Re: [PATCH v1 01/10] mm: Expose clear_huge_page() unconditionally

[Ryan Roberts <ryan.roberts@arm.com>]

On 27/06/2023 19:26, Yu Zhao wrote:
> On Tue, Jun 27, 2023 at 3:41 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> On 27/06/2023 09:29, Yu Zhao wrote:
>>> On Tue, Jun 27, 2023 at 1:21 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>
>>>> On 27/06/2023 02:55, Yu Zhao wrote:
>>>>> On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>>
>>>>>> In preparation for extending vma_alloc_zeroed_movable_folio() to
>>>>>> allocate a arbitrary order folio, expose clear_huge_page()
>>>>>> unconditionally, so that it can be used to zero the allocated folio in
>>>>>> the generic implementation of vma_alloc_zeroed_movable_folio().
>>>>>>
>>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>>>>> ---
>>>>>>  include/linux/mm.h | 3 ++-
>>>>>>  mm/memory.c        | 2 +-
>>>>>>  2 files changed, 3 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>>>> index 7f1741bd870a..7e3bf45e6491 100644
>>>>>> --- a/include/linux/mm.h
>>>>>> +++ b/include/linux/mm.h
>>>>>> @@ -3684,10 +3684,11 @@ enum mf_action_page_type {
>>>>>>   */
>>>>>>  extern const struct attribute_group memory_failure_attr_group;
>>>>>>
>>>>>> -#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>>>>>>  extern void clear_huge_page(struct page *page,
>>>>>>                             unsigned long addr_hint,
>>>>>>                             unsigned int pages_per_huge_page);
>>>>>> +
>>>>>> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
>>>>>
>>>>> We might not want to depend on THP eventually. Right now, we still
>>>>> have to, unless splitting is optional, which seems to contradict
>>>>> 06/10. (deferred_split_folio()  is a nop without THP.)
>>>>
>>>> Yes, I agree - for large anon folios to work, we depend on THP. But I don't
>>>> think that helps us here.
>>>>
>>>> In the next patch, I give vma_alloc_zeroed_movable_folio() an extra `order`
>>>> parameter. So the generic/default version of the function now needs a way to
>>>> clear a compound page.
>>>>
>>>> I guess I could do something like:
>>>>
>>>>  static inline
>>>>  struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
>>>>                                    unsigned long vaddr, gfp_t gfp, int order)
>>>>  {
>>>>         struct folio *folio;
>>>>
>>>>         folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp,
>>>>                                         order, vma, vaddr, false);
>>>>         if (folio) {
>>>> #ifdef CONFIG_LARGE_FOLIO
>>>>                 clear_huge_page(&folio->page, vaddr, 1U << order);
>>>> #else
>>>>                 BUG_ON(order != 0);
>>>>                 clear_user_highpage(&folio->page, vaddr);
>>>> #endif
>>>>         }
>>>>
>>>>         return folio;
>>>>  }
>>>>
>>>> But that's pretty messy and there's no reason why other users might come along
>>>> that pass order != 0 and will be surprised by the BUG_ON.
>>>
>>> #ifdef CONFIG_LARGE_ANON_FOLIO // depends on CONFIG_TRANSPARENT_HUGE_PAGE
>>> struct folio *alloc_anon_folio(struct vm_area_struct *vma, unsigned
>>> long vaddr, int order)
>>> {
>>>   // how do_huge_pmd_anonymous_page() allocs and clears
>>>   vma_alloc_folio(..., *true*);
>>
>> This controls the mem allocation policy (see mempolicy.c::vma_alloc_folio()) not
>> clearing. Clearing is done in __do_huge_pmd_anonymous_page():
>>
>>   clear_huge_page(page, vmf->address, HPAGE_PMD_NR);
> 
> Sorry for rushing this previously. This is what I meant. The #ifdef
> makes it safe to use clear_huge_page() without 01/10. I highlighted
> the last parameter to vma_alloc_folio() only because it's different
> from what you chose (not implying it clears the folio).>
>>> }
>>> #else
>>> #define alloc_anon_folio(vma, addr, order)
>>> vma_alloc_zeroed_movable_folio(vma, addr)
>>> #endif
>>
>> Sorry I don't get this at all... If you are suggesting to bypass
>> vma_alloc_zeroed_movable_folio() entirely for the LARGE_ANON_FOLIO case
> 
> Correct.
> 
>> I don't
>> think that works because the arch code adds its own gfp flags there. For
>> example, arm64 adds __GFP_ZEROTAGS for VM_MTE VMAs.
> 
> I think it's the opposite: it should be safer to reuse the THP code because
> 1. It's an existing case that has been working for PMD_ORDER folios
> mapped by PTEs, and it's an arch-independent API which would be easier
> to review.
> 2. Use vma_alloc_zeroed_movable_folio() for large folios is a *new*
> case. It's an arch-*dependent* API which I have no idea what VM_MTE
> does (should do) to large folios and don't plan to answer that for
> now.

I've done some archaology on this now, and convinced myself that your suggestion
is a good one - sorry for doubting it!

If you are interested here are the details: Only arm64 and ia64 do something
non-standard in vma_alloc_zeroed_movable_folio(). ia64 flushes the dcache for
the folio - but given it does not support THP this is not a problem for the THP
path. arm64 adds the __GFP_ZEROTAGS flag which means that the MTE tags will be
zeroed at the same time as the page is zeroed. This is a perf optimization - if
its not performed then it will be done at set_pte_at(), which is how this works
for the THP path.

So on that basis, I agree we can use your proposed alloc_anon_folio() approach.
arm64 will lose the MTE optimization but that can be added back later if needed.
So no need to unconditionally expose clear_huge_page() and no need to modify all
the arch vma_alloc_zeroed_movable_folio() implementations.

Thanks,
Ryan


> 
>> Perhaps we can do away with an arch-owned vma_alloc_zeroed_movable_folio() and
>> replace it with a new arch_get_zeroed_movable_gfp_flags() then
>> alloc_anon_folio() add in those flags?
>>
>> But I still think the cleanest, simplest change is just to unconditionally
>> expose clear_huge_page() as I've done it.
> 
> The fundamental choice there as I see it is to whether the first step
> of large anon folios should lean toward the THP code base or the base
> page code base (I'm a big fan of the answer "Neither -- we should
> create something entirely new instead"). My POV is that the THP code
> base would allow us to move faster, since it's proven to work for a
> very similar case (PMD_ORDER folios mapped by PTEs).