On Thu, Jul 28 2016, Mikulas Patocka wrote:
> On Wed, 27 Jul 2016, NeilBrown wrote:
>
>> On Tue, Jul 26 2016, Mikulas Patocka wrote:
>>
>> > On Sat, 23 Jul 2016, NeilBrown wrote:
>> >
>> >> "dirtying ... from the reclaim context" ??? What does that mean?
>> >> According to
>> >> Commit: 26eecbf3543b ("[PATCH] vm: pageout throttling")
>> >> From the history tree, the purpose of throttle_vm_writeout() is to
>> >> limit the amount of memory that is concurrently under I/O.
>> >> That seems strange to me because I thought it was the responsibility of
>> >> each backing device to impose a limit - a maximum queue size of some
>> >> sort.
>> >
>> > Device mapper doesn't impose any limit for in-flight bios.
>>
>> I would suggest that it probably should. At least it should
>> "set_wb_congested()" when the number of in-flight bios reaches some
>> arbitrary threshold.
>
> If we set the device mapper device as congested, it can again trigger that
> mempool alloc throttling bug.
>
> I.e. suppose that we swap to a dm-crypt device. The dm-crypt device
> becomes clogged and sets its state as congested. The underlying block
> device is not congested.
>
> The mempool_alloc function in the dm-crypt workqueue sets the
> PF_LESS_THROTTLE flag, and tries to allocate memory, but according to
> Michal's patches, processes with PF_LESS_THROTTLE may still get throttled.
>
> So if we set the dm-crypt device as congested, it can incorrectly throttle
> the dm-crypt workqueue that does allocations of temporary pages and
> encryption.
>
> I think that approach with PF_LESS_THROTTLE in mempool_alloc is incorrect
> and that mempool allocations should never be throttled.I very much agree with that last statement! It may be that to get to that point we will need all backing devices to signal congestion correctly. > >> > I've made some patches that limit in-flight bios for device mapper in >> > the past, but there were not integrated into upstream. >> >> I second the motion to resurrect these. > > I uploaded those patches here: > > http://people.redhat.com/~mpatocka/patches/kernel/dm-limit-outstanding-bios/ Thanks! I'll have a look. NeilBrown
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