Re: [PATCH v4 34/36] rmap: add folio_add_file_rmap_range()

[Ryan Roberts]

On 15/03/2023 13:34, Ryan Roberts wrote:
> On 15/03/2023 05:14, Matthew Wilcox (Oracle) wrote:
>> From: Yin Fengwei <fengwei.yin@xxxxxxxxx>
>>
>> folio_add_file_rmap_range() allows to add pte mapping to a specific
>> range of file folio. Comparing to page_add_file_rmap(), it batched
>> updates __lruvec_stat for large folio.
>>
>> Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
>> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
>> ---
>>  include/linux/rmap.h |  2 ++
>>  mm/rmap.c            | 60 +++++++++++++++++++++++++++++++++-----------
>>  2 files changed, 48 insertions(+), 14 deletions(-)
>>
>> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
>> index b87d01660412..a3825ce81102 100644
>> --- a/include/linux/rmap.h
>> +++ b/include/linux/rmap.h
>> @@ -198,6 +198,8 @@ void folio_add_new_anon_rmap(struct folio *, struct vm_area_struct *,
>>  		unsigned long address);
>>  void page_add_file_rmap(struct page *, struct vm_area_struct *,
>>  		bool compound);
>> +void folio_add_file_rmap_range(struct folio *, struct page *, unsigned int nr,
>> +		struct vm_area_struct *, bool compound);
>>  void page_remove_rmap(struct page *, struct vm_area_struct *,
>>  		bool compound);
>>  
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 4898e10c569a..a91906b28835 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1301,31 +1301,39 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
>>  }
>>  
>>  /**
>> - * page_add_file_rmap - add pte mapping to a file page
>> - * @page:	the page to add the mapping to
>> + * folio_add_file_rmap_range - add pte mapping to page range of a folio
>> + * @folio:	The folio to add the mapping to
>> + * @page:	The first page to add
>> + * @nr_pages:	The number of pages which will be mapped
>>   * @vma:	the vm area in which the mapping is added
>>   * @compound:	charge the page as compound or small page
>>   *
>> + * The page range of folio is defined by [first_page, first_page + nr_pages)
>> + *
>>   * The caller needs to hold the pte lock.
>>   */
>> -void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
>> -		bool compound)
>> +void folio_add_file_rmap_range(struct folio *folio, struct page *page,
>> +			unsigned int nr_pages, struct vm_area_struct *vma,
>> +			bool compound)
>>  {
>> -	struct folio *folio = page_folio(page);
>>  	atomic_t *mapped = &folio->_nr_pages_mapped;
>> -	int nr = 0, nr_pmdmapped = 0;
>> -	bool first;
>> +	unsigned int nr_pmdmapped = 0, first;
>> +	int nr = 0;
>>  
>> -	VM_BUG_ON_PAGE(compound && !PageTransHuge(page), page);
>> +	VM_WARN_ON_FOLIO(compound && !folio_test_pmd_mappable(folio), folio);
>>  
>>  	/* Is page being mapped by PTE? Is this its first map to be added? */
>>  	if (likely(!compound)) {
>> -		first = atomic_inc_and_test(&page->_mapcount);
>> -		nr = first;
>> -		if (first && folio_test_large(folio)) {
>> -			nr = atomic_inc_return_relaxed(mapped);
>> -			nr = (nr < COMPOUND_MAPPED);
>> -		}
>> +		do {
>> +			first = atomic_inc_and_test(&page->_mapcount);
>> +			if (first && folio_test_large(folio)) {
>> +				first = atomic_inc_return_relaxed(mapped);
>> +				first = (nr < COMPOUND_MAPPED);
> 
> This still contains the typo that Yin Fengwei spotted in the previous version:
> https://lore.kernel.org/linux-mm/20230228213738.272178-1-willy@xxxxxxxxxxxxx/T/#m84673899e25bc31356093a1177941f2cc35e5da8
> 
> FYI, I'm seeing a perf regression of about 1% when compiling the kernel on
> Ampere Altra (arm64) with this whole series on top of v6.3-rc1 (In a VM using
> ext4 filesystem). Looks like instruction aborts are taking much longer and a
> selection of syscalls are a bit slower. Still hunting down the root cause. Will
> report once I have conclusive diagnosis.

I'm sorry - I'm struggling to find the exact cause. But its spending over 2x the
amount of time in the instruction abort handling code once patches 32-36 are
included. Everything in the flame graph is just taking longer. Perhaps we are
getting more instruction aborts somehow? I have the flamegraphs if anyone wants
them - just shout and I'll email them separately.

> 
> Thanks,
> Ryan
> 
> 
>> +			}
>> +
>> +			if (first)
>> +				nr++;
>> +		} while (page++, --nr_pages > 0);
>>  	} else if (folio_test_pmd_mappable(folio)) {
>>  		/* That test is redundant: it's for safety or to optimize out */
>>  
>> @@ -1354,6 +1362,30 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
>>  	mlock_vma_folio(folio, vma, compound);
>>  }
>>  
>> +/**
>> + * page_add_file_rmap - add pte mapping to a file page
>> + * @page:	the page to add the mapping to
>> + * @vma:	the vm area in which the mapping is added
>> + * @compound:	charge the page as compound or small page
>> + *
>> + * The caller needs to hold the pte lock.
>> + */
>> +void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
>> +		bool compound)
>> +{
>> +	struct folio *folio = page_folio(page);
>> +	unsigned int nr_pages;
>> +
>> +	VM_WARN_ON_ONCE_PAGE(compound && !PageTransHuge(page), page);
>> +
>> +	if (likely(!compound))
>> +		nr_pages = 1;
>> +	else
>> +		nr_pages = folio_nr_pages(folio);
>> +
>> +	folio_add_file_rmap_range(folio, page, nr_pages, vma, compound);
>> +}
>> +
>>  /**
>>   * page_remove_rmap - take down pte mapping from a page
>>   * @page:	page to remove mapping from
>