(sbi, F2FS_DIRTY_NODES) +
-					get_pages(sbi, F2FS_DIRTY_DENTS);
-	unsigned int dent_blocks = get_pages(sbi, F2FS_DIRTY_DENTS);
+
 	unsigned int segno, left_blocks;
 	int i;
 
@@ -605,19 +604,28 @@ static inline bool has_curseg_enough_space(struct f2fs_sb_info *sbi)
 static inline bool has_not_enough_free_secs(struct f2fs_sb_info *sbi,
 					int freed, int needed)
 {
-	int node_secs = get_blocktype_secs(sbi, F2FS_DIRTY_NODES);
-	int dent_secs = get_blocktype_secs(sbi, F2FS_DIRTY_DENTS);
-	int imeta_secs = get_blocktype_secs(sbi, F2FS_DIRTY_IMETA);
+	unsigned int total_node_blocks = get_pages(sbi, F2FS_DIRTY_NODES) +
+					get_pages(sbi, F2FS_DIRTY_DENTS) +
+					get_pages(sbi, F2FS_DIRTY_IMETA);
+	unsigned int total_dent_blocks = get_pages(sbi, F2FS_DIRTY_DENTS);
+	unsigned int node_secs = total_node_blocks / BLKS_PER_SEC(sbi);
+	unsigned int dent_secs = total_dent_blocks / BLKS_PER_SEC(sbi);
+	unsigned int node_blocks = total_node_blocks % BLKS_PER_SEC(sbi);
+	unsigned int dent_blocks = total_dent_blocks % BLKS_PER_SEC(sbi);
+	unsigned int free, need_lower, need_upper;
 
 	if (unlikely(is_sbi_flag_set(sbi, SBI_POR_DOING)))
 		return false;
 
-	if (free_sections(sbi) + freed == reserved_sections(sbi) + needed &&
-			has_curseg_enough_space(sbi))
+	free = free_sections(sbi) + freed;
+	need_lower = node_secs + dent_secs + reserved_sections(sbi) + needed;
+	need_upper = need_lower + node_blocks ? 1 : 0 + dent_blocks ? 1 : 0;
+
+	if (free > need_upper)
 		return false;
-	return (free_sections(sbi) + freed) <=
-		(node_secs + 2 * dent_secs + imeta_secs +
-		reserved_sections(sbi) + needed);
+	else if (free <= need_lower)
+		return true;
+	return !has_curseg_enough_space(sbi, node_blocks, dent_blocks);
 }
 
 static inline bool f2fs_is_checkpoint_ready(struct f2fs_sb_info *sbi)
-- 
2.32.0