btrfs: increase the metadata allowance for the free_space_cache

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.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 802e06d6303dcc692e95464e5498dbd165258d9a..f75eb737e713db87f473b0c8beeaab397f258a5f 100644 (file)
--- 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 1GiB 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