On Wed, Apr 20, 2022 at 10:48 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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
