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

[Zi Yan <ziy@nvidia.com>]

On 15 Apr 2026, at 21:45, Baolin Wang wrote:

> 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?

Filesystems wishing to turn on large folios in the pagecache should call
``mapping_set_large_folios`` when initializing the incore inode.

You mean tmpfs with huge option set is a FS wishing to turn on large
folios in the pagecache, otherwise it is a FS wishing not to have large folio
in the pagecache. tmpfs with different options is seen as different FSes.

Best Regards,
Yan, Zi