linux-kernel - Re: [PATCH v4 34/36] rmap: add folio_add_file_rmap

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Date:   Fri, 17 Mar 2023 21:28:57 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Ryan Roberts <ryan.roberts@....com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        <linux-arch@...r.kernel.org>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 34/36] rmap: add folio_add_file_rmap_range()



On 3/17/2023 8:46 PM, Ryan Roberts wrote:
> 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?).
OK. I will try to reproduce the issue on my local env to see whether I could
reproduce it on x86_64 env.


Regards
Yin, Fengwei

> 
> 
>>
>> 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
>>>>>>
>>>>>
>>>
>>>
>