[PATCH 09/12] btrfs: increase the metadata allowance for the free_space_cache

Dennis Zhou
2020-01-02T21:27:04Z

Currently, there is no way for the free space cache to recover from
being serviced by purely bitmaps because the extent threshold is set to
0 in recalculate_thresholds() when we surpass the metadata allowance.

This adds a recovery mechanism by keeping large extents out of the
bitmaps and increases the metadata upper bound to 64KB. The recovery
mechanism bypasses this upper bound, thus making it a soft upper bound.
But, with the bypass being 1MB or greater, it shouldn't add unbounded
overhead.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
fs/btrfs/free-space-cache.c | 26 +++++++++++---------------
1 file changed, 11 insertions(+), 15 deletions(-)

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 8a4a3b9cd544..665f6eb6c828 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -24,7 +24,8 @@
#include "discard.h"

#define BITS_PER_BITMAP (PAGE_SIZE * 8UL)
-#define MAX_CACHE_BYTES_PER_GIG SZ_32K
+#define MAX_CACHE_BYTES_PER_GIG SZ_64K
+#define FORCE_EXTENT_THRESHOLD SZ_1M

struct btrfs_trim_range {
u64 start;
@@ -1694,26 +1695,17 @@ static void recalculate_thresholds(struct btrfs_free_space_ctl *ctl)
ASSERT(ctl->total_bitmaps <= max_bitmaps);

/*
- * The goal is to keep the total amount of memory used per 1gb of space
- * at or below 32k, so we need to adjust how much memory we allow to be
- * used by extent based free space tracking
+ * We are trying to keep the total amount of memory used per 1gb of
+ * space to be MAX_CACHE_BYTES_PER_GIG. However, with a reclamation
+ * mechanism of pulling extents >= FORCE_EXTENT_THRESHOLD out of
+ * bitmaps, we may end up using more memory than this.
*/
if (size < SZ_1G)
max_bytes = MAX_CACHE_BYTES_PER_GIG;
else
max_bytes = MAX_CACHE_BYTES_PER_GIG * div_u64(size, SZ_1G);

- /*
- * we want to account for 1 more bitmap than what we have so we can make
- * sure we don't go over our overall goal of MAX_CACHE_BYTES_PER_GIG as
- * we add more bitmaps.
- */
- bitmap_bytes = (ctl->total_bitmaps + 1) * ctl->unit;
-
- if (bitmap_bytes >= max_bytes) {
- ctl->extents_thresh = 0;
- return;
- }
+ bitmap_bytes = ctl->total_bitmaps * ctl->unit;

/*
* we want the extent entry threshold to always be at most 1/2 the max
@@ -2099,6 +2091,10 @@ static bool use_bitmap(struct btrfs_free_space_ctl *ctl,
forced = true;
#endif

+ /* This is a way to reclaim large regions from the bitmaps. */
+ if (!forced && info->bytes >= FORCE_EXTENT_THRESHOLD)
+ return false;
+
/*
* If we are below the extents threshold then we can add this as an
* extent, and don't have to deal with the bitmap
--
2.17.1