Message-Id: <D57RWGA2IIFD.2EWIDM7HVYF5U@kruces.com>
Date: Mon, 28 Oct 2024 22:56:14 +0100
From: "Daniel Gomez" <d@...ces.com>
To: "David Hildenbrand" <david@...hat.com>, "Baolin Wang"
 <baolin.wang@...ux.alibaba.com>, "Daniel Gomez" <da.gomez@...sung.com>,
 "Kirill A. Shutemov" <kirill@...temov.name>
Cc: "Matthew Wilcox" <willy@...radead.org>, <akpm@...ux-foundation.org>,
 <hughd@...gle.com>, <wangkefeng.wang@...wei.com>, <21cnbao@...il.com>,
 <ryan.roberts@....com>, <ioworker0@...il.com>, <linux-mm@...ck.org>,
 <linux-kernel@...r.kernel.org>, "Kirill A . Shutemov"
 <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH v3 0/4] Support large folios for tmpfs

On Fri Oct 25, 2024 at 10:21 PM CEST, David Hildenbrand wrote:
> 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.

Assuming a system without swap, what's the difference you are concern
about between using the current tmpfs allocation method vs large folios
implementation?

>
> 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)"

This is the default behaviour in other fs too. I don't think is
necessary to make it explicit.

>
> 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.

The previous implementation with large folios allocation was wrong
and was actually wasting memory by rounding up while trying to find
the order. Matthew already pointed it out [1]. So, with that fixed, we
should not end up wasting memory.

https://lore.kernel.org/all/ZvVQoY8Tn_BNc79T@casper.infradead.org/

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