Re: [PATCH v5] mm: support madvise(MADV_FREE)

[Rik van Riel <riel@redhat.com>]

On 05/09/2014 02:28 AM, Minchan Kim wrote:
> On Fri, May 09, 2014 at 03:17:14PM +0900, Minchan Kim wrote:
>> Hello Rik,
>>
>> On Thu, May 08, 2014 at 04:04:33PM -0400, Rik van Riel wrote:
>>> On 04/20/2014 09:56 PM, Minchan Kim wrote:
>>>
>>>> In summary, MADV_FREE is about 2 time faster than MADV_DONTNEED.
>>>
>>> This is awesome.
>>
>> Thanks!
>>
>>>
>>> I have a few nitpicks with the patch, though :)
>>>
>>>> +static long madvise_lazyfree(struct vm_area_struct *vma,
>>>> +			     struct vm_area_struct **prev,
>>>> +			     unsigned long start, unsigned long end)
>>>> +{
>>>> +	*prev = vma;
>>>> +	if (vma->vm_flags & (VM_LOCKED|VM_HUGETLB|VM_PFNMAP))
>>>> +		return -EINVAL;
>>>> +
>>>> +	/* MADV_FREE works for only anon vma at the moment */
>>>> +	if (vma->vm_file)
>>>> +		return -EINVAL;
>>>> +
>>>> +	lazyfree_range(vma, start, end - start);
>>>> +	return 0;
>>>> +}
>>>
>>> This code checks whether lazyfree_range would work on
>>> the VMA...
>>>
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index c4b5bc250820..ca427f258204 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -1270,6 +1270,104 @@ static inline unsigned long zap_pud_range(struct mmu_gather *tlb,
>>>>   	return addr;
>>>>   }
>>>>
>>>> +static unsigned long lazyfree_pte_range(struct mmu_gather *tlb,
>>>> +				struct vm_area_struct *vma, pmd_t *pmd,
>>>> +				unsigned long addr, unsigned long end)
>>>> +{
>>>> +	struct mm_struct *mm = tlb->mm;
>>>> +	spinlock_t *ptl;
>>>> +	pte_t *start_pte;
>>>> +	pte_t *pte;
>>>> +
>>>> +	start_pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
>>>> +	pte = start_pte;
>>>> +	arch_enter_lazy_mmu_mode();
>>>> +	do {
>>>> +		pte_t ptent = *pte;
>>>> +
>>>> +		if (pte_none(ptent))
>>>> +			continue;
>>>> +
>>>> +		if (!pte_present(ptent))
>>>> +			continue;
>>>> +
>>>> +		ptent = pte_mkold(ptent);
>>>> +		ptent = pte_mkclean(ptent);
>>>> +		set_pte_at(mm, addr, pte, ptent);
>>>> +		tlb_remove_tlb_entry(tlb, pte, addr);
>>>
>>> This may not work on PPC, which has a weird hash table for
>>> its TLB. You will find that tlb_remove_tlb_entry does
>>> nothing for PPC64, and set_pte_at does not remove the hash
>>> table entry either.
>>
>> Hmm, I didn't notice that. Thanks Rik.
>>
>> Maybe I need this in asm-generic.
>>
>> static inline void ptep_set_lazyfree(struct mm_struct *mm, unsigned addr, pte_t *ptep)
>> {
>>          pte_t ptent = *ptep;
>>          ptent = pte_mkold(ptent);
>>          ptent = pte_mkclean(ptent);
>>          set_pte_at(mm, addr, ptep, ptent);
>> }
>>
>> For arch/powerpc/include/asm/pgtable.h
>>
>> static inline void ptep_set_lazyfree(struct mm_struct *mm, unsigned long addr,
>>                          pte_t *ptep)
>> {
>>          pte_update(mm, addr, ptep, _PAGE_DIRTY|_PAGE_ACCESSED, 0, 0);
>> }
>>
>>>
>>>> @@ -1370,6 +1485,31 @@ void unmap_vmas(struct mmu_gather *tlb,
>>>>   }
>>>>
>>>>   /**
>>>> + * lazyfree_range - clear dirty bit of pte in a given range
>>>> + * @vma: vm_area_struct holding the applicable pages
>>>> + * @start: starting address of pages
>>>> + * @size: number of bytes to do lazyfree
>>>> + *
>>>> + * Caller must protect the VMA list
>>>> + */
>>>> +void lazyfree_range(struct vm_area_struct *vma, unsigned long start,
>>>> +		unsigned long size)
>>>> +{
>>>> +	struct mm_struct *mm = vma->vm_mm;
>>>> +	struct mmu_gather tlb;
>>>> +	unsigned long end = start + size;
>>>> +
>>>> +	lru_add_drain();
>>>> +	tlb_gather_mmu(&tlb, mm, start, end);
>>>> +	update_hiwater_rss(mm);
>>>> +	mmu_notifier_invalidate_range_start(mm, start, end);
>>>> +	for ( ; vma && vma->vm_start < end; vma = vma->vm_next)
>>>> +		lazyfree_single_vma(&tlb, vma, start, end);
>>>> +	mmu_notifier_invalidate_range_end(mm, start, end);
>>>> +	tlb_finish_mmu(&tlb, start, end);
>>>> +}
>>>
>>> This function, called by madvise_lazyfree, can iterate
>>> over multiple VMAs.
>>>
>>> However, madvise_lazyfree only checked one of them.
>>
>> Oops, the check should have been lazyfree_range.
>> Will fix.
>
> Now that I see the code, madvise_vma always pass *a* vma so madvise_lazyfree
> doesn't cover multiple vma all at once so the current sematic is same with
> dontneed. So, I don't see any problem. If I miss something, let me know it.
>

Does that mean lazyfree_range is unnecessary, and everything
can be done inside lazyfree_single_vma ?

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