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Present known kernel bugs in table form with issue descriptions, fixed and broken kernel versions, and references to fixes. Update kernel version recommendations to include information on kernel versions up to 5.8.14. Reduce emphasis on data corruption bugs which are 1) two or more years old now, and 2) much less bad than the bugs in kernel 5.1. Add deprecation warning for kernels before 4.15. Signed-off-by: Zygo Blaxell <bees@furryterror.org>
156 lines
11 KiB
Markdown
156 lines
11 KiB
Markdown
Recommended Kernel Version for bees
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===================================
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First, a warning that is not specific to bees:
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> **Kernel 5.1, 5.2, and 5.3 should not be used with btrfs due to a
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severe regression that can lead to fatal metadata corruption.**
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This issue is fixed in kernel 5.4.14 and later.
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**Recommended kernel versions for bees are 4.19, 5.4, 5.7, or 5.8, with
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recent LTS and -stable updates.** The latest released kernel as of this
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writing is 5.8.14.
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4.14, 4.9, and 4.4 LTS kernels with recent updates are OK, but older
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kernels will be somewhat slower, and not all fixes are backported.
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Obsolete non-LTS kernels have a variety of unfixed issues. For details
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see the table below.
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bees requires btrfs kernel API version 4.2 or higher, and does not work
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on older kernels.
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bees will detect and use btrfs kernel API up to version 4.15 if present.
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In some future bees release, this API version may become mandatory.
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Kernel Bug Tracking Table
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-------------------------
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These bugs are particularly popular among bees users:
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First bad kernel | Last bad kernel | Issue Description | Fixed Kernel Versions | Fix Commit
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:---: | :---: | --- | :---: | ---
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| - | 4.10 | garbage inserted in read data when reading compressed inline extent followed by a hole | 3.18.89, 4.1.49, 4.4.107, 4.9.71, 4.11 and later | e1699d2d7bf6 btrfs: add missing memset while reading compressed inline extents
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| - | 4.14 | spurious warnings from `fs/btrfs/backref.c` in `find_parent_nodes` | 3.16.57, 4.14.29, 4.15.12, 4.16 and later | c8195a7b1ad5 btrfs: remove spurious WARN_ON(ref->count < 0) in find_parent_nodes
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| 4.15 | 4.18 | compression ratio and performance regression on bees test corpus | improved in 4.19 | 4.14 performance not fully restored yet
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| - | 5.0 | silently corrupted data returned when reading compressed extents around a punched hole (bees dedupes all-zero data blocks with holes which can produce a similar effect to hole punching) | 3.16.70, 3.18.137, 4.4.177, 4.9.165, 4.14.108, 4.19.31, 5.0.4, 5.1 and later | 8e928218780e Btrfs: fix corruption reading shared and compressed extents after hole punching
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| - | 5.0 | deadlock when dedupe and rename are used simultaneously on the same files | 5.0.4, 5.1 and later | 4ea748e1d2c9 Btrfs: fix deadlock between clone/dedupe and rename
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| - | 5.1 | send failure or kernel crash while running send and dedupe on same snapshot at same time | 5.0.18, 5.1.4, 5.2 and later | 62d54f3a7fa2 Btrfs: fix race between send and deduplication that lead to failures and crashes
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| - | 5.2 | alternating send and dedupe results in incremental send failure | 4.9.188, 4.14.137, 4.19.65, 5.2.7, 5.3 and later | b4f9a1a87a48 Btrfs: fix incremental send failure after deduplication
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| - | 5.3 | balance convert to single rejected with error on 32-bit CPUs | 5.3.7, 5.4 and later | 7a54789074a5 btrfs: fix balance convert to single on 32-bit host CPUs
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| - | 5.4 | send performance failure when shared extents have too many references | 4.9.207, 4.14.159, 4.19.90, 5.3.17, 5.4.4, 5.5 and later | fd0ddbe25095 Btrfs: send, skip backreference walking for extents with many references
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| 5.1 | 5.4 | metadata corruption resulting in loss of filesystem when a write operation occurs while balance starts a new block group. **Do not use kernel 5.1 with btrfs.** Kernel 5.2 and 5.3 have workarounds that may detect corruption in progress and abort before it becomes permanent, but do not prevent corruption from occurring. | 5.4.14, 5.5 and later | 6282675e6708 btrfs: relocation: fix reloc_root lifespan and access
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| 4.5, backported to 3.18.31, 4.1.22, 4.4.4 | 5.5 | `df` incorrectly reports 0 free space while data space is available. Triggered by changes in metadata size, including those typical of large-scale dedupe. Occurs more often starting in 5.3 and especially 5.4 | 4.4.213, 4.9.213, 4.14.170, 4.19.102, 5.4.18, 5.5.2, 5.6 and later | d55966c4279b btrfs: do not zero f_bavail if we have available space
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| - | 5.5 | kernel crashes due to various tree mod log issues (often triggered by bees) | 3.16.84, 4.4.214, 4.9.214, 4.14.171, 4.19.103, 5.4.19, 5.5.3, 5.6 and later | at least 3, last is 7227ff4de55d Btrfs: fix race between adding and putting tree mod seq elements and nodes
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| 5.0 | 5.5 | last extent in file not removed by dedupe if file size is not a multiple of 4K | 5.4.19, 5.5.3, 5.6 and later | 831d2fa25ab8 Btrfs: make deduplication with range including the last block work
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| - | 5.6 | deadlock when enumerating file references to physical extent addresses while some references still exist in deleted subvols | 5.7 and later | 39dba8739c4e btrfs: do not resolve backrefs for roots that are being deleted
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| - | 5.6 | deadlock when many extent reference updates are pending and available memory is low | 4.14.177, 4.19.116, 5.4.33, 5.5.18, 5.6.5, 5.7 and later | 351cbf6e4410 btrfs: use nofs allocations for running delayed items
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| - | 5.6 | excessive CPU usage and btrfs write latency when translating from extent physical address to list of referencing files and offsets (`LOGICAL_INO` ioctl) | 5.7 and later | many backref code changes in kernel 5.7, also improvements in many earlier kernels
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| - | 5.7 | filesystem becomes read-only if out of space while deleting snapshot | 4.9.238, 4.14.200, 4.19.149, 5.4.69, 5.8 and later | 7c09c03091ac btrfs: don't force read-only after error in drop snapshot
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| 5.1 | 5.7 | balance, device delete, or filesystem shrink operations loop endlessly on a single block group, extent count does not decrease | 5.4.54, 5.7.11, 5.8 and later | 1dae7e0e58b4 btrfs: reloc: clear DEAD\_RELOC\_TREE bit for orphan roots to prevent runaway balance
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| - | 5.8 | deadlock in `TREE_SEARCH` ioctl (core component of bees filesystem scanner), followed by regression in deadlock fix | 4.4.237, 4.9.237, 4.14.199, 4.19.146, 5.4.66, 5.8.10 and later | a48b73eca4ce btrfs: fix potential deadlock in the search ioctl, 1c78544eaa46 btrfs: fix wrong address when faulting in pages in the search ioctl
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| 4.15 | - | spurious warnings from `fs/fs-writeback.c` when `flushoncommit` is enabled | - | workaround: comment out the `WARN_ON`
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| 5.7 | - | kernel crash if balance receives fatal signal at wrong point during start of new block group | - | workaround: keep `btrfs balance` from being killed or Ctrl-Ced
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"Last bad kernel" refers to that version's last stable update from
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kernel.org. Distro kernels may backport additional fixes. Consult
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your distro's kernel support for details.
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When the same version appears in both "last bad kernel" and "fixed kernel
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version" columns, it means the bug appears in the `.0` release and is
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fixed in the stated `.y` release. e.g. a "last bad kernel" of 5.4 and
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a "fixed kernel version" of 5.4.14 has the bug in kernel versions 5.4.0
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through 5.4.13 inclusive.
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A "-" for "first bad kernel" indicates the bug has been present since
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the relevant feature first appeared in btrfs.
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A "-" for "last bad kernel" indicates the bug has not yet been fixed as
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of 5.8.14.
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In cases where issues are fixed by commits spread out over multiple
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kernel versions, "fixed kernel version" refers to the version that
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contains all components of the fix.
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Workarounds for known kernel bugs
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---------------------------------
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* **Tree mod log issues**: bees will detect that a btrfs balance is
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running, and pause bees activity until the balance is done. This avoids
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running both the `LOGICAL_INO` ioctl and btrfs balance at the same time,
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which avoids kernel crashes on old kernel versions.
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This workaround is not necessary for kernels 5.4.19, 5.5.3, 5.6 and later.
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* **Slow backrefs** (aka toxic extents): Under certain conditions,
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if the number of references to a single shared extent grows too high,
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the kernel consumes more and more CPU while holding locks that block
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access to the filesystem. bees avoids this bug by measuring the time
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the kernel spends performing `LOGICAL_INO` operations and permanently
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blacklisting any extent or hash involved where the kernel starts
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to get slow. In the bees log, such blocks are labelled as 'toxic'
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hash/block addresses. Toxic extents are rare (about 1 in 100,000
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extents become toxic), but toxic extents can become 8 orders of
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magnitude more expensive to process than the fastest non-toxic
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extents. This seems to affect all dedupe agents on btrfs; at this
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time of writing only bees has a workaround for this bug.
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Update (5.8.14): this issue may be a race condition that occurs if
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two or more threads attempt to modify the same extent or immediately
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adjacent extents. It has not been observed on a kernel version later
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than 5.7 (after backref code changes in the kernel).
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* **`btrfs send` is incompatible with dedupe in old kernels**.
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The bees option `--workaround-btrfs-send` prevents any modification
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of read-only subvols in order to avoid breaking `btrfs send`.
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This workaround is not necessary for kernels 4.9.188, 4.14.137, 4.19.65,
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5.2.7, 5.3 and later.
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Unfixed kernel bugs
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-------------------
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As of 5.8.14:
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* **`btrfs send` still cannot run at the same time as dedupe**, even
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with all current fixes. Recent kernels refuse the dedupe operation with
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error `EAGAIN`. If `btrfs send` is invoked at the instant when a dedupe
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operation is running, `send` will fail to start and return an error.
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bees has not been updated to handle the new dedupe behavior optimally.
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Optimal behavior is to defer dedupe operations until after the send is
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finished. Current bees behavior is to complain loudly about the dedupe
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failure in log messages, and abandon the duplicate data references
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in the sending snapshot. A future bees version shall have better
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handling for this situation.
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Workaround: send `SIGSTOP` to bees, or terminate the bees process,
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while `btrfs send` is running. This workaround is not required if
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snapshot is deleted after sending--in that case, any duplicate data
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blocks that were not removed by dedupe will be removed by snapshot
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delete instead.
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`btrfs receive` is not affected by these issues.
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* **Spurious warnings in `fs/fs-writeback.c`** on kernel 4.15 and later
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when filesystem is mounted with `flushoncommit`. These
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seem to be harmless (there are other locks which prevent
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concurrent umount of the filesystem), but the underlying
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problems that trigger the `WARN_ON` are [not trivial to
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fix](https://www.spinics.net/lists/linux-btrfs/msg87752.html).
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The warnings can be especially voluminous when bees is running.
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Workarounds:
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1. mount with `-o noflushoncommit`
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2. patch kernel to remove warning in `fs/fs-writeback.c`.
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Note that using kernels 4.14 and earlier is *not* a viable workaround
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for this issue, because kernels 4.14 and earlier will eventually
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deadlock when a filesystem is mounted with `-o flushoncommit` (a single
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commit fixes one bug and introduces the other).
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