On 05/24/2017 04:28 PM, Pavel Emelyanov wrote: > On 05/24/2017 02:31 PM, Vlastimil Babka wrote: >> On 05/24/2017 12:39 PM, Mike Rapoport wrote: >>>> 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 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
