[PATCH v2 0/5] mm: reduce mmap_lock contention and improve page fault performance

[Barry Song]

On Thu, May 21, 2026 at 5:35 AM David Hildenbrand (Arm)
<david at kernel.org> wrote:
>
> On 5/20/26 23:15, Matthew Wilcox wrote:
> > On Thu, May 21, 2026 at 05:14:20AM +0800, Barry Song wrote:
> >> My understanding is that we should not blame applications here. This is 2026:
> >> there are basically only two kinds of applications — single-threaded and
> >> multi-threaded — and single-threaded applications are nearly extinct.
> >
> > all of the applications i run are either single threaded or don't fork.
> > what multithreaded applications call fork?
>
> Traditionally the problem was random libraries using fork+execve to launch other
> programs ... instead of using alternatives like posix_spwan (some use cases
> require more work done before execve and cannot yet switch to that). I'd hope
> that that is less of a problem on Android.
>
> I assume Android zygote might be multi threaded? Maybe sshd as well? Systemd?
> But I'd be surprised if there are really performance implications.

I am trying to answer the question above:

1. zygote, multi-threaded on my phone using Android13.
/ # ls /proc/`pidof zygote64`/task/
1359  22728  22729  22730  22731  22732

/proc/1359/task # cat 22728/comm
Jit thread pool
/proc/1359/task # cat 22730/comm
ReferenceQueueD
/proc/1359/task # cat 22731/comm
FinalizerDaemon
/proc/1359/task # cat 22732/comm
FinalizerWatchd
/proc/1359/task # cat 1359/comm
main

But on another phone of mine running Android 16, zygote64 is
single-threaded.
Not sure if it is due to the Android team making some changes
related to threads from Android 13 to Android 16.

2. sshd, multi-processes instead of multi-threads:
$ ps aux | grep sshd
root        1192  0.0  0.0  15444  9032 ?        Ss   09:42   0:00
sshd: /usr/sbin/sshd -D [listener] 0 of 10-100 startups
root        2465  0.0  0.0  17164 10760 ?        Ss   09:42   0:00
sshd: barry [priv]
barry       2632  0.0  0.0  17164  7852 ?        S    09:42   0:00
sshd: barry at pts/0
root        3305  2.5  0.0  17164 10772 ?        Ss   09:44   0:00
sshd: barry [priv]
barry       3406  0.0  0.0  17164  7940 ?        S    09:44   0:00
sshd: barry at pts/1

3. systemd, also multi-processes

$ ps ax | grep systemd
    350 ?        S<s    0:00 /lib/systemd/systemd-journald
    387 ?        Ss     0:00 /lib/systemd/systemd-udevd
    666 ?        Ss     0:00 /lib/systemd/systemd-oomd
    667 ?        Ss     0:00 /lib/systemd/systemd-resolved
    728 ?        Ss     0:00 @dbus-daemon --system --address=systemd:
--nofork --nopidfile --systemd-activation --syslog-only
    751 ?        Ss     0:00 /lib/systemd/systemd-logind
    753 ?        Ssl    0:00 /usr/sbin/thermald --systemd
--dbus-enable --adaptive
   1350 ?        Ss     0:00 /lib/systemd/systemd --user
   1428 ?        Ss     0:00 /usr/bin/dbus-daemon --session
--address=systemd: --nofork --nopidfile --systemd-activation
--syslog-only
   1900 ?        Ssl    0:00 /usr/libexec/gnome-session-binary
--systemd-service --session=ubuntu
   2141 ?        Ssl    0:00 /lib/systemd/systemd-timesyncd

>
> Not sure about webbroswers .... I think most of them switched to fork servers,
> where I would assume fork servers would be single-threaded.

On my phone, Chrome is multi-process, but its parent process
chrome_zygote (10774) is single-threaded:

 ps -A | grep chrome
u0_i15        9883 10774 321066464 119452 do_epoll_wait     0 S
com.android.chrome:sandboxed_process0:org.chromium.content.app.SandboxedProcessService0:15
u0_a142      10164  1359 35110548 277640 do_epoll_wait      0 S
com.android.chrome
u0_a278      10724  1359 9779864 104988 do_epoll_wait       0 S
com.google.android.apps.chromecast.app
u0_a142      10774  1359 32803908 64076 do_sys_poll         0 S
com.android.chrome_zygote
u0_a142      11173  1359 34208592 142192 do_epoll_wait      0 S
com.android.chrome:privileged_process0

/proc/10774/task # ls
10774

>
> So, yeah, getting a clear understanding how this ends up being a problem on
> Android would be great.

I guess the real issue is that in the Android market, there
are so many applications that are out of our control？

Here are some trace examples from Nanzhe:

iQIYI plugin
vma reader thread:
PbMisc-0, pid=27183, tgid=26444

vma writer thread:
i.video:plugin1, pid=27298, tgid=26444
writer blocked: 440394938 ns (440 ms)

reader stack:
vma_start_read
lock_vma_under_rcu
do_page_fault
do_translation_fault
do_mem_abort
el0_da
el0t_64_sync_handler
el0t_64_sync

writer stack:
__vma_start_write
dup_mmap
copy_mm
copy_process
kernel_clone
__arm64_sys_clone
invoke_syscall
el0_svc_common
do_el0_svc
el0_svc


Baidu Tieba
vma reader thread:
elastic_pms_pro, pid=7731, tgid=7575

vma writer thread:
com.baidu.tieba, pid=8005, tgid=7575
writer blocked: 514975545 ns(515 ms)

reader stack:
vma_start_read
lock_vma_under_rcu
do_page_fault
do_translation_fault
do_mem_abort
el0_da
el0t_64_sync_handler
el0t_64_sync

writer stack:
__vma_start_write
dup_mmap
copy_mm
copy_process
kernel_clone
__arm64_sys_clone
invoke_syscall
el0_svc_common
do_el0_svc
el0_svc

Thanks
Barry