Re: [PATCH] hugetlb: block hugetlb file creation if hugetlb is not set up

[Jann Horn <jannh@google.com>]

On Tue, Jun 3, 2025 at 7:43 AM Oscar Salvador <osalvador@suse.de> wrote:
> On Tue, Jun 03, 2025 at 06:29:24AM +0200, Jann Horn wrote:
> > Yes, this change changes kernel behavior that is userspace-visible,
> > and causes syscalls to return errors where they worked before.
>
> Yes, that is what make me unease about this.
>
> It is true that most of the hugetlb cases out there work on
> pre-allocated pages, because the later it gets the harder to get large
> pages from the system.
>
> But as you say below, there might be applications out there that tweak
> the sys knobs themselves, and with this change those might break.
> Now, how valid are those? Heh, hard to anwser.
>
> So I guess it boils down to how hard and effective is to actually exploit
> whatever we manage to create by allowing this.
> But if we take that route, I think that hinting the user about this behaviour
> change is the right thing to do.

I guess the unusual part of hugetlb are really mostly the shared page
tables, which are also the reason why walks of hugetlb page tables
require extra locks. I guess I'll think about this some more and maybe
send a slightly different take on this that only prevents page fault
handling from getting far enough to establish shared PMDs, or
something like that.