Re: [RFC PATCH v3 0/4] Support large folios for tmpfs

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

On 2024/10/21 16:54, Kirill A. Shutemov wrote:
> On Mon, Oct 21, 2024 at 02:24:18PM +0800, Baolin Wang wrote:
>>
>>
>> On 2024/10/17 19:26, Kirill A. Shutemov wrote:
>>> On Thu, Oct 17, 2024 at 05:34:15PM +0800, Baolin Wang wrote:
>>>> + Kirill
>>>>
>>>> On 2024/10/16 22:06, Matthew Wilcox wrote:
>>>>> On Thu, Oct 10, 2024 at 05:58:10PM +0800, Baolin Wang wrote:
>>>>>> Considering that tmpfs already has the 'huge=' option to control the THP
>>>>>> allocation, it is necessary to maintain compatibility with the 'huge='
>>>>>> option, as well as considering the 'deny' and 'force' option controlled
>>>>>> by '/sys/kernel/mm/transparent_hugepage/shmem_enabled'.
>>>>>
>>>>> No, it's not.  No other filesystem honours these settings.  tmpfs would
>>>>> not have had these settings if it were written today.  It should simply
>>>>> ignore them, the way that NFS ignores the "intr" mount option now that
>>>>> we have a better solution to the original problem.
>>>>>
>>>>> To reiterate my position:
>>>>>
>>>>>     - When using tmpfs as a filesystem, it should behave like other
>>>>>       filesystems.
>>>>>     - When using tmpfs to implement MAP_ANONYMOUS | MAP_SHARED, it should
>>>>>       behave like anonymous memory.
>>>>
>>>> I do agree with your point to some extent, but the ‘huge=’ option has
>>>> existed for nearly 8 years, and the huge orders based on write size may not
>>>> achieve the performance of PMD-sized THP in some scenarios, such as when the
>>>> write length is consistently 4K. So, I am still concerned that ignoring the
>>>> 'huge' option could lead to compatibility issues.
>>>
>>> Yeah, I don't think we are there yet to ignore the mount option.
>>
>> OK.
>>
>>> Maybe we need to get a new generic interface to request the semantics
>>> tmpfs has with huge= on per-inode level on any fs. Like a set of FADV_*
>>> handles to make kernel allocate PMD-size folio on any allocation or on
>>> allocations within i_size. I think this behaviour is useful beyond tmpfs.
>>>
>>> Then huge= implementation for tmpfs can be re-defined to set these
>>> per-inode FADV_ flags by default. This way we can keep tmpfs compatible
>>> with current deployments and less special comparing to rest of
>>> filesystems on kernel side.
>>
>> I did a quick search, and I didn't find any other fs that require PMD-sized
>> huge pages, so I am not sure if FADV_* is useful for filesystems other than
>> tmpfs. Please correct me if I missed something.
> 
> What do you mean by "require"? THPs are always opportunistic.
> 
> IIUC, we don't have a way to hint kernel to use huge pages for a file on
> read from backing storage. Readahead is not always the right way.

IIUC, most file systems use method similar to iomap buffered IO (see 
iomap_get_folio()) to allocate huge pages. What I mean is that, it would 
be better to have a real use case to add a hint for allocating THP 
(other than tmpfs).

>>> If huge= is not set, tmpfs would behave the same way as the rest of
>>> filesystems.
>>
>> So if 'huge=' is not set, tmpfs write()/fallocate() can still allocate large
>> folios based on the write size? If yes, that means it will change the
>> default huge behavior for tmpfs. Because previously having 'huge=' is not
>> set means the huge option is 'SHMEM_HUGE_NEVER', which is similar to what I
>> mentioned:
>> "Another possible choice is to make the huge pages allocation based on write
>> size as the *default* behavior for tmpfs, ..."
> 
> I am more worried about breaking existing users of huge pages. So changing
> behaviour of users who don't specify huge is okay to me.

OK. Good.