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Chris Mason d58562ca6c iomap: skip pages past eof in iomap_do_writepage() 
iomap_do_writepage() sends pages past i_size through
folio_redirty_for_writepage(), which normally isn't a problem because
truncate and friends clean them very quickly.

When the system has cgroups configured, we can end up in situations
where one cgroup has almost no dirty pages at all, and other cgroups
consume the entire background dirty limit.  This is especially common in
our XFS workloads in production because they have cgroups using O_DIRECT
for almost all of the IO mixed in with cgroups that do more traditional
buffered IO work.

We've hit storms where the redirty path hits millions of times in a few
seconds, on all a single file that's only ~40 pages long.  This leads to
long tail latencies for file writes because the pdflush workers are
hogging the CPU from some kworkers bound to the same CPU.

Reproducing this on 5.18 was tricky because 869ae85dae ("xfs: flush new
eof page on truncate...") ends up writing/waiting most of these dirty pages
before truncate gets a chance to wait on them.

The actual repro looks like this:

/*
 * run me in a cgroup all alone.  Start a second cgroup with dd
 * streaming IO into the block device.
 */
int main(int ac, char **av) {
	int fd;
	int ret;
	char buf[BUFFER_SIZE];
	char *filename = av[1];

	memset(buf, 0, BUFFER_SIZE);

	if (ac != 2) {
		fprintf(stderr, "usage: looper filename\n");
		exit(1);
	}
	fd = open(filename, O_WRONLY | O_CREAT, 0600);
	if (fd < 0) {
		err(errno, "failed to open");
	}
	fprintf(stderr, "looping on %s\n", filename);
	while(1) {
		/*
		 * skip past page 0 so truncate doesn't write and wait
		 * on our extent before changing i_size
		 */
		ret = lseek(fd, 8192, SEEK_SET);
		if (ret < 0)
			err(errno, "lseek");
		ret = write(fd, buf, BUFFER_SIZE);
		if (ret != BUFFER_SIZE)
			err(errno, "write failed");
		/* start IO so truncate has to wait after i_size is 0 */
		ret = sync_file_range(fd, 16384, 4095, SYNC_FILE_RANGE_WRITE);
		if (ret < 0)
			err(errno, "sync_file_range");
		ret = ftruncate(fd, 0);
		if (ret < 0)
			err(errno, "truncate");
		usleep(1000);
	}
}

And this bpftrace script will show when you've hit a redirty storm:

kretprobe:xfs_vm_writepages {
    delete(@dirty[pid]);
}

kprobe:xfs_vm_writepages {
    @dirty[pid] = 1;
}

kprobe:folio_redirty_for_writepage /@dirty[pid] > 0/ {
    $inode = ((struct folio *)arg1)->mapping->host->i_ino;
    @inodes[$inode] = count();
    @redirty++;
    if (@redirty > 90000) {
        printf("inode %d redirty was %d", $inode, @redirty);
        exit();
    }
}

This patch has the same number of failures on xfstests as unpatched 5.18:
Failures: generic/648 xfs/019 xfs/050 xfs/168 xfs/299 xfs/348 xfs/506
xfs/543

I also ran it through a long stress of multiple fsx processes hammering.

(Johannes Weiner did significant tracing and debugging on this as well)

Signed-off-by: Chris Mason <clm@fb.com>
Co-authored-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Reported-by: Domas Mituzas <domas@fb.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
2022-06-30 09:52:57 -07:00
arch ARM: SoC fixes for 5.19 2022-06-26 14:12:56 -07:00
block block: remove WARN_ON() from bd_link_disk_holder 2022-06-23 07:48:05 -06:00
certs Certs changes 2022-06-21 12:13:53 -05:00
crypto Certs changes 2022-06-21 12:13:53 -05:00
Documentation Minor things, mainly - mailmap updates, MAINTAINERS updates, etc. 2022-06-26 14:00:55 -07:00
drivers ARM: SoC fixes for 5.19 2022-06-26 14:12:56 -07:00
fs iomap: skip pages past eof in iomap_do_writepage() 2022-06-30 09:52:57 -07:00
include ARM: SoC fixes for 5.19 2022-06-26 14:12:56 -07:00
init gcc-12: disable '-Warray-bounds' universally for now 2022-06-09 10:11:12 -07:00
ipc These changes update the ipc sysctls so that they are fundamentally 2022-06-03 15:54:57 -07:00
kernel Minor things, mainly - mailmap updates, MAINTAINERS updates, etc. 2022-06-26 14:00:55 -07:00
lib Build tool updates: 2022-06-19 09:54:16 -05:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm Minor things, mainly - mailmap updates, MAINTAINERS updates, etc. 2022-06-26 14:00:55 -07:00
net Networking fixes for 5.19-rc4, including fixes from bpf and netfilter. 2022-06-23 09:01:01 -05:00
samples fprobe, samples: Add use_trace option and show hit/missed counter 2022-06-17 21:53:29 +02:00
scripts kbuild: Ignore __this_module in gen_autoksyms.sh 2022-06-26 06:15:05 +09:00
security selinux: free contexts previously transferred in selinux_add_opt() 2022-06-15 21:20:45 -04:00
sound sound fixes for 5.19-rc4 2022-06-23 08:44:00 -05:00
tools tools headers UAPI: Synch KVM's svm.h header with the kernel 2022-06-26 12:32:55 -03:00
usr Not a lot of material this cycle. Many singleton patches against various 2022-05-27 11:22:03 -07:00
virt KVM: x86: disable preemption around the call to kvm_arch_vcpu_{un|}blocking 2022-06-09 10:52:20 -04:00
.clang-format clang-format: Fix space after for_each macros 2022-05-20 19:27:16 +02:00
.cocciconfig
.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 kbuild: split the second line of *.mod into *.usyms 2022-05-08 03:16:59 +09:00
.mailmap mailmap: add entry for Christian Marangi 2022-06-16 19:11:32 -07:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: replace a Microchip AT91 maintainer 2022-02-09 11:30:01 +01:00
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 ARM: SoC fixes for 5.19 2022-06-26 14:12:56 -07:00
Makefile Linux 5.19-rc4 2022-06-26 14:22:10 -07:00
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.