linux-kernel - Re: [PATCH v5] mm/rmap: do not add fully unmapped large folio to deferred split list

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Date: Fri, 26 Apr 2024 15:20:42 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Yang Shi <shy828301@...il.com>, Ryan Roberts <ryan.roberts@....com>,
 Barry Song <21cnbao@...il.com>, Lance Yang <ioworker0@...il.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On 26 Apr 2024, at 15:08, David Hildenbrand wrote:

> On 26.04.24 21:02, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>>
>> In __folio_remove_rmap(), a large folio is added to deferred split list
>> if any page in a folio loses its final mapping. But it is possible that
>> the folio is fully unmapped and adding it to deferred split list is
>> unnecessary.
>>
>> For PMD-mapped THPs, that was not really an issue, because removing the
>> last PMD mapping in the absence of PTE mappings would not have added the
>> folio to the deferred split queue.
>>
>> However, for PTE-mapped THPs, which are now more prominent due to mTHP,
>> they are always added to the deferred split queue. One side effect
>> is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be
>> unintentionally increased, making it look like there are many partially
>> mapped folios -- although the whole folio is fully unmapped stepwise.
>>
>> Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs
>> where possible starting from commit b06dc281aa99 ("mm/rmap: introduce
>> folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-mapped
>> folio is unmapped in one go and can avoid being added to deferred split
>> list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still be
>> noise when we cannot batch-unmap a complete PTE-mapped folio in one go
>> -- or where this type of batching is not implemented yet, e.g., migration.
>>
>> To avoid the unnecessary addition, folio->_nr_pages_mapped is checked
>> to tell if the whole folio is unmapped. If the folio is already on
>> deferred split list, it will be skipped, too.
>>
>> Note: commit 98046944a159 ("mm: huge_memory: add the missing
>> folio_test_pmd_mappable() for THP split statistics") tried to exclude
>> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
>> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still
>> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
>> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
>> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable().
>>
>> Suggested-by: David Hildenbrand <david@...hat.com>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>>   mm/rmap.c | 12 +++++++++---
>>   1 file changed, 9 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 2608c40dffad..a9bd64ebdd9a 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1495,6 +1495,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>   {
>>   	atomic_t *mapped = &folio->_nr_pages_mapped;
>>   	int last, nr = 0, nr_pmdmapped = 0;
>> +	bool partially_mapped = false;
>>   	enum node_stat_item idx;
>>    	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
>> @@ -1515,6 +1516,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>   					nr++;
>>   			}
>>   		} while (page++, --nr_pages > 0);
>> +
>> +		partially_mapped = !!nr && !!atomic_read(mapped);
>
> Nit: The && should remove the need for both !!.

My impression was that !! is needed to convert from int to bool and I do
find "!!int && !!int" use in the kernel. If this is unnecessary, Andrew
can apply the fixup below. I can send a new version if it is really needed.

diff --git a/mm/rmap.c b/mm/rmap.c
index a9bd64ebdd9a..c1fd5828409b 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1517,7 +1517,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
                        }
                } while (page++, --nr_pages > 0);

-               partially_mapped = !!nr && !!atomic_read(mapped);
+               partially_mapped = nr && atomic_read(mapped);
                break;
        case RMAP_LEVEL_PMD:
                atomic_dec(&folio->_large_mapcount);

>
> Reviewed-by: David Hildenbrand <david@...hat.com>

Thanks.

--
Best Regards,
Yan, Zi

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