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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 20 Apr 2022 23:02:48 -0700
Message-ID: <CAHk-=wjYabTPnKiHgVzeKCaRkQaGVunwPbS+QeVb09Bm=YUEow@mail.gmail.com>
Subject: Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE_VMAP
To:     Nicholas Piggin <npiggin@gmail.com>
Cc:     Mike Rapoport <rppt@kernel.org>,
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On Wed, Apr 20, 2022 at 10:48 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> The lagepage thing needs to be opt-in, and needs a lot more care.

Side note: part of the opt-in really should be about the performance impact.

It clearly can be quite noticeable, as outlined by that powerpc case
in commit 8abddd968a30 ("powerpc/64s/radix: Enable huge vmalloc
mappings"), but it presumably is some _particular_ case that actually
matters.

But it's equalyl clearly not the module code/data case, since
__module_alloc() explicitly disables largepages on powerpc.

At a guess, it's one or more of the large hash-table allocations.

And it would actually be interesting to hear *which*one*. From the
'git diff' workload, I'd expect it to be the dentry lookup hash table
- I can't think of anything else that would be vmalloc'ed that would
be remotely interesting - but who knows.

So I think the whole "opt in" isn't _purely_ about the "oh, random
cases are broken for odd reasons, so let's not enable it by default".

I think it would actually be good to literally mark the cases that
matter (and have the performance numbers for those cases).

               Linus