btrfs: account stripe_alloc trapped free space for honest statfs
Free space in the partially filled stripes of a stripe_alloc (raid56
write-hole-safe) block group is real free space -- it is in the free
space tree and cache -- but the stripe-exclusive allocator cannot hand it
out until the whole stripe frees. statfs therefore over-reports available
space, promising free space that a later allocation refuses with ENOSPC.
Track this trapped space per block group as stripe_unusable, summed into
space_info->bytes_stripe_unusable. It is derived, so there is no on-disk
format change and the free space tree and cache are left untouched
(preserving the extent-tree/free-space-tree consistency btrfs check
verifies).
stripe_unusable is defined by a scan that accumulates every free byte of
the block group into a per-stripe array and then sums the stripes that are
partially filled. A per-stripe accumulator, rather than a streaming sweep
or a per-stripe cache search, is what makes the result correct regardless
of the order the free space cache yields its ranges -- an offset-sorted
bitmap entry can emit runs that lie past a following extent entry, so the
ranges are not globally monotonic -- and regardless of whether free space
is stored as extents or bitmaps. A debug-build assertion checks that the
scan distributes exactly the cache's free space. Because trapped space
only settles at commit -- when the retire path returns partially filled
stripes and deleted extents are unpinned -- the scan is recomputed at
commit for groups whose free space changed (flagged cheaply on the
allocation and free paths), which suits the timescale of the reclaim it
feeds far better than a running per-extent tally. The space_info total is
recomputed as the sum of the armed groups after those rescans, so it
cannot drift.
statfs subtracts bytes_stripe_unusable from the data f_bavail so df
reports what can actually be allocated, and the counter is exposed at
/sys/fs/btrfs/<uuid>/allocation/data/bytes_stripe_unusable. It is
deliberately not part of btrfs_space_info_used(), so the reservation layer
is unchanged and this carries no ENOSPC-behaviour risk.
The counter is armed once a group's free space cache is loaded, so a group
contributes zero until then and statfs starts optimistic and settles to
honest as groups cache and commit. It is disarmed when a group turns
read-only (its free space is then accounted as read-only and already
excluded from statfs) and re-armed by a rescan on the way back to
read-write, so a balance both recovers trapped space and updates the
counter. A CONFIG_BTRFS_DEBUG-only sysfs trigger, stripe_unusable_rescan,
forces a full recompute for auditing the accounting.
Two deliberate imprecisions, matching existing behaviour rather than
bettering it: statfs subtracts bytes_stripe_unusable but not
bytes_zone_unusable, so stripe_alloc df reports availability before reclaim
while zoned df reports it after; and superblock stripes, permanently
unusable in every profile, are left to the existing bytes_super accounting
rather than separately reported here. Both await a maintainer decision on
a common convention.
projects / linux / commit