linux-kernel - [PATCH] f2fs: fix to avoid migrating empty section

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Message-ID: <20250901020416.2172182-1-chao@kernel.org>
Date: Mon,  1 Sep 2025 10:04:15 +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>,
	Daeho Jeong <daehojeong@...gle.com>
Subject: [PATCH] f2fs: fix to avoid migrating empty section

It reports a bug from device w/ zufs:

F2FS-fs (dm-64): Inconsistent segment (173822) type [1, 0] in SSA and SIT
F2FS-fs (dm-64): Stopped filesystem due to reason: 4

Thread A				Thread B
- f2fs_expand_inode_data
 - f2fs_allocate_pinning_section
  - f2fs_gc_range
   - do_garbage_collect w/ segno #x
					- writepage
					 - f2fs_allocate_data_block
					  - new_curseg
					   - allocate segno #x

The root cause is: fallocate on pinning file may race w/ block allocation
as above, result in do_garbage_collect() from fallocate() may migrate
segment which is just allocated by a log, the log will update segment type
in its in-memory structure, however GC will get segment type from on-disk
SSA block, once segment type changes by log, we can detect such
inconsistency, then shutdown filesystem.

In this case, on-disk SSA shows type of segno #173822 is 1 (SUM_TYPE_NODE),
however segno #173822 was just allocated as data type segment, so in-memory
SIT shows type of segno #173822 is 0 (SUM_TYPE_DATA).

Change as below to fix this issue:
- check whether current section is empty before gc
- add sanity checks on do_garbage_collect() to avoid any race case, result
in migrating segment used by log.
- btw, it fixes misc issue in printed logs: "SSA and SIT" -> "SIT and SSA".

Fixes: 9703d69d9d15 ("f2fs: support file pinning for zoned devices")
Cc: Daeho Jeong <daehojeong@...gle.com>
Signed-off-by: Chao Yu <chao@...nel.org>
---
 fs/f2fs/gc.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index ed3acbfc83ca..a7708cf80c04 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -1794,6 +1794,13 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
 		struct folio *sum_folio = filemap_get_folio(META_MAPPING(sbi),
 					GET_SUM_BLOCK(sbi, segno));
 
+		if (is_cursec(sbi, GET_SEC_FROM_SEG(sbi, segno))) {
+			f2fs_err(sbi, "%s: segment %u is used by log",
+							__func__, segno);
+			f2fs_bug_on(sbi, 1);
+			goto skip;
+		}
+
 		if (get_valid_blocks(sbi, segno, false) == 0)
 			goto freed;
 		if (gc_type == BG_GC && __is_large_section(sbi) &&
@@ -1805,7 +1812,7 @@ static int do_garbage_collect(struct f2fs_sb_info *sbi,
 
 		sum = folio_address(sum_folio);
 		if (type != GET_SUM_TYPE((&sum->footer))) {
-			f2fs_err(sbi, "Inconsistent segment (%u) type [%d, %d] in SSA and SIT",
+			f2fs_err(sbi, "Inconsistent segment (%u) type [%d, %d] in SIT and SSA",
 				 segno, type, GET_SUM_TYPE((&sum->footer)));
 			f2fs_stop_checkpoint(sbi, false,
 				STOP_CP_REASON_CORRUPTED_SUMMARY);
@@ -2068,6 +2075,13 @@ int f2fs_gc_range(struct f2fs_sb_info *sbi,
 			.iroot = RADIX_TREE_INIT(gc_list.iroot, GFP_NOFS),
 		};
 
+		/*
+		 * avoid migrating empty section, as it can be allocated by
+		 * log in parallel.
+		 */
+		if (!get_valid_blocks(sbi, segno, true))
+			continue;
+
 		if (is_cursec(sbi, GET_SEC_FROM_SEG(sbi, segno)))
 			continue;
 
-- 
2.49.0