On 15.05.25 20:08, Lorenzo Stoakes wrote:
On Thu, May 15, 2025 at 06:11:55PM +0200, David Hildenbrand wrote:
So if you're not overriding VM_NOHUGEPAGE, the whole point of this exercise
is to override global 'never'?
Again, I am not overriding never.
hugepage_global_always and hugepage_global_enabled will evaluate to false
and you will not get a hugepage.
Yeah, again ack, but I kind of hate that we set VM_HUGEPAGE everywhere even
if the policy is never.
I think it should behave just as if someone does manually an madvise(). So
whatever we do here during an madvise, we should try to do the same thing
here.
Ack I agree with this.
It actually simplifies things a LOT to view it this way - we're saying 'by
default apply madvise(...) to new VMAs'.
Hm I wonder if we could have a more generic version of this...
Note though that we're not _quite_ doing this.
So in hugepage_madvise():
int hugepage_madvise(struct vm_area_struct *vma,
unsigned long *vm_flags, int advice)
{
...
switch (advice) {
case MADV_HUGEPAGE:
*vm_flags &= ~VM_NOHUGEPAGE;
*vm_flags |= VM_HUGEPAGE;
...
break;
...
}
...
}
So here we're actually clearing VM_NOHUGEPAGE and overriding it, but in the
proposed code we're not.
Yeah, I think I suggested that, but probably we should just do exactly
what madvise() does.
So we're back into confusing territory again :)
I wonder if we could...
1. Add an MADV_xxx that mimics the desired behaviour here.
2. Add a generic 'madvise() by default' thing at a process level?
Is this crazy?
I think that's what I had in mind, just a bit twisted.
What could work is
1) prctl to set the default
2) madvise() to adjust all existing VMAs
We might have to teach 2) to ignore non-compatible VMAs / holes. Maybe
not, worth an investigation.
--
Cheers,
David / dhildenb
