[PATCH v4 1/2] btrfs: fix periodic reclaim condition

[Sun YangKai <sunk67188@gmail.com>]

Problems with current implementation:
1. reclaimable_bytes is signed while chunk_sz is unsigned, causing
   negative reclaimable_bytes to trigger reclaim unexpectedly
2. The "space must be freed between scans" assumption breaks the
   two-scan requirement: first scan marks block groups, second scan
   reclaims them. Without the second scan, no reclamation occurs.

Instead, track actual reclaim progress: pause reclaim when block groups
will be reclaimed, and resume only when progress is made. This ensures
reclaim continues until no further progress can be made. And resume
perioidc reclaim when there's enough free space.

And we take care if reclaim is making any progress now, so it's
unnecessary to set periodic_reclaim_ready to false when failed to reclaim
a blockgroup.

Fixes: 813d4c6422516 ("btrfs: prevent pathological periodic reclaim loops")
Suggested-by: Boris Burkov <boris@bur.io>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Sun YangKai <sunk67188@gmail.com>
---
 fs/btrfs/block-group.c |  6 ++++--
 fs/btrfs/space-info.c  | 21 ++++++++++++---------
 2 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
index e417aba4c4c7..79d86b233dda 100644
--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -1871,6 +1871,7 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
 	while (!list_empty(&fs_info->reclaim_bgs)) {
 		u64 used;
 		u64 reserved;
+		u64 old_total;
 		int ret = 0;
 
 		bg = list_first_entry(&fs_info->reclaim_bgs,
@@ -1936,6 +1937,7 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
 		}
 
 		spin_unlock(&bg->lock);
+		old_total = space_info->total_bytes;
 		spin_unlock(&space_info->lock);
 
 		/*
@@ -1988,14 +1990,14 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
 			reserved = 0;
 			spin_lock(&space_info->lock);
 			space_info->reclaim_errors++;
-			if (READ_ONCE(space_info->periodic_reclaim))
-				space_info->periodic_reclaim_ready = false;
 			spin_unlock(&space_info->lock);
 		}
 		spin_lock(&space_info->lock);
 		space_info->reclaim_count++;
 		space_info->reclaim_bytes += used;
 		space_info->reclaim_bytes += reserved;
+		if (space_info->total_bytes < old_total)
+			btrfs_set_periodic_reclaim_ready(space_info, true);
 		spin_unlock(&space_info->lock);
 
 next:
diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index 7b7b7255f7d8..7d2386ea86c5 100644
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -2079,11 +2079,11 @@ static bool is_reclaim_urgent(struct btrfs_space_info *space_info)
 	return unalloc < data_chunk_size;
 }
 
-static void do_reclaim_sweep(struct btrfs_space_info *space_info, int raid)
+static bool do_reclaim_sweep(struct btrfs_space_info *space_info, int raid)
 {
 	struct btrfs_block_group *bg;
 	int thresh_pct;
-	bool try_again = true;
+	bool will_reclaim = false;
 	bool urgent;
 
 	spin_lock(&space_info->lock);
@@ -2101,7 +2101,7 @@ static void do_reclaim_sweep(struct btrfs_space_info *space_info, int raid)
 		spin_lock(&bg->lock);
 		thresh = mult_perc(bg->length, thresh_pct);
 		if (bg->used < thresh && bg->reclaim_mark) {
-			try_again = false;
+			will_reclaim = true;
 			reclaim = true;
 		}
 		bg->reclaim_mark++;
@@ -2118,12 +2118,13 @@ static void do_reclaim_sweep(struct btrfs_space_info *space_info, int raid)
 	 * If we have any staler groups, we don't touch the fresher ones, but if we
 	 * really need a block group, do take a fresh one.
 	 */
-	if (try_again && urgent) {
-		try_again = false;
+	if (!will_reclaim && urgent) {
+		urgent = false;
 		goto again;
 	}
 
 	up_read(&space_info->groups_sem);
+	return will_reclaim;
 }
 
 void btrfs_space_info_update_reclaimable(struct btrfs_space_info *space_info, s64 bytes)
@@ -2133,7 +2134,8 @@ void btrfs_space_info_update_reclaimable(struct btrfs_space_info *space_info, s6
 	lockdep_assert_held(&space_info->lock);
 	space_info->reclaimable_bytes += bytes;
 
-	if (space_info->reclaimable_bytes >= chunk_sz)
+	if (space_info->reclaimable_bytes > 0 &&
+	    space_info->reclaimable_bytes >= chunk_sz)
 		btrfs_set_periodic_reclaim_ready(space_info, true);
 }
 
@@ -2160,7 +2162,6 @@ static bool btrfs_should_periodic_reclaim(struct btrfs_space_info *space_info)
 
 	spin_lock(&space_info->lock);
 	ret = space_info->periodic_reclaim_ready;
-	btrfs_set_periodic_reclaim_ready(space_info, false);
 	spin_unlock(&space_info->lock);
 
 	return ret;
@@ -2174,8 +2175,10 @@ void btrfs_reclaim_sweep(const struct btrfs_fs_info *fs_info)
 	list_for_each_entry(space_info, &fs_info->space_info, list) {
 		if (!btrfs_should_periodic_reclaim(space_info))
 			continue;
-		for (raid = 0; raid < BTRFS_NR_RAID_TYPES; raid++)
-			do_reclaim_sweep(space_info, raid);
+		for (raid = 0; raid < BTRFS_NR_RAID_TYPES; raid++) {
+			if (do_reclaim_sweep(space_info, raid))
+				btrfs_set_periodic_reclaim_ready(space_info, false);
+		}
 	}
 }
 
-- 
2.52.0