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7ee19a59a7
qgroup_snapshot_quick_inherit() detects conditions where the snapshot
destination would land in the same parent qgroup as the snapshot source
subvolume. In this case we can avoid costly qgroup calculations and just
add the nodesize of the new snapshot to the parent.
However, in the case of squotas this is actually a double count, and
also an undercount for deeper qgroup nestings.
The following annotated script shows the issue:
btrfs quota enable --simple "$mnt"
# Create 2-level qgroup hierarchy
btrfs qgroup create 2/100 "$mnt" # Q2 (level 2)
btrfs qgroup create 1/100 "$mnt" # Q1 (level 1)
btrfs qgroup assign 1/100 2/100 "$mnt"
# Create base subvolume
btrfs subvolume create "$mnt/base" >/dev/null
base_id=$(btrfs subvolume show "$mnt/base" | grep 'Subvolume ID:' | awk '{print $3}')
# Create intermediate snapshot and add to Q1
btrfs subvolume snapshot "$mnt/base" "$mnt/intermediate" >/dev/null
inter_id=$(btrfs subvolume show "$mnt/intermediate" | grep 'Subvolume ID:' | awk '{print $3}')
btrfs qgroup assign "0/$inter_id" 1/100 "$mnt"
# Create working snapshot with --inherit (auto-adds to Q1)
# src=intermediate (in only Q1)
# dst=snap (inheriting only into Q1)
# This double counts the 16k nodesize of the snapshot in Q1, and
# undercounts it in Q2.
btrfs subvolume snapshot -i 1/100 "$mnt/intermediate" "$mnt/snap" >/dev/null
snap_id=$(btrfs subvolume show "$mnt/snap" | grep 'Subvolume ID:' | awk '{print $3}')
# Fully complete snapshot creation
sync
# Delete working snapshot
# Q1 and Q2 will lose the full snap usage
btrfs subvolume delete "$mnt/snap" >/dev/null
# Delete intermediate and remove from Q1
# Q1 and Q2 will lose the full intermediate usage
btrfs qgroup remove "0/$inter_id" 1/100 "$mnt"
btrfs subvolume delete "$mnt/intermediate" >/dev/null
# Q1 should be at 0, but still has 16k. Q2 is "correct" at 0 (for now...)
# Trigger cleaner, wait for deletions
mount -o remount,sync=1 "$mnt"
btrfs subvolume sync "$mnt" "$snap_id"
btrfs subvolume sync "$mnt" "$inter_id"
# Remove Q1 from Q2
# Frees 16k more from Q2, underflowing it to 16EiB
btrfs qgroup remove 1/100 2/100 "$mnt"
# And show the bad state:
btrfs qgroup show -pc "$mnt"
Qgroupid Referenced Exclusive Parent Child Path
-------- ---------- --------- ------ ----- ----
0/5 16.00KiB 16.00KiB - - <toplevel>
0/256 16.00KiB 16.00KiB - - base
1/100 16.00KiB 16.00KiB - - <0 member qgroups>
2/100 16.00EiB 16.00EiB - - <0 member qgroups>
Fix this by simply not doing this quick inheritance with squotas.
I suspect that it is also wrong in normal qgroups to not recurse up the
qgroup tree in the quick inherit case, though other consistency checks
will likely fix it anyway.
Fixes:
|
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.