[BUG] Page allocation failures with newest kernels

From: Marcin Wojtas
Date: Mon May 30 2016 - 23:03:07 EST


After rebasing platform support of two different ARMv8 SoC's from v4.1
baseline to v4.4 it occurred that stressed systems tend to have page
allocation problems, related to creating new slabs:


Steps to reproduce:
- use SATA drive (on-board or over PCIe) with 2 btrfs 50G partitions
- run a couple of loops of following script:
mount /dev/sd${1}1 /mnt
mount /dev/sd${1}2 /mnt2
while [[ $i -lt ${2} ]]
echo -e "i = ${i}\n"
dd if=/dev/zero of=/mnt/3g bs=3M count=1024 &
dd if=/dev/zero of=/mnt/2g bs=2M count=1024 &
dd if=/dev/zero of=/mnt/1g bs=1M count=1024 &
dd if=/dev/zero of=/mnt2/2g bs=2M count=1024 &
dd if=/dev/zero of=/mnt2/1g bs=1M count=1024 &
dd if=/dev/zero of=/mnt2/3g bs=3M count=1024
let "i++"

The issue also reproduced on v4.6. Usually problems occur within first
iteration and then the rest is done without errors, also kernel remain
stable. I got an information, that page alloc problem were observed
also on Marvell ARMv7 platfrom (Armada38x).

About the debug itself - after adding simplest possible trace in
trace/events/kmem.h (single argument u64 for counter or whatever kind
of number), it was shown both on v4.1 and v4.4 following condition is
achieved multiple times during test:
In __alloc_pages_nodemask(), during the test kernel jumps huge amount
of times (~250k times in v4.1 and ~570k in v4.4 per one script loop)
into following 'unlikely' condition:
page = get_page_from_freelist(alloc_mask, order, alloc_flags, &ac);
if (unlikely(!page)) {
page = __alloc_pages_slowpath(alloc_mask, order, &ac);

The further difference is seen in __alloc_pages_slowpath().
warn_alloc_page() (routine responsible for printing page alloc failure
message) is reached via following condition:
if (!can_direct_reclaim) {
goto nopage;
In v4.1 ~5 times and in v4.4 ~40 times per one script loop.

Printing message however can be blocked by following condition in
if ((gfp_mask & _GFP_NOWARN) || !_ratelimit(&nopage_rs) ||
debug_guardpage_minorder() > 0)
Only first two are relevant. As ratelimit is derived directly from
CONFIG_HZ and this parameter differ between v4.1 and v4.4 (100 vs 250,
also CONFIG_SCHED_HRTICK is enabled only in v4.4) the configs were
swapped, but no change in behavior.

Also within 'faulty' revision there is a difference, depending on
filesystem used - with buildroot the dumps occur, but with same test
under ubuntu - it's impossible see the failure output (and it's not a
question of dmesg level:)). Comparing /proc/sys/vm contents didn't
show anything meaningful.

I tried to analyze changes around mm/ folder between v4.1 and v4.4
that may cause such difference, but wasn't able to find out what may
be causing the issue. Have anyone encountered such problems in recent
revisions? I would be very grateful for any hint or comment. Also if
any other data can be captured, please let know.

Best regards,
Marcin Wojtas