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Qu Wenruo 0bb020dca6 btrfs: defrag: avoid unnecessary defrag caused by incorrect extent size 
commit e42b9d8b9ea2672811285e6a7654887ff64d23f3 upstream.

[BUG]
With the following file extent layout, defrag would do unnecessary IO
and result more on-disk space usage.

  # mkfs.btrfs -f $dev
  # mount $dev $mnt
  # xfs_io -f -c "pwrite 0 40m" $mnt/foobar
  # sync
  # xfs_io -f -c "pwrite 40m 16k" $mnt/foobar
  # sync

Above command would lead to the following file extent layout:

        item 6 key (257 EXTENT_DATA 0) itemoff 15816 itemsize 53
                generation 7 type 1 (regular)
                extent data disk byte 298844160 nr 41943040
                extent data offset 0 nr 41943040 ram 41943040
                extent compression 0 (none)
        item 7 key (257 EXTENT_DATA 41943040) itemoff 15763 itemsize 53
                generation 8 type 1 (regular)
                extent data disk byte 13631488 nr 16384
                extent data offset 0 nr 16384 ram 16384
                extent compression 0 (none)

Which is mostly fine. We can allow the final 16K to be merged with the
previous 40M, but it's upon the end users' preference.

But if we defrag the file using the default parameters, it would result
worse file layout:

 # btrfs filesystem defrag $mnt/foobar
 # sync

        item 6 key (257 EXTENT_DATA 0) itemoff 15816 itemsize 53
                generation 7 type 1 (regular)
                extent data disk byte 298844160 nr 41943040
                extent data offset 0 nr 8650752 ram 41943040
                extent compression 0 (none)
        item 7 key (257 EXTENT_DATA 8650752) itemoff 15763 itemsize 53
                generation 9 type 1 (regular)
                extent data disk byte 340787200 nr 33292288
                extent data offset 0 nr 33292288 ram 33292288
                extent compression 0 (none)
        item 8 key (257 EXTENT_DATA 41943040) itemoff 15710 itemsize 53
                generation 8 type 1 (regular)
                extent data disk byte 13631488 nr 16384
                extent data offset 0 nr 16384 ram 16384
                extent compression 0 (none)

Note the original 40M extent is still there, but a new 32M extent is
created for no benefit at all.

[CAUSE]
There is an existing check to make sure we won't defrag a large enough
extent (the threshold is by default 32M).

But the check is using the length to the end of the extent:

	range_len = em->len - (cur - em->start);

	/* Skip too large extent */
	if (range_len >= extent_thresh)
		goto next;

This means, for the first 8MiB of the extent, the range_len is always
smaller than the default threshold, and would not be defragged.
But after the first 8MiB, the remaining part would fit the requirement,
and be defragged.

Such different behavior inside the same extent caused the above problem,
and we should avoid different defrag decision inside the same extent.

[FIX]
Instead of using @range_len, just use @em->len, so that we have a
consistent decision among the same file extent.

Now with this fix, we won't touch the extent, thus not making it any
worse.

Reported-by: Filipe Manana <fdmanana@suse.com>
Fixes: 0cb5950f3f ("btrfs: fix deadlock when reserving space during defrag")
CC: stable@vger.kernel.org # 6.1+
Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
2024-03-01 13:34:58 +01:00
arch LoongArch: Update cpu_sibling_map when disabling nonboot CPUs 2024-03-01 13:34:58 +01:00
block block: Fix WARNING in _copy_from_iter 2024-03-01 13:34:49 +01:00
certs certs: Reference revocation list for all keyrings 2023-08-17 20:12:41 +00:00
crypto crypto: algif_hash - Remove bogus SGL free on zero-length error path 2024-02-23 09:25:11 +01:00
Documentation docs: Instruct LaTeX to cope with deeper nesting 2024-03-01 13:34:58 +01:00
drivers IB/hfi1: Fix sdma.h tx->num_descs off-by-one error 2024-03-01 13:34:58 +01:00
fs btrfs: defrag: avoid unnecessary defrag caused by incorrect extent size 2024-03-01 13:34:58 +01:00
include xen/events: reduce externally visible helper functions 2024-03-01 13:34:57 +01:00
init update workarounds for gcc "asm goto" issue 2024-02-23 09:24:47 +01:00
io_uring io_uring/net: fix multishot accept overflow handling 2024-02-23 09:25:10 +01:00
ipc Add x86 shadow stack support 2023-08-31 12:20:12 -07:00
kernel sched/rt: Disallow writing invalid values to sched_rt_period_us 2024-03-01 13:34:47 +01:00
lib Revert "kobject: Remove redundant checks for whether ktype is NULL" 2024-02-23 09:24:58 +01:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm blk-wbt: Fix detection of dirty-throttled tasks 2024-02-23 09:25:16 +01:00
net mptcp: corner case locking for rx path fields initialization 2024-03-01 13:34:57 +01:00
rust rust: upgrade to Rust 1.73.0 2024-02-16 19:10:43 +01:00
samples work around gcc bugs with 'asm goto' with outputs 2024-02-23 09:24:47 +01:00
scripts modpost: Add '.ltext' and '.ltext.*' to TEXT_SECTIONS 2024-02-23 09:25:03 +01:00
security lsm: fix the logic in security_inode_getsecctx() 2024-02-23 09:25:02 +01:00
sound ALSA: usb-audio: Ignore clock selector errors for single connection 2024-03-01 13:34:52 +01:00
tools tools: selftests: riscv: Fix compile warnings in mm tests 2024-03-01 13:34:48 +01:00
usr initramfs: Encode dependency on KBUILD_BUILD_TIMESTAMP 2023-06-06 17:54:49 +09:00
virt ARM: 2023-09-07 13:52:20 -07:00
.clang-format iommu: Add for_each_group_device() 2023-05-23 08:15:51 +02:00
.cocciconfig
.get_maintainer.ignore get_maintainer: add Alan to .get_maintainer.ignore 2022-08-20 15:17:44 -07:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore kbuild: rpm-pkg: rename binkernel.spec to kernel.spec 2023-07-25 00:59:33 +09:00
.mailmap 20 hotfixes. 12 are cc:stable and the remainder address post-6.5 issues 2023-10-24 09:52:16 -10:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING
CREDITS USB: Remove Wireless USB and UWB documentation 2023-08-09 14:17:32 +02:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: add Catherine as xfs maintainer for 6.6.y 2024-02-16 19:10:43 +01:00
Makefile Linux 6.6.18 2024-02-23 09:25:28 +01:00
README

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.