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

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

On 2024/10/21 21:34, Daniel Gomez wrote:
> On Mon Oct 21, 2024 at 10:54 AM CEST, 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.
>>
>>>> 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.
> 
> I think moving tmpfs to allocate large folios opportunistically by
> default (as it was proposed initially) doesn't necessary conflict with
> the default behaviour (huge=never). We just need to clarify that in
> the documentation.
> 
> However, and IIRC, one of the requests from Hugh was to have a way to
> disable large folios which is something other FS do not have control
> of as of today. Ryan sent a proposal to actually control that globally
> but I think it didn't move forward. So, what are we missing to go back
> to implement large folios in tmpfs in the default case, as any other fs
> leveraging large folios?

IMHO, as I discussed with Kirill, we still need maintain compatibility 
with the 'huge=' mount option. This means that if 'huge=never' is set 
for tmpfs, huge page allocation will still be prohibited (which can 
address Hugh's request?). However, if 'huge=' is not set, we can 
allocate large folios based on the write size.