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

[David Hildenbrand <david@redhat.com>]

Sorry for the late reply!

>>>>> 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.
> 
> So, in order to make tmpfs behave like other filesystems, we need to
> allocate large folios by default. Not setting 'huge=' is the same as
> setting it to 'huge=never' as per documentation. But 'huge=' is meant to
> control THP, not large folios, so it should not have a conflict here, or
> else, what case are you thinking?

I think we really have to move away from "huge/thp == PMD", that's a 
historical artifact. Everything else will simply be inconsistent and 
confusing in the future -- and I don't see any real need for that. For 
anonymous memory and anon shmem we managed the transition. (there is a 
longer writeup from me about this topic, so I won't go into detail).


I think I raised this in the past, but tmpfs/shmem is just like any 
other file system .. except it sometimes really isn't and behaves much 
more like (swappable) anonymous memory. (or mlocked files)

There are many systems out there that run without swap enabled, or with 
extremely minimal swap (IIRC until recently kubernetes was completely 
incompatible with swapping). Swap can even be disabled today for shmem 
using a mount option.

That's a big difference to all other file systems where you are 
guaranteed to have backend storage where you can simply evict under 
memory pressure (might temporarily fail, of course).

I *think* that's the reason why we have the "huge=" parameter that also 
controls the THP allocations during page faults (IOW possible memory 
over-allocation). Maybe also because it was a new feature, and we only 
had a single THP size.

There is, of course also the "fallocate() might not free up memory if 
there is an unexpected reference on the page because splitting it will 
fail" problem, that even exists when not over-allocating memory in the 
first place ...


So ...I don't think tmpfs behaves like other file system in some cases. 
And I don't think ignoring these points is a good idea.

Fortunately I don't maintain that code :)


If we don't want to go with the shmem_enabled toggles, we should 
probably still extend the documentation to cover "all THP sizes", like 
we did elsewhere.

huge=never: no THPs of any size
huge=always: THPs of any size (fault/write/etc)
huge=fadvise: like "always" but only with fadvise/madvise
huge=within_size: like "fadvise" but respect i_size

We could think about adding a "nowaste" extension and try make it the 
default.

For example

"huge=always:nowaste: THPs of any size as long as we don't over-allocate 
memory (write)"

The sysfs toggles have their beauty as well and could be useful (I'm 
pretty sure they will be useful :) ):

"huge=always;sysfs": THPs of any size (fault/write/etc) as configured in 
sysfs.

Too many options here to explore, too little time I have to spend on 
this. Just to throw out some ideas.

What I can really suggest is not making this one of the remaining 
interfaces where "huge" means "PMD-sized" once other sizes exist.

> 
>>>>
>>>> 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.
> 
> I agree. That aligns with what I recall Hugh requested. However, I
> believe if that is the way to go, we shouldn't limit it to tmpfs.
> Otherwise, why should tmpfs be prevented from allocating large folios if
> other filesystems in the system are allowed to allocate them?

See above. On systems without/little swap you might not want them for 
shmem/tmpfs, but would happily use them elsewhere.

The "write() won't waste memory" case is really interesting, the 
"fallocate cannot free the memory" still exists. A shrinker might help.

-- 
Cheers,

David / dhildenb