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Qu Wenruo e42b9d8b9e btrfs: defrag: avoid unnecessary defrag caused by incorrect extent size 
[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>
2024-02-19 11:19:58 +01:00
arch Current release - regressions: 2023-12-14 13:11:49 -08:00
block block-6.7-2023-12-01 2023-12-02 06:39:30 +09:00
certs This update includes the following changes: 2023-11-02 16:15:30 -10:00
crypto This push fixes a regression in ahash and hides the Kconfig sub-options for the jitter RNG. 2023-11-09 17:04:58 -08:00
Documentation fuse fixes for 6.7-rc6 2023-12-12 11:06:41 -08:00
drivers platform-drivers-x86 for v6.7-4 2023-12-14 17:15:33 -08:00
fs btrfs: defrag: avoid unnecessary defrag caused by incorrect extent size 2024-02-19 11:19:58 +01:00
include btrfs: defrag: reject unknown flags of btrfs_ioctl_defrag_range_args 2024-01-12 02:04:19 +01:00
init As usual, lots of singleton and doubleton patches all over the tree and 2023-11-02 20:53:31 -10:00
io_uring io_uring/af_unix: disable sending io_uring over sockets 2023-12-09 21:20:33 +00:00
ipc Many singleton patches against the MM code. The patch series which are 2023-11-02 19:38:47 -10:00
kernel - Make sure tasks are thawed exactly and only once to avoid their state 2023-12-10 11:09:16 -08:00
lib 31 hotfixes. 10 of these address pre-6.6 issues and are marked cc:stable. 2023-12-08 08:36:23 -08:00
LICENSES LICENSES: Add the copyleft-next-0.3.1 license 2022-11-08 15:44:01 +01:00
mm mm/madvise: add cond_resched() in madvise_cold_or_pageout_pte_range() 2023-12-06 16:12:50 -08:00
net appletalk: Fix Use-After-Free in atalk_ioctl 2023-12-14 12:02:45 +01:00
rust Kbuild updates for v6.7 2023-11-04 08:07:19 -10:00
samples Landlock updates for v6.7-rc1 2023-11-03 09:28:53 -10:00
scripts sign-file: Fix incorrect return values check 2023-12-13 12:55:11 -08:00
security + Features 2023-11-03 09:48:17 -10:00
sound ALSA: hda/realtek: Add quirk for Lenovo Yoga Pro 7 2023-12-07 19:21:08 +01:00
tools Generic: 2023-12-10 10:46:46 -08:00
usr arch: Remove Itanium (IA-64) architecture 2023-09-11 08:13:17 +00:00
virt Revert "KVM: Prevent module exit until all VMs are freed" 2023-12-01 08:12:30 -08:00
.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: generate kernel.spec in rpmbuild/SPECS/ 2023-10-03 20:49:09 +09:00
.mailmap hid-for-linus-2023121201 2023-12-12 17:02:56 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: drop Antti Palosaari 2023-12-06 16:12:49 -08:00
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 Current release - regressions: 2023-12-14 13:11:49 -08:00
Makefile Linux 6.7-rc5 2023-12-10 14:33:40 -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.