Btrfs devel - Jun 2014 - [PATCH] Btrfs-progs: fsck: reduce memory usage for extents check

Steps to reproduce:

 # mkfs.btrfs -f /dev/sda9 -b 2g
 # mount /dev/sda9 /mnt
 # dd if=/dev/zero of=/mnt/data bs=4k oflag=direct
 # btrfs file df /mnt

    Data, single: total=1.66GiB, used=1.66GiB
    System, single: total=4.00MiB, used=16.00KiB
    Metadata, single: total=200.00MiB, used=67.88MiB

For a filesystem without snapshots, 70M metadata, extent
checking eats max memory about 110M, this is a nightmare
for some system with low memory.

It is very likely that extent record can be freed quickly
for a filesystem without snapshots, improve this by trying
if it can free memory after adding data/tree backrefs.

This patch reduces max memory cost from 110M to 40M for
extents checking for the above case.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
---
 cmds-check.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/cmds-check.c b/cmds-check.c
index 26baab0..ac97e48 100644
--- a/cmds-check.c
+++ b/cmds-check.c
@@ -2980,6 +2980,7 @@ static int add_tree_backref(struct cache_tree
*extent_cache, u64 bytenr,
 		}
 		back->node.found_extent_tree = 1;
 	}
+	maybe_free_extent_rec(extent_cache, rec);
 	return 0;
 }
 
@@ -3046,6 +3047,7 @@ static int add_data_backref(struct cache_tree
*extent_cache, u64 bytenr,
 		back->num_refs = num_refs;
 		back->node.found_extent_tree = 1;
 	}
+	maybe_free_extent_rec(extent_cache, rec);
 	return 0;
 }
 
-- 
1.9.0

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