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 9:36 AM, Zi Yan wrote:
> On 15 Apr 2026, at 21:22, Baolin Wang wrote:
> 
>> 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.
> 
> IIUC, generic_perform_write() needs to use a small chunk if tmpfs denies huge.
> It seems that Kefeng did that in the first try[1]. But willy suggested
> the current fix.
> 
> I wonder if we should revisit Kefeng’s first version.
> 
> [1] https://lore.kernel.org/all/20240914140613.2334139-1-wangkefeng.wang@huawei.com/

Personally, I still prefer the current fix (commit 5a90c155defa). We 
should honor the tmpfs mount option. If it explicitly says no large 
folios, we shouldn’t call mapping_set_large_folios(). Isn’t that more 
consistent with its semantics?