Re: [PATCH v2 2/9] mm/vmscan: remove unneeded can_split_huge_page check

[Miaohe Lin]

On 2022/4/15 11:07, ying.huang@xxxxxxxxx wrote:
> On Wed, 2022-04-13 at 09:26 +0800, ying.huang@xxxxxxxxx wrote:
>> On Tue, 2022-04-12 at 16:59 +0200, David Hildenbrand wrote:
>>> On 12.04.22 15:42, Miaohe Lin wrote:
>>>> On 2022/4/12 16:59, Oscar Salvador wrote:
>>>>> On Sat, Apr 09, 2022 at 05:34:53PM +0800, Miaohe Lin wrote:
>>>>>> We don't need to check can_split_folio() because folio_maybe_dma_pinned()
>>>>>> is checked before. It will avoid the long term pinned pages to be swapped
>>>>>> out. And we can live with short term pinned pages. Without can_split_folio
>>>>>> checking we can simplify the code. Also activate_locked can be changed to
>>>>>> keep_locked as it's just short term pinning.
>>>>>
>>>>> What do you mean by "we can live with short term pinned pages"?
>>>>> Does it mean that it was not pinned when we check
>>>>> folio_maybe_dma_pinned() but now it is?
>>>>>
>>>>> To me it looks like the pinning is fluctuating and we rely on
>>>>> split_folio_to_list() to see whether we succeed or not, and if not
>>>>> we give it another spin in the next round?
>>>>
>>>> Yes. Short term pinned pages is relative to long term pinned pages and these pages won't be
>>>> pinned for a noticeable time. So it's expected to split the folio successfully in the next
>>>> round as the pinning is really fluctuating. Or am I miss something?
>>>>
>>>
>>> Just so we're on the same page. folio_maybe_dma_pinned() only capture
>>> FOLL_PIN, but not FOLL_GET. You can have long-term FOLL_GET right now
>>> via vmsplice().
>>
>> Per my original understanding, folio_maybe_dma_pinned() can be used to
>> detect long-term pinned pages.  And it seems reasonable to skip the
>> long-term pinned pages and try short-term pinned pages during page
>> reclaiming.  But as you pointed out, vmsplice() doesn't use FOLL_PIN. 
>> So if vmsplice() is expected to pin pages for long time, and we have no
>> way to detect it, then we should keep can_split_folio() in the original
>> code.
>>
>> Copying more people who have worked on long-term pinning for comments.
> 
> Checked the discussion in the following thread,
> 
> https://lore.kernel.org/lkml/CA+CK2bBffHBxjmb9jmSKacm0fJMinyt3Nhk8Nx6iudcQSj80_w@xxxxxxxxxxxxxx/
> 
> It seems that from the practical point of view, folio_maybe_dma_pinned()
> can identify most long-term pinned pages that may block memory hot-
> remove or CMA allocation.  Although as David pointed out, some pages may
> still be GUPed for long time (e.g. via vmsplice) even if
> !folio_maybe_dma_pinned().
> 
> But from another point of view, can_split_huge_page() is cheap and THP
> swapout is expensive (swap space, disk IO, and hard to be recovered), so
> it may be better to keep can_split_huge_page() in shink_page_list().

Many thanks for your explanation. Looks convincing for me. Is it worth a comment about the above
stuff? Anyway, will drop this patch. Thanks!

> 
> Best Regards,
> Huang, Ying
> 
>>
>>> can_split_folio() is more precise then folio_maybe_dma_pinned(), but
>>> both are racy as long as the page is still mapped.
>>>
>>>
>>
> 
> 
> .
>