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

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

On 2024/10/22 23:31, David Hildenbrand wrote:
> On 22.10.24 05:41, Baolin Wang wrote:
>>
>>
>> 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.
> 
> I consider allocating large folios in shmem/tmpfs on the write path less 
> controversial than allocating them on the page fault path -- especially 
> as long as we stay within the size to-be-written.
> 
> I think in RHEL THP on shmem/tmpfs are disabled as default (e.g., 
> shmem_enabled=never). Maybe because of some rather undesired 
> side-effects (maybe some are historical?): I recall issues with VMs with 
> THP+ memory ballooning, as we cannot reclaim pages of folios if 
> splitting fails). I assume most of these problematic use cases don't use 
> tmpfs as an ordinary file system (write()/read()), but mmap() the whole 
> thing.
> 
> Sadly, I don't find any information about shmem/tmpfs + THP in the RHEL 
> documentation; most documentation is only concerned about anon THP. 
> Which makes me conclude that they are not suggested as of now.
> 
> I see more issues with allocating them on the page fault path and not 
> having a way to disable it -- compared to allocating them on the write() 
> path.

I may not understand your issues. IIUC, you can disable allocating huge 
pages on the page fault path by using the 'huge=never' mount option or 
setting shmem_enabled=deny. No?