On 05/24/2017 04:28 PM, Pavel Emelyanov wrote:

On 05/24/2017 02:31 PM, Vlastimil Babka wrote:

quoted

On 05/24/2017 12:39 PM, Mike Rapoport wrote:

quoted

quoted

Hm so the prctl does:

                if (arg2)
                        me->mm->def_flags |= VM_NOHUGEPAGE;
                else
                        me->mm->def_flags &= ~VM_NOHUGEPAGE;

That's rather lazy implementation IMHO. Could we change it so the flag
is stored elsewhere in the mm, and the code that decides to (not) use
THP will check both the per-vma flag and the per-mm flag?

I afraid I don't understand how that can help.
What we need is an ability to temporarily disable collapse of the pages in
VMAs that do not have VM_*HUGEPAGE flags set and that after we re-enable
THP, the vma->vm_flags for those VMAs will remain intact.

That's what I'm saying - instead of implementing the prctl flag via
mm->def_flags (which gets permanently propagated to newly created vma's
but e.g. doesn't affect already existing ones), it would be setting a
flag somewhere in mm, which khugepaged (and page faults) would check in
addition to the per-vma flags.

I do not insist, but this would make existing paths (checking for flags) be 
2 times slower -- from now on these would need to check two bits (vma flags
and mm flags) which are 100% in different cache lines.

I'd expect you already have mm struct cached during a page fault. And
THP-eligible page fault is just one per pmd, the overhead should be
practically zero.

What Mike is proposing is the way to fine-tune the existing vma flags. This
would keep current paths as fast (or slow ;) ) as they are now. All the
complexity would go to rare cases when someone needs to turn thp off for a
while and then turn it back on.

Yeah but it's extending user-space API for a corner case. We should do
that only when there's no other option.

-- Pavel

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