Subject: Re: [PATCH V2] powerpc/mm: Fix Multi hit ERAT cause by recent THP update

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Mon, 08 Feb 2016 19:34:32 +0530

"Kirill A. Shutemov" <[email-masked]> writes:

> On Mon, Feb 08, 2016 at 11:44:22AM +0530, Aneesh Kumar K.V wrote:
>> With ppc64 we use the deposited pgtable_t to store the hash pte slot
>> information. We should not withdraw the deposited pgtable_t without
>> marking the pmd none. This ensure that low level hash fault handling
>> will skip this huge pte and we will handle them at upper levels.
>>
>> Recent change to pmd splitting changed the above in order to handle the
>> race between pmd split and exit_mmap. The race is explained below.
>>
>> Consider following race:
>>
>> 		CPU0				CPU1
>> shrink_page_list()
>>   add_to_swap()
>>     split_huge_page_to_list()
>>       __split_huge_pmd_locked()
>>         pmdp_huge_clear_flush_notify()
>> 	// pmd_none() == true
>> 					exit_mmap()
>> 					  unmap_vmas()
>> 					    zap_pmd_range()
>> 					      // no action on pmd since pmd_none() == true
>> 	pmd_populate()
>>
>> As result the THP will not be freed. The leak is detected by check_mm():
>>
>> 	BUG: Bad rss-counter state mm:ffff880058d2e580 idx:1 val:512
>>
>> The above required us to not mark pmd none during a pmd split.
>>
>> The fix for ppc is to clear the huge pte of _PAGE_USER, so that low
>> level fault handling code skip this pte. At higher level we do take ptl
>> lock. That should serialze us against the pmd split. Once the lock is
>> acquired we do check the pmd again using pmd_same. That should always
>> return false for us and hence we should retry the access.
>
> I guess it worth mention that this serialization against ptl happens in
> huge_pmd_set_accessed(), if I didn't miss anything.

Ok will update the commit message with the below

"We do the pmd_same check in all case after taking plt with
THP (do_huge_pmd_wp_page, do_huge_pmd_numa_page and
huge_pmd_set_accessed)"
>
>>
>> Also make sure we wait for irq disable section in other cpus to finish
>> before flipping a huge pte entry with a regular pmd entry. Code paths
>> like find_linux_pte_or_hugepte depend on irq disable to get
>> a stable pte_t pointer. A parallel thp split need to make sure we
>> don't convert a pmd pte to a regular pmd entry without waiting for the
>> irq disable section to finish.
>>
>> Signed-off-by: Aneesh Kumar K.V <[email-masked]>

.....
...

>>  #ifndef __HAVE_ARCH_PTE_SAME
>>  static inline int pte_same(pte_t pte_a, pte_t pte_b)
>>  {
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 36c070167b71..b52d16a86e91 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2860,6 +2860,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>>  	young = pmd_young(*pmd);
>>  	dirty = pmd_dirty(*pmd);
>>
>> +	pmdp_huge_splitting_flush(vma, haddr, pmd);
>
> Let's call it pmdp_huge_split_prepare().
>
> "_flush" part is ppc-specific implementation detail and generic code
> should not expect tlb to be flushed there.

Ok done

>
> Otherwise,
>
> Acked-by: Kirill A. Shutemov <[email-masked]>
>
>>  	pgtable = pgtable_trans_huge_withdraw(mm, pmd);
>>  	pmd_populate(mm, &_pmd, pgtable);
>>
>> --
>> 2.5.0
>>

-aneesh