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

[Yin, Fengwei]

Hi Matthew,

On 3/16/2023 12:08 AM, Ryan Roberts wrote:
> 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 a lot to Ryan for sharing the flamegraphs to me. I found the __do_fault()
is called with patch 32-36 while no __do_fault() just with first 31 patches. I 
suspect the folio_add_file_rmap_range() missed some PTEs population. Please give
me few days to find the root cause and fix. Sorry for this.


Regards
Yin, Fengwei

> 
>>
>> 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
>>
>