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Message-ID: <e1ee2a95-aff1-461b-7e9b-9dd2fd0e70d3@arm.com>
Date: Mon, 10 Jul 2023 10:39:58 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Matthew Wilcox <willy@...radead.org>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Yin Fengwei <fengwei.yin@...el.com>,
David Hildenbrand <david@...hat.com>,
Yu Zhao <yuzhao@...gle.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>,
Anshuman Khandual <anshuman.khandual@....com>,
Yang Shi <shy828301@...il.com>,
linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org
Subject: Re: [PATCH v2 2/5] mm: Allow deferred splitting of arbitrary large
anon folios
On 10/07/2023 10:01, Huang, Ying wrote:
> Ryan Roberts <ryan.roberts@....com> writes:
>
>> On 10/07/2023 06:37, Huang, Ying wrote:
>>> Ryan Roberts <ryan.roberts@....com> writes:
>>>
>>>> Somehow I managed to reply only to the linux-arm-kernel list on first attempt so
>>>> resending:
>>>>
>>>> On 07/07/2023 09:21, Huang, Ying wrote:
>>>>> Ryan Roberts <ryan.roberts@....com> writes:
>>>>>
>>>>>> With the introduction of large folios for anonymous memory, we would
>>>>>> like to be able to split them when they have unmapped subpages, in order
>>>>>> to free those unused pages under memory pressure. So remove the
>>>>>> artificial requirement that the large folio needed to be at least
>>>>>> PMD-sized.
>>>>>>
>>>>>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>>>>>> Reviewed-by: Yu Zhao <yuzhao@...gle.com>
>>>>>> Reviewed-by: Yin Fengwei <fengwei.yin@...el.com>
>>>>>> ---
>>>>>> mm/rmap.c | 2 +-
>>>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
>>>>>> index 82ef5ba363d1..bbcb2308a1c5 100644
>>>>>> --- a/mm/rmap.c
>>>>>> +++ b/mm/rmap.c
>>>>>> @@ -1474,7 +1474,7 @@ void page_remove_rmap(struct page *page, struct vm_area_struct *vma,
>>>>>> * page of the folio is unmapped and at least one page
>>>>>> * is still mapped.
>>>>>> */
>>>>>> - if (folio_test_pmd_mappable(folio) && folio_test_anon(folio))
>>>>>> + if (folio_test_large(folio) && folio_test_anon(folio))
>>>>>> if (!compound || nr < nr_pmdmapped)
>>>>>> deferred_split_folio(folio);
>>>>>> }
>>>>>
>>>>> One possible issue is that even for large folios mapped only in one
>>>>> process, in zap_pte_range(), we will always call deferred_split_folio()
>>>>> unnecessarily before freeing a large folio.
>>>>
>>>> Hi Huang, thanks for reviewing!
>>>>
>>>> I have a patch that solves this problem by determining a range of ptes covered
>>>> by a single folio and doing a "batch zap". This prevents the need to add the
>>>> folio to the deferred split queue, only to remove it again shortly afterwards.
>>>> This reduces lock contention and I can measure a performance improvement for the
>>>> kernel compilation benchmark. See [1].
>>>>
>>>> However, I decided to remove it from this patch set on Yu Zhao's advice. We are
>>>> aiming for the minimal patch set to start with and wanted to focus people on
>>>> that. I intend to submit it separately later on.
>>>>
>>>> [1] https://lore.kernel.org/linux-mm/20230626171430.3167004-8-ryan.roberts@arm.com/
>>>
>>> Thanks for your information! "batch zap" can solve the problem.
>>>
>>> And, I agree with Matthew's comments to fix the large folios interaction
>>> issues before merging the patches to allocate large folios as in the
>>> following email.
>>>
>>> https://lore.kernel.org/linux-mm/ZKVdUDuwNWDUCWc5@casper.infradead.org/
>>>
>>> If so, we don't need to introduce the above problem or a large patchset.
>>
>> I appreciate Matthew's and others position about not wanting to merge a minimal
>> implementation while there are some fundamental features (e.g. compaction) it
>> doesn't play well with - I'm working to create a definitive list so these items
>> can be tracked and tackled.
>
> Good to know this, Thanks!
>
>> That said, I don't see this "batch zap" patch as an example of this. It's just a
>> performance enhancement that improves things even further than large anon folios
>> on their own. I'd rather concentrate on the core changes first then deal with
>> this type of thing later. Does that work for you?
>
> IIUC, allocating large folios upon page fault depends on splitting large
> folios in page_remove_rmap() to avoid memory wastage. Splitting large
> folios in page_remove_rmap() depends on "batch zap" to avoid performance
> regression in zap_pte_range(). So we need them to be done earlier. Or
> I miss something?
My point was just that large anon folios improves performance significantly
overall, despite a small perf regression in zap_pte_range(). That regression is
reduced further by a patch from Yin Fengwei to reduce the lock contention [1].
So it doesn't seem urgent to me to get the "batch zap" change in.
I'll add it to my list, then prioritize it against the other stuff.
[1] https://lore.kernel.org/linux-mm/20230429082759.1600796-1-fengwei.yin@intel.com/
>
> Best Regards,
> Huang, Ying
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