[PATCH 3/3] xfs: improve xfs_zoned_need_gc()

[Damien Le Moal <dlemoal@kernel.org>]

If zonegc_low_space is set to zero (which is the default), the second
condition in xfs_zoned_need_gc() that triggers GC never evaluates to
true, so there is no need to calculate the test threshold and to
evaluate that condition.

While at it, add comments to document the intent of each of the 3 tests
used to determine the return value to control the execution of garbage
collection.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
---
 fs/xfs/xfs_zone_gc.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/fs/xfs/xfs_zone_gc.c b/fs/xfs/xfs_zone_gc.c
index 309f70098524..0ff710fa0ee7 100644
--- a/fs/xfs/xfs_zone_gc.c
+++ b/fs/xfs/xfs_zone_gc.c
@@ -170,25 +170,37 @@ xfs_zoned_need_gc(
 	s64			available, free, threshold;
 	s32			remainder;
 
+	/* If we have no reclaimable blocks, running GC is useless. */
 	if (!xfs_zoned_have_reclaimable(mp->m_zone_info))
 		return false;
 
+	/*
+	 * In order to avoid file fragmentation as much as possible, we should
+	 * make sure that we can open enough zones. So trigger GC if the number
+	 * of blocks immediately available for writes is lower than the total
+	 * number of blocks from all possible open zones.
+	 */
 	available = xfs_estimate_freecounter(mp, XC_FREE_RTAVAILABLE);
-
 	if (available <
 	    xfs_rtgs_to_rfsbs(mp, mp->m_max_open_zones - XFS_OPEN_GC_ZONES))
 		return true;
 
-	free = xfs_estimate_freecounter(mp, XC_FREE_RTEXTENTS);
+	/*
+	 * For cases where the user wants to be more aggressive with GC,
+	 * the sysfs attribute zonegc_low_space may be set to a non zero value,
+	 * to indicate that GC should try to maintain at least zonegc_low_space
+	 * percent of the free space to be directly available for writing. Check
+	 * this here.
+	 */
+	if (!mp->m_zonegc_low_space)
+		return false;
 
+	free = xfs_estimate_freecounter(mp, XC_FREE_RTEXTENTS);
 	threshold = div_s64_rem(free, 100, &remainder);
 	threshold = threshold * mp->m_zonegc_low_space +
 		    remainder * div_s64(mp->m_zonegc_low_space, 100);
 
-	if (available < threshold)
-		return true;
-
-	return false;
+	return available < threshold;
 }
 
 static struct xfs_zone_gc_data *
-- 
2.53.0