Re: [PATCH v2] mm: fault in complete folios instead of individual pages for tmpfs

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/7/7 21:33, Lorenzo Stoakes wrote:
> On Sun, Jul 06, 2025 at 10:02:35AM +0800, Baolin Wang wrote:
>>
>>
>> On 2025/7/5 06:18, Andrew Morton wrote:
>>> On Fri,  4 Jul 2025 11:19:26 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
>>>
>>>> After commit acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs"),
>>>> tmpfs can also support large folio allocation (not just PMD-sized large
>>>> folios).
>>>>
>>>> However, when accessing tmpfs via mmap(), although tmpfs supports large folios,
>>>> we still establish mappings at the base page granularity, which is unreasonable.
>>>>
>>>> We can map multiple consecutive pages of a tmpfs folios at once according to
>>>> the size of the large folio. On one hand, this can reduce the overhead of page
>>>> faults; on the other hand, it can leverage hardware architecture optimizations
>>>> to reduce TLB misses, such as contiguous PTEs on the ARM architecture.
>>>>
>>>> Moreover, tmpfs mount will use the 'huge=' option to control large folio
>>>> allocation explicitly. So it can be understood that the process's RSS statistics
>>>> might increase, and I think this will not cause any obvious effects for users.
>>>>
>>>> Performance test:
>>>> I created a 1G tmpfs file, populated with 64K large folios, and write-accessed it
>>>> sequentially via mmap(). I observed a significant performance improvement:
>>>
>>> That doesn't sound like a crazy thing to do.
>>>
>>>> Before the patch:
>>>> real	0m0.158s
>>>> user	0m0.008s
>>>> sys	0m0.150s
>>>>
>>>> After the patch:
>>>> real	0m0.021s
>>>> user	0m0.004s
>>>> sys	0m0.017s
>>>
>>> And look at that.
>>>
>>>> diff --git a/mm/memory.c b/mm/memory.c
>>>> index 0f9b32a20e5b..9944380e947d 100644
>>>> --- a/mm/memory.c
>>>> +++ b/mm/memory.c
>>>> @@ -5383,10 +5383,10 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>>>>    	/*
>>>>    	 * Using per-page fault to maintain the uffd semantics, and same
>>>> -	 * approach also applies to non-anonymous-shmem faults to avoid
>>>> +	 * approach also applies to non shmem/tmpfs faults to avoid
>>>>    	 * inflating the RSS of the process.
>>>>    	 */
>>>> -	if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma)) ||
>>>> +	if (!vma_is_shmem(vma) || unlikely(userfaultfd_armed(vma)) ||
>>>>    	    unlikely(needs_fallback)) {
>>>>    		nr_pages = 1;
>>>>    	} else if (nr_pages > 1) {
>>>
>>> and that's it?
>>>
>>> I'm itching to get this into -stable, really.  What LTS user wouldn't
>>> want this?
>>
>> This is an improvement rather than a bugfix, so I don't think it needs to go
>> into LTS.
>>
>> Could it be viewed as correcting an oversight in
>>> acd7ccb284b8?
>>
>> Yes, I should have added this optimization in the series of the commit
>> acd7ccb284b8. But obviously, I missed this :(.
> 
> Buuut if this was an oversight for that patch that causes an unnecessary
> perf degradation, surely this should have fixes tag + cc stable no?

IMO, this commit acd7ccb284b8 won't cause perf degradation, instead it 
is used to introduce a new feature, while the current patch is a further 
reasonable optimization. As I mentioned, this is an improvement, not a 
bugfix or a patch to address performance regression.