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

[David Hildenbrand <david@redhat.com>]

On 23.10.24 10:04, Baolin Wang wrote:
> 
> 
> 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?

That's what I am saying: if there is some way to disable it that will 
keep working, great.

-- 
Cheers,

David / dhildenb