Created attachment 1030811 [details]
virt-rescue log, including udevadm strace

Description of problem:

'udevadm settle' in systemd >= 220 runs very slowly when there are a
large number (256) of disks and you add a partition to at least one
of the disks.  This behaviour was not seen in earlier versions.

The libguestfs test which fails is:
https://github.com/libguestfs/libguestfs/blob/master/tests/disks/test-max-disks.pl

Version-Release number of selected component (if applicable):

systemd-220-2.fc23.x86_64
kernel-4.1.0-0.rc5.git0.1.fc23.x86_64

How reproducible:

100%

Steps to Reproduce:

TBH this is very tricky to reproduce with a general guest, but here is one way
to reproduce it quite easily with libguestfs:

1. dnf install libguestfs-tools-c libguestfs-rescue

2. Run the following command as non-root:

virt-rescue --scratch=255

This command creates a simple appliance with 255 disks (/dev/sda, /dev/sdb etc)

3. In the rescue shell, run the following commands:

parted -s -- /dev/sda mklabel msdos
parted -s -- /dev/sda mkpart primary 64s 127s
udevadm --debug settle

Actual results:

The udevadm above will hang for quite a long time (at least a few minutes).

Additional info:

I ran udevadm under strace.  The full strace is attached, but the bit where
it hangs just does this over and over:

access("/run/udev/queue", F_OK)         = 0
poll([{fd=3, events=POLLIN}], 1, 1000)  = 0 (Timeout)
access("/run/udev/queue", F_OK)         = 0
poll([{fd=3, events=POLLIN}], 1, 1000)  = 0 (Timeout)