Subject: Re: [PATCH] powerpc/THP: Wait for all hash_page calls to finish before invalidating HPTE entries

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Wed, 19 Jun 2013 12:34:24 +0530

Michael Neuling <[email-masked]> writes:

> Aneesh Kumar K.V <[email-masked]> wrote:
>
>> From: "Aneesh Kumar K.V" <[email-masked]>
>> 
>> When we collapse normal pages to hugepage, we first clear the pmd, then invalidate all
>> the PTE entries. The assumption here is that any low level page fault will see pmd as
>> none and take the slow path that will wait on mmap_sem. But we could very well be in
>> a hash_page with local ptep pointer value. Such a hash page can result in adding new
>> HPTE entries for normal subpages/small page. That means we could be modifying the
>> page content as we copy them to a huge page. Fix this by waiting on hash_page to finish
>> after marking the pmd none and bfore invalidating HPTE entries. We use the heavy
>> kick_all_cpus_sync(). This should be ok as we do this in the background khugepaged
>> thread and not in application context. But we block page fault handling for this time.
>> Also if we find collapse slow we can ideally increase the scan rate.
>
> 80 columns here
>
>> 
>> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
>> ---
>>  arch/powerpc/mm/pgtable_64.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>> 
>> diff --git a/arch/powerpc/mm/pgtable_64.c b/arch/powerpc/mm/pgtable_64.c
>> index bbecac4..4bb44c3 100644
>> --- a/arch/powerpc/mm/pgtable_64.c
>> +++ b/arch/powerpc/mm/pgtable_64.c
>> @@ -543,6 +543,14 @@ pmd_t pmdp_clear_flush(struct vm_area_struct *vma, unsigned long address,
>>  		pmd = *pmdp;
>>  		pmd_clear(pmdp);
>>  		/*
>> +		 * Wait for all pending hash_page to finish
>> +		 * We can do this by waiting for a context switch to happen on
>> +		 * the cpus. Any new hash_page after this will see pmd none
>> +		 * and fallback to code that takes mmap_sem and hence will block
>> +		 * for collapse to finish.
>> +		 */
>> +		kick_all_cpus_sync();
>> +		/*
>
> This doesn't apply on mainline... I assume it's needs your TPH
> patches?

yes, They are on top V10 THP series

>
> Also, dumb question. Is this a bug we're fixing or just an optimisation?

This is a bug fix. The details can be found at 

http://article.gmane.org/gmane.linux.ports.ppc.embedded/60266

-aneesh