[PATCH v3 2/3] btrfs: space-info: Use per-profile available space in can_overcommit()

Qu Wenruo
2020-01-06T06:13:56Z

For the following disk layout, can_overcommit() can cause false
confidence in available space:

devid 1 unallocated: 1T
devid 2 unallocated: 10T
metadata type: RAID1

As can_overcommit() simply uses unallocated space with factor to
calculate the allocatable metadata chunk size.

can_overcommit() believes we still have 5.5T for metadata chunks, while
the truth is, we only have 1T available for metadata chunks.
This can lead to ENOSPC at run_delalloc_range() and cause transaction
abort.

Since factor based calculation can't distinguish RAID1/RAID10 and DUP at
all, we need proper chunk-allocator level awareness to do such estimation.

Thankfully, we have per-profile available space already calculated, just
use that facility to avoid such false confidence.

Reported-by: Marc Lehmann <schmorp@schmorp.de>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
---
fs/btrfs/space-info.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/space-info.c b/fs/btrfs/space-info.c
index f09aa6ee9113..c26aba9e7124 100644
--- a/fs/btrfs/space-info.c
+++ b/fs/btrfs/space-info.c
@@ -164,10 +164,10 @@ static int can_overcommit(struct btrfs_fs_info *fs_info,
enum btrfs_reserve_flush_enum flush,
bool system_chunk)
{
+ enum btrfs_raid_types index;
u64 profile;
u64 avail;
u64 used;
- int factor;

/* Don't overcommit when in mixed mode. */
if (space_info->flags & BTRFS_BLOCK_GROUP_DATA)
@@ -179,16 +179,15 @@ static int can_overcommit(struct btrfs_fs_info *fs_info,
profile = btrfs_metadata_alloc_profile(fs_info);

used = btrfs_space_info_used(space_info, true);
- avail = atomic64_read(&fs_info->free_chunk_space);

/*
- * If we have dup, raid1 or raid10 then only half of the free
- * space is actually usable. For raid56, the space info used
- * doesn't include the parity drive, so we don't have to
- * change the math
+ * Grab avail space from per-profile array which should be as accurate
+ * as chunk allocator.
*/
- factor = btrfs_bg_type_to_factor(profile);
- avail = div_u64(avail, factor);
+ index = btrfs_bg_flags_to_raid_index(profile);
+ spin_lock(&fs_info->fs_devices->per_profile_lock);
+ avail = fs_info->fs_devices->per_profile_avail[index];
+ spin_unlock(&fs_info->fs_devices->per_profile_lock);

/*
* If we aren't flushing all things, let us overcommit up to
--
2.24.1