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

[Ryan Roberts]

On 17/03/2023 08:23, Yin, Fengwei wrote:
[...]

>>>>> 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.
>>
>> You're welcome. Give me a shout once you have a re-spin and I'll rerun the tests.
> Could you please help to try following changes? Thanks in advance.
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 40be33b5ee46..137011320c68 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3504,15 +3504,16 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
>  		if (!pte_none(vmf->pte[count]))
>  			goto skip;
>  
> -		if (vmf->address == addr)
> -			ret = VM_FAULT_NOPAGE;
> -
>  		count++;
>  		continue;
>  skip:
>  		if (count) {
>  			set_pte_range(vmf, folio, page, count, addr);
>  			folio_ref_add(folio, count);
> +			if ((vmf->address < (addr + count * PAGE_SIZE)) &&
> +					(vmf->address >= addr))
> +				ret = VM_FAULT_NOPAGE;
> +
>  		}
>  
>  		count++;
> @@ -3525,6 +3526,9 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
>  	if (count) {
>  		set_pte_range(vmf, folio, page, count, addr);
>  		folio_ref_add(folio, count);
> +		if ((vmf->address < (addr + count * PAGE_SIZE)) &&
> +				(vmf->address >= addr))
> +			ret = VM_FAULT_NOPAGE;
>  	}
>  
>  	vmf->pte = old_ptep;
> 

I'm afraid this hasn't fixed it, and I still see __do_fault(). I'll send the
flame graph over separately.

Given I'm running on ext4, I wasn't expecting to see any large page cache
folios? So I don't think we would have expected this patch to help anyway? (or
perhaps there are still THP folios? But I think they will get PMD mapped?).


> 
> Regards
> Yin, Fengwei
> 
>>
>>>
>>>
>>> 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
>>>>>
>>>>
>>
>>