linux-kernel - [PATCH v2] f2fs: fix missing discard for active segments

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Message-Id: <20250109122755.177926-1-guochunhai@vivo.com>
Date: Thu,  9 Jan 2025 05:27:55 -0700
From: Chunhai Guo <guochunhai@...o.com>
To: chao@...nel.org,
	jaegeuk@...nel.org
Cc: linux-f2fs-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org,
	Chunhai Guo <guochunhai@...o.com>
Subject: [PATCH v2] f2fs: fix missing discard for active segments

During a checkpoint, the current active segment X may not be handled
properly. This occurs when segment X has 0 valid blocks and a non-zero
number of discard blocks, for the following reasons:

locate_dirty_segment() does not mark any active segment as a prefree
segment. As a result, segment X is not included in dirty_segmap[PRE], and
f2fs_clear_prefree_segments() skips it when handling prefree segments.

add_discard_addrs() skips any segment with 0 valid blocks, so segment X is
also skipped.

Consequently, no `struct discard_cmd` is actually created for segment X.
However, the ckpt_valid_map and cur_valid_map of segment X are synced by
seg_info_to_raw_sit() during the current checkpoint process. As a result,
it cannot find the missing discard bits even in subsequent checkpoints.
Consequently, the value of sbi->discard_blks remains non-zero. Thus, when
f2fs is umounted, CP_TRIMMED_FLAG will not be set due to the non-zero
sbi->discard_blks.

Relevant code process:

f2fs_write_checkpoint()
    f2fs_flush_sit_entries()
         list_for_each_entry_safe(ses, tmp, head, set_list) {
             for_each_set_bit_from(segno, bitmap, end) {
                 ...
                 add_discard_addrs(sbi, cpc, false); // skip segment X due to its 0 valid blocks
                 ...
                 seg_info_to_raw_sit(); // sync ckpt_valid_map with cur_valid_map for segment X
                 ...
             }
         }
    f2fs_clear_prefree_segments(); // segment X is not included in dirty_segmap[PRE] and is skipped

Since add_discard_addrs() can handle active segments with non-zero valid
blocks, it is reasonable to fix this issue by allowing it to also handle
active segments with 0 valid blocks. 

Fixes: b29555505d81 ("f2fs: add key functions for small discards")
Signed-off-by: Chunhai Guo <guochunhai@...o.com>
---
v1: https://lore.kernel.org/linux-f2fs-devel/20241203065108.2763436-1-guochunhai@vivo.com/
v1->v2:
 - Modify the commit message to make it easier to understand.
 - Add fixes to the commit.
---
 fs/f2fs/segment.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 86e547f008f9..13ee73a3c481 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -2090,7 +2090,9 @@ static bool add_discard_addrs(struct f2fs_sb_info *sbi, struct cp_control *cpc,
 		return false;

 	if (!force) {
-		if (!f2fs_realtime_discard_enable(sbi) || !se->valid_blocks ||
+		if (!f2fs_realtime_discard_enable(sbi) ||
+			(!se->valid_blocks &&
+				!IS_CURSEG(sbi, cpc->trim_start)) ||
 			SM_I(sbi)->dcc_info->nr_discards >=
 				SM_I(sbi)->dcc_info->max_discards)
 			return false;
-- 
2.34.1