Re: [PATCHv2, RFC 05/30] thp, mm: avoid PageUnevictable on active/inactive lru lists

[Ric Mason <ric.masonn@gmail.com>]

Hi Kirill,
On 03/22/2013 06:11 PM, Kirill A. Shutemov wrote:
> Dave Hansen wrote:
>> On 03/14/2013 10:50 AM, Kirill A. Shutemov wrote:
>>> active/inactive lru lists can contain unevicable pages (i.e. ramfs pages
>>> that have been placed on the LRU lists when first allocated), but these
>>> pages must not have PageUnevictable set - otherwise shrink_active_list
>>> goes crazy:
>> ...
>>> For lru_add_page_tail(), it means we should not set PageUnevictable()
>>> for tail pages unless we're sure that it will go to LRU_UNEVICTABLE.
>>> The tail page will go LRU_UNEVICTABLE if head page is not on LRU or if
>>> it's marked PageUnevictable() too.
>> This is only an issue once you're using lru_add_page_tail() for
>> non-anonymous pages, right?
> I'm not sure about that. Documentation/vm/unevictable-lru.txt:
>
> Some examples of these unevictable pages on the LRU lists are:
>
>   (1) ramfs pages that have been placed on the LRU lists when first allocated.
>
>   (2) SHM_LOCK'd shared memory pages.  shmctl(SHM_LOCK) does not attempt to
>       allocate or fault in the pages in the shared memory region.  This happens
>       when an application accesses the page the first time after SHM_LOCK'ing
>       the segment.
>
>   (3) mlocked pages that could not be isolated from the LRU and moved to the
>       unevictable list in mlock_vma_page().
>
>   (4) Pages mapped into multiple VM_LOCKED VMAs, but try_to_munlock() couldn't
>       acquire the VMA's mmap semaphore to test the flags and set PageMlocked.
>       munlock_vma_page() was forced to let the page back on to the normal LRU
>       list for vmscan to handle.
>
>>> diff --git a/mm/swap.c b/mm/swap.c
>>> index 92a9be5..31584d0 100644
>>> --- a/mm/swap.c
>>> +++ b/mm/swap.c
>>> @@ -762,7 +762,8 @@ void lru_add_page_tail(struct page *page, struct page *page_tail,
>>>   			lru = LRU_INACTIVE_ANON;
>>>   		}
>>>   	} else {
>>> -		SetPageUnevictable(page_tail);
>>> +		if (!PageLRU(page) || PageUnevictable(page))
>>> +			SetPageUnevictable(page_tail);
>>>   		lru = LRU_UNEVICTABLE;
>>>   	}
>> You were saying above that ramfs pages can get on the normal
>> active/inactive lists.  But, this will end up getting them on the
>> unevictable list, right?  So, we have normal ramfs pages on the
>> active/inactive lists, but ramfs pages after a huge-page-split on the
>> unevictable list.  That seems a bit inconsistent.
> Yeah, it's confusing.
>
> I was able to trigger another bug on this code:
> if page_evictable(page_tail) is false and PageLRU(page) is true, page_tail
> will go to the same lru as page, but nobody cares to sync page_tail
> active/inactive state with page. So we can end up with inactive page on
> active lru...
>
> I've updated the patch for the next interation. You can check it in git.
> It should be cleaner. Description need to be updated.

Hope you can send out soon. ;-)

>