Re: Hangs in balance_dirty_pages with arm-32 LPAE + highmem

[Laura Abbott <labbott@redhat.com>]

On 03/14/2018 02:08 AM, Michal Hocko wrote:
> On Mon 05-03-18 13:04:24, Laura Abbott wrote:
>> On 02/26/2018 06:28 AM, Michal Hocko wrote:
>>> On Fri 23-02-18 11:51:41, Laura Abbott wrote:
>>>> Hi,
>>>>
>>>> The Fedora arm-32 build VMs have a somewhat long standing problem
>>>> of hanging when running mkfs.ext4 with a bunch of processes stuck
>>>> in D state. This has been seen as far back as 4.13 but is still
>>>> present on 4.14:
>>>>
>>> [...]
>>>> This looks like everything is blocked on the writeback completing but
>>>> the writeback has been throttled. According to the infra team, this problem
>>>> is _not_ seen without LPAE (i.e. only 4G of RAM). I did see
>>>> https://patchwork.kernel.org/patch/10201593/ but that doesn't seem to
>>>> quite match since this seems to be completely stuck. Any suggestions to
>>>> narrow the problem down?
>>>
>>> How much dirtyable memory does the system have? We do allow only lowmem
>>> to be dirtyable by default on 32b highmem systems. Maybe you have the
>>> lowmem mostly consumed by the kernel memory. Have you tried to enable
>>> highmem_is_dirtyable?
>>>
>>
>> Setting highmem_is_dirtyable did fix the problem. The infrastructure
>> people seemed satisfied enough with this (and are happy to have the
>> machines back). I'll see if they are willing to run a few more tests
>> to get some more state information.
> 
> Please be aware that highmem_is_dirtyable is not for free. There are
> some code paths which can only allocate from lowmem (e.g. block device
> AFAIR) and those could fill up the whole lowmem without any throttling.
> 

Good to note. This particular setup is one basically everyone dislikes
so I think this is only encouragement to move to something else.

Thanks,
Laura