Re: [External] Re: vmscan: Reclaim unevictable pages

[Pankaj Suryawanshi]

From: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Sent: 18 March 2019 15:17:56
To: Pankaj Suryawanshi; Vlastimil Babka; Michal Hocko; aneesh.kumar@xxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx; minchan@xxxxxxxxxx; linux-mm@xxxxxxxxx; khandual@xxxxxxxxxxxxxxxxxx; hillf.zj@xxxxxxxxxxxxxxx
Subject: Re: [External] Re: vmscan: Reclaim unevictable pages
  

On 18.03.2019 12:43, Pankaj Suryawanshi wrote:
> Hi Kirill Tkhai,
>

Please, do not top posting:  https://kernelnewbies.org/mailinglistguidelines

Okay.

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> Please see mm/vmscan.c in which it first added to list and than throw the error :
> --------------------------------------------------------------------------------------------------
> keep:
>                 list_add(&page->lru, &ret_pages);
>                 VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page), page);
> ---------------------------------------------------------------------------------------------------
> 
> Before throwing error, pages are added to list, this is under iteration of shrink_page_list().

I say about about the list, which is passed to shrink_page_list() as first argument.
Did you mean candidate list which is passed to shrink_page_list().

shrink_inactive_list()
{
        isolate_lru_pages(&page_list); // <-- you can't obtain unevictable pages here.
        shrink_page_list(&page_list);
}

below is the overview of flow of calls for your information.

cma_alloc() ->
alloc_contig_range() ->
start_isolate_page_range() ->
__alloc_contig_migrate_range() ->
isolate_migratepages_range() ->
reclaim_clean_pages_from_list() ->
shrink_page_list()
 
> From: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> Sent: 18 March 2019 15:03:15
> To: Pankaj Suryawanshi; Vlastimil Babka; Michal Hocko; aneesh.kumar@xxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx; minchan@xxxxxxxxxx; linux-mm@xxxxxxxxx; khandual@xxxxxxxxxxxxxxxxxx; hillf.zj@xxxxxxxxxxxxxxx
> Subject: Re: [External] Re: vmscan: Reclaim unevictable pages
>   
> 
> Hi, Pankaj,
> 
> On 18.03.2019 12:09, Pankaj Suryawanshi wrote:
>>
>> Hello
>>
>> shrink_page_list() returns , number of pages reclaimed, when pages is unevictable it returns VM_BUG_ON_PAGE(PageLRU(page) || PageUnevicatble(page),page);
> 
> the general idea is shrink_page_list() can't iterate PageUnevictable() pages.
> PageUnevictable() pages are never being added to lists, which shrink_page_list()
> uses for iteration. Also, a page can't be marked as PageUnevictable(), when
> it's attached to a shrinkable list.
> 
> So, the problem should be somewhere outside shrink_page_list().
> 
> I won't suggest you something about CMA, since I haven't dived in that code.
> 
>> We can add the unevictable pages in reclaim list in shrink_page_list(), return total number of reclaim pages including unevictable pages, let the caller handle unevictable pages.
>>
>> I think the problem is shrink_page_list is awkard. If page is unevictable it goto activate_locked->keep_locked->keep lables, keep lable list_add the unevictable pages and throw the VM_BUG instead of passing it to caller while it relies on caller for non-reclaimed-non-unevictable    page's putback.
>> I think we can make it consistent so that shrink_page_list could return non-reclaimed pages via page_list and caller can handle it. As an advance, it could try to migrate mlocked pages without retrial.
>>
>>
>> Below is the issue of CMA_ALLOC of large size buffer : (Kernel version - 4.14.65 (On Android pie [ARM])).
>>
>> [   24.718792] page dumped because: VM_BUG_ON_PAGE(PageLRU(page) || PageUnevictable(page))
>> [   24.726949] page->mem_cgroup:bd008c00
>> [   24.730693] ------------[ cut here ]------------
>> [   24.735304] kernel BUG at mm/vmscan.c:1350!
>> [   24.739478] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
>>
>>
>> Below is the patch which solved this issue :
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index be56e2e..12ac353 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -998,7 +998,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>                 sc->nr_scanned++;
>>  
>>                 if (unlikely(!page_evictable(page)))
>> -                       goto activate_locked;
>> +                      goto cull_mlocked;
>>  
>>                 if (!sc->may_unmap && page_mapped(page))
>>                         goto keep_locked;
>> @@ -1331,7 +1331,12 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>                 } else
>>                         list_add(&page->lru, &free_pages);
>>                 continue;
>> -
>> +cull_mlocked:
>> +                if (PageSwapCache(page))
>> +                        try_to_free_swap(page);
>> +                unlock_page(page);
>> +                list_add(&page->lru, &ret_pages);
>> +                continue;
>>  activate_locked:
>>                 /* Not a candidate for swapping, so reclaim swap space. */
>>                 if (PageSwapCache(page) && (mem_cgroup_swap_full(page) ||
>>
>>
>>
>>
>> It fixes the below issue.
>>
>> 1. Large size buffer allocation using cma_alloc successful with unevictable pages.
>>
>> cma_alloc of current kernel will fail due to unevictable page
>>
>> Please let me know if anything i am missing.
>>
>> Regards,
>> Pankaj
>>    
>> From: Vlastimil Babka <vbabka@xxxxxxx>
>> Sent: 18 March 2019 14:12:50
>> To: Pankaj Suryawanshi; Kirill Tkhai; Michal Hocko; aneesh.kumar@xxxxxxxxxxxxx
>> Cc: linux-kernel@xxxxxxxxxxxxxxx; minchan@xxxxxxxxxx; linux-mm@xxxxxxxxx; khandual@xxxxxxxxxxxxxxxxxx; hillf.zj@xxxxxxxxxxxxxxx
>> Subject: Re: [External] Re: vmscan: Reclaim unevictable pages
>>   
>>
>> On 3/15/19 11:11 AM, Pankaj Suryawanshi wrote:
>>>
>>> [ cc Aneesh kumar, Anshuman, Hillf, Vlastimil]
>>
>> Can you send a proper patch with changelog explaining the change? I
>> don't know the context of this thread.
>>
>>> From: Pankaj Suryawanshi
>>> Sent: 15 March 2019 11:35:05
>>> To: Kirill Tkhai; Michal Hocko
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx; minchan@xxxxxxxxxx; linux-mm@xxxxxxxxx
>>> Subject: Re: Re: [External] Re: vmscan: Reclaim unevictable pages
>>>
>>>
>>>
>>> [ cc linux-mm ]
>>>
>>>
>>> From: Pankaj Suryawanshi
>>> Sent: 14 March 2019 19:14:40
>>> To: Kirill Tkhai; Michal Hocko
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx; minchan@xxxxxxxxxx
>>> Subject: Re: Re: [External] Re: vmscan: Reclaim unevictable pages
>>>
>>>
>>>
>>> Hello ,
>>>
>>> Please ignore the curly braces, they are just for debugging.
>>>
>>> Below is the updated patch.
>>>
>>>
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index be56e2e..12ac353 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -998,7 +998,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>>                  sc->nr_scanned++;
>>>
>>>                  if (unlikely(!page_evictable(page)))
>>> -                       goto activate_locked;
>>> +                      goto cull_mlocked;
>>>
>>>                  if (!sc->may_unmap && page_mapped(page))
>>>                          goto keep_locked;
>>> @@ -1331,7 +1331,12 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>>                  } else
>>>                          list_add(&page->lru, &free_pages);
>>>                  continue;
>>> -
>>> +cull_mlocked:
>>> +                if (PageSwapCache(page))
>>> +                        try_to_free_swap(page);
>>> +                unlock_page(page);
>>> +                list_add(&page->lru, &ret_pages);
>>> +                continue;
>>>   activate_locked:
>>>                  /* Not a candidate for swapping, so reclaim swap space. */
>>>                  if (PageSwapCache(page) && (mem_cgroup_swap_full(page) ||
>>>
>>>
>>>
>>> Regards,
>>> Pankaj
>>>
>>>
>>> From: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
>>> Sent: 14 March 2019 14:55:34
>>> To: Pankaj Suryawanshi; Michal Hocko
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx; minchan@xxxxxxxxxx
>>> Subject: Re: Re: [External] Re: vmscan: Reclaim unevictable pages
>>>
>>>
>>> On 14.03.2019 11:52, Pankaj Suryawanshi wrote:
>>>>
>>>> I am using kernel version 4.14.65 (on Android pie [ARM]).
>>>>
>>>> No additional patches applied on top of vanilla.(Core MM).
>>>>
>>>> If  I change in the vmscan.c as below patch, it will work.
>>>
>>> Sorry, but 4.14.65 does not have braces around trylock_page(),
>>> like in your patch below.
>>>
>>> See        https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/mm/vmscan.c?h=v4.14.65
>>>
>>> [...]
>>>
>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>>> index be56e2e..2e51edc 100644
>>>>> --- a/mm/vmscan.c
>>>>> +++ b/mm/vmscan.c
>>>>> @@ -990,15 +990,17 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>>>>>                   page = lru_to_page(page_list);
>>>>>                   list_del(&page->lru);
>>>>>
>>>>>                  if (!trylock_page(page)) {
>>>>>                           goto keep;
>>>>>                  }
>>>
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>>>
>>
>>      
>>
>     
>