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Josef Bacik b7774425e0 btrfs: remove bogus BUG_ON in alloc_reserved_tree_block 
The fix 361048f586 ("Btrfs: fix full backref problem when inserting
shared block reference") added a delayed ref flushing at subvolume
creation time in order to avoid hitting this particular BUG_ON().

Before this fix, we were tripping the BUG_ON() by

1. Modify snapshot A, which creates blocks with a normal reference for
   snapshot A, as A is the owner of these blocks.  We now have delayed
   refs for these blocks.
2. Create a snapshot of A named B, which pushes references for the
   children blocks of the root node for the new root B, thus creating
   more delayed refs for newly allocated blocks.
3. A is modified, and because the metadata blocks can now be shared, it
   must push FULL_BACKREF references to the children of any block that A
   COWs down it's path to its target key.
4. Delayed refs are run.  Because these are newly allocated blocks, we
   have ->must_insert_reserved reserved set on the delayed ref head, we
   call into alloc_reserved_tree_block() to add the extent item, and
   then add our ref.  At the time of this fix, we were ordering
   FULL_BACKREF delayed ref operations first, so we'd go to add this
   reference and then BUG_ON() because we didn't have the FULL_BACKREF
   flag set.

The patch fixed this problem by making sure we ran the delayed refs
before we had the chance to modify A.  This meant that any *new* blocks
would have had their extent items created _before_ we would ever
actually COW down and generate FULL_BACKREF entries.  Thus the problem
went away.

However this BUG_ON() is actually completely bogus.  The existence of a
full backref doesn't necessarily mean that FULL_BACKREF must be set on
that block, it must only be set on the actual parent itself.  Consider
the example provided above.  If we COW down one path from A, any nodes
are going to have a FULL_BACKREF ref pushed down to _all_ of their
children, but not all of the children are going to have FULL_BACKREF
set.  It is completely valid to have an extent item with normal and full
backrefs without FULL_BACKREF actually set on the block itself.

As a final note, I have been testing with the patch (applied after this
one)

  btrfs: stop running all delayed refs during snapshot

which removed this flushing.  My test was a torture test which did a lot
of operations while snapshotting and deleting snapshots as well as
relocation, and I never tripped this BUG_ON().  This is actually because
at the time of 361048f586, we ordered SHARED keys _before_ normal
references, and thus they would get run first.  However currently they
are ordered _after_ normal references, so we'd do the initial creation
without having a shared reference, and thus not hit this BUG_ON(), which
explains why I didn't start hitting this problem during my testing with
my other patch applied.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>
History 2021-02-08 22:58:57 +01:00
arch - For syscall user dispatch, separate ptctl operation from syscall 2021-02-07 10:16:24 -08:00
block block-5.11-2021-02-05 2021-02-06 14:40:27 -08:00
certs .gitignore: add SPDX License Identifier 2020-03-25 11:50:48 +01:00
crypto X.509: Fix crash caused by NULL pointer 2021-01-20 11:33:51 -08:00
Documentation - For syscall user dispatch, separate ptctl operation from syscall 2021-02-07 10:16:24 -08:00
drivers libnvdimm for 5.11-rc7 2021-02-07 10:45:26 -08:00
fs btrfs: remove bogus BUG_ON in alloc_reserved_tree_block 2021-02-08 22:58:57 +01:00
include lib/zstd: convert constants to defines 2021-02-08 22:58:55 +01:00
init init/gcov: allow CONFIG_CONSTRUCTORS on UML to fix module gcov 2021-02-05 11:03:47 -08:00
ipc Merge branch 'akpm' (patches from Andrew) 2020-12-15 12:53:37 -08:00
kernel tracing: Fix output of top level event "enable" file 2021-02-08 11:32:39 -08:00
lib - Revert an attempt to not spread IRQ threads on isolated CPUs which has 2021-02-07 10:03:43 -08:00
LICENSES LICENSES: Add the CC-BY-4.0 license 2020-12-08 10:33:27 -07:00
mm mm: hugetlb: fix missing put_page in gather_surplus_pages() 2021-02-05 11:03:47 -08:00
net Fixes: 2021-02-05 10:11:14 -08:00
samples ARM: SoC drivers for v5.11 2020-12-16 16:38:41 -08:00
scripts kallsyms: fix nonconverging kallsyms table with lld 2021-02-05 17:53:28 +09:00
security cap: fix conversions on getxattr 2021-01-28 10:22:48 +01:00
sound ALSA: hda/via: Apply the workaround generically for Clevo machines 2021-01-26 18:05:03 +01:00
tools libnvdimm for 5.11-rc7 2021-02-07 10:45:26 -08:00
usr Merge branch 'work.fdpic' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2020-08-07 13:29:39 -07:00
virt KVM/arm64 fixes for 5.11, take #2 2021-01-25 18:52:01 -05:00
.clang-format clang-format: Update with the latest for_each macro list 2021-01-29 15:00:23 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: docs: ignore sphinx_*/ directories 2020-09-10 10:44:31 -06:00
.mailmap block-5.11-2021-02-05 2021-02-06 14:40:27 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: dccp: move Gerrit Renker to CREDITS 2021-01-14 10:53:49 -08:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS/.mailmap: use my @kernel.org address 2021-02-05 11:03:47 -08:00
Makefile Linux 5.11-rc7 2021-02-07 13:57:38 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

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 Restructured Text 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.