Re: [PATCH v4 19/19] fs: handle inode->i_version more efficiently

From: Krzysztof Kozlowski
Date: Mon Jan 08 2018 - 08:21:16 EST

On Mon, Jan 8, 2018 at 1:56 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Sun, 2018-01-07 at 13:44 +0100, Krzysztof Kozlowski wrote:
>> On Fri, Dec 22, 2017 at 1:05 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
>> > From: Jeff Layton <jlayton@xxxxxxxxxx>
>> >
>> > Since i_version is mostly treated as an opaque value, we can exploit that
>> > fact to avoid incrementing it when no one is watching. With that change,
>> > we can avoid incrementing the counter on writes, unless someone has
>> > queried for it since it was last incremented. If the a/c/mtime don't
>> > change, and the i_version hasn't changed, then there's no need to dirty
>> > the inode metadata on a write.
>> >
>> > Convert the i_version counter to an atomic64_t, and use the lowest order
>> > bit to hold a flag that will tell whether anyone has queried the value
>> > since it was last incremented.
>> >
>> > When we go to maybe increment it, we fetch the value and check the flag
>> > bit. If it's clear then we don't need to do anything if the update
>> > isn't being forced.
>> >
>> > If we do need to update, then we increment the counter by 2, and clear
>> > the flag bit, and then use a CAS op to swap it into place. If that
>> > works, we return true. If it doesn't then do it again with the value
>> > that we fetch from the CAS operation.
>> >
>> > On the query side, if the flag is already set, then we just shift the
>> > value down by 1 bit and return it. Otherwise, we set the flag in our
>> > on-stack value and again use cmpxchg to swap it into place if it hasn't
>> > changed. If it has, then we use the value from the cmpxchg as the new
>> > "old" value and try again.
>> >
>> > This method allows us to avoid incrementing the counter on writes (and
>> > dirtying the metadata) under typical workloads. We only need to increment
>> > if it has been queried since it was last changed.
>> >
>> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
>> > ---
>> > include/linux/fs.h | 2 +-
>> > include/linux/iversion.h | 208 ++++++++++++++++++++++++++++++++++-------------
>> > 2 files changed, 154 insertions(+), 56 deletions(-)
>> >
>>
>> Hi,
>>
>> On recent linux-next my ARM/Exynos boards fail to boot over nfsroot.
>> This commit popped up through bisect (log at the end). Systemd
>> timeouts on some device-specific services, including mounting ext4
>> /home:
>>
>> [ *** ] (1 of 4) A start job is running forâress polling (1min 41s / no limit)
>> [ TIME ] Timed out waiting for device dev-ttySAC2.device.
>> Jan 07 13:29:38 [DEPEND] Dependency failed for Serial Getty on ttySAC2.
>> Jan 07 13:29:38 [ TIME ] Timed out waiting for device
>> dev-disk-by\x2dlabel-home.device.
>> Jan 07 13:29:38 [DEPEND] Dependency failed for /home.
>> Jan 07 13:29:38 [DEPEND] Dependency failed for Local File Systems.
>> Jan 07 13:29:38 [DEPEND] Dependency failed for File System Check on
>> /dev/disk/by-label/home.
>> Jan 07 13:30:02 [ *** ] (1 of 2) A start job is running forâress
>> polling (1min 53s / no limit)
>>
>> Kernel command line:
>> console=tty1 console=ttySAC2,115200n8
>> ip=192.168.1.11:192.168.1.10:192.168.1.1:255.255.255.0::eth0:none
>> nfsrootdebug root=/dev/nfs
>> nfsroot=192.168.1.10:/srv/nfs/odroidxu3,vers=4,nolock rootwait rw
>> smsc95xx.macaddr=00:1e:06:61:7a:93 no_console_suspend
>>
>> /home is /dev/mmcblk1p2:
>> kozik@odroidxu3:~$ tune2fs -l /dev/mmcblk1p2
>> tune2fs 1.43.7 (16-Oct-2017)
>> Filesystem volume name: home
>> Last mounted on: /home
>> Filesystem UUID: 3f9dbeba-2738-45d3-807e-c1b2e21128ed
>> Filesystem magic number: 0xEF53
>> Filesystem revision #: 1 (dynamic)
>> Filesystem features: has_journal ext_attr resize_inode dir_index
>> filetype needs_recovery extent flex_bg sparse_super large_file
>> uninit_bg dir_nlink extra_isize
>> Filesystem flags: signed_directory_hash
>> Default mount options: user_xattr acl
>> Filesystem state: clean
>> Errors behavior: Continue
>> Filesystem OS type: Linux
>> Inode count: 1430800
>> Block count: 5717760
>> Reserved block count: 285888
>> Free blocks: 5467576
>> Free inodes: 1428301
>> First block: 0
>> Block size: 4096
>> Fragment size: 4096
>> Reserved GDT blocks: 1022
>> Blocks per group: 32768
>> Fragments per group: 32768
>> Inodes per group: 8176
>> Inode blocks per group: 511
>> Flex block group size: 16
>> Filesystem created: Thu May 21 12:17:05 2015
>> Last mount time: Thu Dec 21 13:31:26 2017
>> Last write time: Thu Dec 21 13:31:26 2017
>> Mount count: 1
>> Maximum mount count: -1
>> Last checked: Thu Dec 21 13:31:25 2017
>> Check interval: 0 (<none>)
>> Lifetime writes: 126 GB
>> Reserved blocks uid: 0 (user root)
>> Reserved blocks gid: 0 (group root)
>> First inode: 11
>> Inode size: 256
>> Required extra isize: 28
>> Desired extra isize: 28
>> Journal inode: 8
>> Default directory hash: half_md4
>> Directory Hash Seed: 42e17e23-86b2-4356-ad63-78aa51651d03
>> Journal backup: inode blocks
>>
>>
>> Full dmesg log:
>> http://www.krzk.eu/#/builders/1/builds/1258/steps/10/logs/serial0
>>
>> The regular boot from rootfs on SD card also fails - but without any
>> serial console logs (just "Starting kernel...") so the real cause is
>> unknown.
>>
>> Any hints?
>>
>> Best regards,
>> Krzysztof
>>
>>
>> bisect log:
>> # bad: [73005e1a35fd67c644b0645c9e4c1efabd0fe62c] Add linux-next
>> specific files for 20180103
>> # good: [30a7acd573899fd8b8ac39236eff6468b195ac7d] Linux 4.15-rc6
>> git bisect start 'next/master' 'next/stable'
>> # bad: [c1d290f9ce8daa2b0a79d2fe48c1b7c3c5370f5a] Merge
>> remote-tracking branch 'crypto/master'
>> git bisect bad c1d290f9ce8daa2b0a79d2fe48c1b7c3c5370f5a
>> # bad: [55695d94f0915121d106cd2d1ab94983a32f3e9a] Merge
>> remote-tracking branch 'hid/for-next'
>> git bisect bad 55695d94f0915121d106cd2d1ab94983a32f3e9a
>> # good: [cffae1eead0dd91be1a3069a8348127bb00158f3] Merge
>> remote-tracking branch 'realtek/for-next'
>> git bisect good cffae1eead0dd91be1a3069a8348127bb00158f3
>> # good: [5f889f1176dc99636c6bf8af7c286decc888c007] Merge
>> remote-tracking branch 'btrfs/next'
>> git bisect good 5f889f1176dc99636c6bf8af7c286decc888c007
>> # good: [984c35877f36bee305e43a1c58176169854d85cf] Merge
>> remote-tracking branch 'xfs/for-next'
>> git bisect good 984c35877f36bee305e43a1c58176169854d85cf
>> # bad: [f9fec502daea2a869232b6dff33ba3de79dd0d61] Merge
>> remote-tracking branch 'printk/for-next'
>> git bisect bad f9fec502daea2a869232b6dff33ba3de79dd0d61
>> # good: [c71d227fc4133f949dae620ed5e3a250b43f2415] make kernel-side
>> POLL... arch-independent
>> git bisect good c71d227fc4133f949dae620ed5e3a250b43f2415
>> # good: [416d20e8c31107f5dfd45d1d80d1e6c8e4871180] Merge branches
>> 'work.get_user_pages_fast', 'work.wmci', 'work.sock_recvmsg',
>> 'misc.netdrv', 'misc.poll', 'work.mqueue', 'work.whack-a-mole' and
>> 'work.misc' into for-next
>> git bisect good 416d20e8c31107f5dfd45d1d80d1e6c8e4871180
>> # good: [325a1de4a691512a48c1426b943a7b0b9f8d6744] xfs: convert to new
>> i_version API
>> git bisect good 325a1de4a691512a48c1426b943a7b0b9f8d6744
>> # good: [a94fe10fb114c169e7ddaecd8251521886409121] checkpatch: add
>> pF/pf deprecation warning
>> git bisect good a94fe10fb114c169e7ddaecd8251521886409121
>> # good: [6b3911dffd1184fdcd63299a5fee59ac000f2067] btrfs: only dirty
>> the inode in btrfs_update_time if something was changed
>> git bisect good 6b3911dffd1184fdcd63299a5fee59ac000f2067
>> # bad: [448f8c749a7a0ae03505823910ec45a112678048] Merge
>> remote-tracking branch 'iversion/iversion-next'
>> git bisect bad 448f8c749a7a0ae03505823910ec45a112678048
>> # bad: [8618bff776758ebff5b55211e7b5a60a0fc119a5] fs: handle
>> inode->i_version more efficiently
>> git bisect bad 8618bff776758ebff5b55211e7b5a60a0fc119a5
>> # first bad commit: [8618bff776758ebff5b55211e7b5a60a0fc119a5] fs:
>> handle inode->i_version more efficiently
>
> That's really strange. I'm afraid I have no idea what could be going on.
>
> With NFS, we really just treat i_version as an opaque value, so I'm not
> sure how this patch in particular would affect anything there. We _do_
> increment it if you have a write delegation in some cases, but not many
> servers hand those out.

About the NFS server, Arch Linux on Raspberry Pi (so 32-bit, ARMv6):
Linux pi 4.9.70-1-ARCH #1 SMP Mon Dec 18 19:38:00 UTC 2017 armv6l GNU/Linux

The /etc/nfs.conf is default except:
[nfsd]
vers2=n
vers3=n

The client logs for nfsroot mounts are:
Jan 08 14:07:25 :: running hook [net_nfs4]
Jan 08 14:07:25 IP-Config: eth0 hardware address ba:17:70:7e:87:d1 mtu 1500
Jan 08 14:07:25 IP-Config: eth0 guessed broadcast address 192.168.1.255
Jan 08 14:07:25 IP-Config: eth0 complete (from 192.168.1.10):
Jan 08 14:07:25 address: 192.168.1.11 broadcast: 192.168.1.255
netmask: 255.255.255.0
Jan 08 14:07:25 gateway: 192.168.1.1 dns0 : 0.0.0.0 dns1 : 0.0.0.0
Jan 08 14:07:25 rootserver: 192.168.1.10 rootpath:
Jan 08 14:07:25 filename :
Jan 08 14:07:25 NFS-Mount: 192.168.1.10:/srv/nfs/odroidxu3
Jan 08 14:07:25 Waiting 10 seconds for device /dev/nfs ...
Jan 08 14:07:36 ERROR: device '/dev/nfs' not found. Skipping fsck.
Jan 08 14:07:36 Mount cmd:
Jan 08 14:07:36 /opt/tools/buildbot/arch-arm-bin/mount.nfs4 -o
vers=4,nolock 192.168.1.10:/srv/nfs/odroidxu3 /new_root

Only root (/) is froom NFS. The /home comes from sdcard (/dev/mmcblk1).

> ext4 will only touch the i_version field if you mount it with '-o
> iversion'. Are you doing that here?

The /home is mounted by systemd from /etc/fstab:
LABEL=home /home ext4 defaults 0 2

>
> Have you run the bisect more than once? Is this maybe an intermittent
> problem, and the bisect has landed on the wrong commit?

I just run tests on commits around again (but not full bisect) on
current next (next-20180108):
fbf97ece47d66d22 - works
3da7bcdd695bae43 ("fs: handle inode->i_version more efficiently") -
fails: http://www.krzk.eu/#/builders/1/builds/1267


Best regards,
Krzysztof