Re: [PATCH v2] f2fs: fix missing discard for active segments

From: Chao Yu
Date: Mon Jan 20 2025 - 07:25:18 EST


On 1/9/25 20:27, Chunhai Guo wrote:
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

How does this happen? Allocator selects a dirty segment w/ SSR? and the
left valid data blocks were deleted later before following checkpoint?

If so, pending discard count in that segment should be in range of (0, 512)?

Thanks,

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@xxxxxxxx>
---
v1: https://lore.kernel.org/linux-f2fs-devel/20241203065108.2763436-1-guochunhai@xxxxxxxx/
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;