Re: [PATCH v2] mm: shmem: don't set large-order range for internal shmem mount

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

On 4/16/26 9:11 AM, Zi Yan wrote:
> On 15 Apr 2026, at 21:05, Baolin Wang wrote:
> 
>> On 4/15/26 10:36 PM, David Hildenbrand (Arm) wrote:
>>> On 4/15/26 12:05, Baolin Wang wrote:
>>>>
>>>>
>>>> On 4/15/26 5:54 PM, David Hildenbrand (Arm) wrote:
>>>>>>
>>>>>> Yes, that makes sense.
>>>>>>
>>>>>> However, it’s also possible that the mapping does not support large
>>>>>> folios, yet anonymous shmem can still allocate large folios via the
>>>>>> sysfs interfaces. That doesn't make sense, right?
>>>>>
>>>>> That's what I am saying: if there could be large folios in there, then
>>>>> let's tell the world.
>>>>>
>>>>> Getting in a scenario where the mapping claims to not support large
>>>>> folios, but then we have large folios in there is inconsistent, not?
>>>>>
>>>>> [...]
>>>>>
>>>>>>
>>>>>> For the current anonymous shmem (tmpfs is already clear, no questions),
>>>>>> I don’t think there will be any "will never have/does never allow"
>>>>>> cases, because it can be changed dynamically via the sysfs interfaces.
>>>>>
>>>>> Right. It's about non-anon shmem with huge=off.
>>>>>
>>>>>>
>>>>>> If we still want that logic, then for anonymous shmem we can treat it as
>>>>>> always "might have large folios".
>>>>
>>>> OK. To resolve the confusion about 1, the logic should be changed as
>>>> follows. Does that make sense to you?
>>>>
>>>> if (sbinfo->huge || (sb->s_flags & SB_KERNMOUNT))
>>>>       mapping_set_large_folios(inode->i_mapping);
>>>
>>> I think that's better.
>>
>> Thanks for your valuable input.
>>
>> But has Willy says, maybe we can just
>>> unconditionally set it and have it even simpler.
>>
>> However, for tmpfs mounts, we should still respect the 'huge=' mount option. See commit 5a90c155defa ("tmpfs: don't enable large folios if not supported").
> 
> Is it possible to get sbinfo->huge during tmpfs’s folio allocation time, so that
> even if all tmpfs has mapping_set_large_folios() but sbinfo->huge can still
> decide whether huge page will be allocated for a tmpfs?

Yes, of course. However, the issue isn’t whether tmpfs allows allocating 
large folios.

The problem commit 5a90c155defa tries to fix is that when tmpfs is 
mounted with the 'huge=never' option, we will not allocate large folios 
for it. Then when writing tmpfs files, generic_perform_write() will call 
mapping_max_folio_size() to get the chunk size and ends up with an 
order-9 size for writing tmpfs files. However, this tmpfs file is 
populated only with small folios, resulting in a performance regression.