Re: [PATCH] mm: memory_hotplug: put migration failure information under DEBUG_VM

[Charan Teja Kalla]

On 11/24/2020 1:11 PM, Michal Hocko wrote:
> On Mon 23-11-20 20:40:40, Charan Teja Kalla wrote:
>>
>> Thanks Michal!
>> On 11/23/2020 7:43 PM, Michal Hocko wrote:
>>> On Mon 23-11-20 19:33:16, Charan Teja Reddy wrote:
>>>> When the pages are failed to get isolate or migrate, the page owner
>>>> information along with page info is dumped. If there are continuous
>>>> failures in migration(say page is pinned) or isolation, the log buffer
>>>> is simply getting flooded with the page owner information. As most of
>>>> the times page info is sufficient to know the causes for failures of
>>>> migration or isolation, place the page owner information under DEBUG_VM.
>>>
>>> I do not see why this path is any different from others that call
>>> dump_page. Page owner can add a very valuable information to debug
>>> the underlying reasons for failures here. It is an opt-in debugging
>>> feature which needs to be enabled explicitly. So I would argue users
>>> are ready to accept a lot of data in the kernel log.
>>
>> Just thinking how frequently failures can happen in those paths. In the
>> memory hotplug path, we can flood the page owner logs just by making one
>> page pinned.
> 
> If you are operating on a movable zone then pages shouldn't be pinned
> for unbound amount of time. Yeah there are some ways to break this
> fundamental assumption but this is a bigger problem that needs a
> solution.
> 
>> Say If it is anonymous page, the page owner information
>> shows is something like below, which is not really telling anything
>> other than how the pinned page is allocated.
> 
> Well you can tell an anonymous page from __dump_page, all right, but
> this is not true universally.
> 
>> page last allocated via order 0, migratetype Movable, gfp_mask
>> 0x100dca(GFP_HIGHUSER_MOVABLE|__GFP_ZERO)
>>   prep_new_page+0x7c/0x1a4
>>   get_page_from_freelist+0x1ac/0x1c4
>>   __alloc_pages_nodemask+0x12c/0x378
>>   do_anonymous_page+0xac/0x3b4
>>   handle_pte_fault+0x2a4/0x3bc
>>   __handle_speculative_fault+0x208/0x3c0
>>   do_page_fault+0x280/0x508
>>   do_translation_fault+0x3c/0x54
>>   do_mem_abort+0x64/0xf4
>>   el0_da+0x1c/0x20
>>  page last free stack trace:
>>   free_pcp_prepare+0x320/0x454
>>   free_unref_page_list+0x9c/0x2a4
>>   release_pages+0x370/0x3c8
>>   free_pages_and_swap_cache+0xdc/0x10c
>>   tlb_flush_mmu+0x110/0x134
>>   tlb_finish_mmu+0x48/0xc0
>>   unmap_region+0x104/0x138
>>   __do_munmap+0x2ec/0x3b4
>>   __arm64_sys_munmap+0x80/0xd8
>>
>> I see at some places in the kernel where they put the dump_page under
>> DEBUG_VM, but in the end I agree that it is up to the users need. Then
>> there are some users who don't care for these page owner logs.
> 
> Well, as I've said page_owner requires an explicit enabling and I would
> expect that if somebody enables this tracking then it is expected to see
> the information when we dump a page state.
> 
>> And an issue on Embedded systems with these continuous logs being
>> printed to the console is the watchdog timeouts, because console logging
>> happens by disabling the interrupts.
> 
> Are you enabling page_owner on those systems unconditionally?
> 

Yes, We do always enable the page owner on just the internal debug
builds for memory analysis, But never on the production kernels. And on
these builds excessive logging, at times because of a pinned page,
causing the watchdog timeouts, is the problem.

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