Re: [PATCH 35/46] hugetlb: add MADV_COLLAPSE for hugetlb

[James Houghton]

> > +             if (curr < end) {
> > +                     /* Don't hold the VMA lock for too long. */
> > +                     hugetlb_vma_unlock_write(vma);
> > +                     cond_resched();
> > +                     hugetlb_vma_lock_write(vma);
>
> The intention is good here but IIUC this will cause vma lock to be taken
> after the i_mmap_rwsem, which can cause circular deadlocks.  If to do this
> properly we'll need to also release the i_mmap_rwsem.

Sorry if you spent a long time debugging this! I sent a reply a week
ago about this too.

>
> However it may make the resched() logic over complicated, meanwhile for 2M
> huge pages I think this will be called for each 2M range which can be too
> fine grained, so it looks like the "cur < end" check is a bit too aggresive.
>
> The other thing is I noticed that the long period of mmu notifier
> invalidate between start -> end will (in reallife VM context) causing vcpu
> threads spinning.
>
> I _think_ it's because is_page_fault_stale() (when during a vmexit
> following a kvm page fault) always reports true during the long procedure
> of MADV_COLLAPSE if to be called upon a large range, so even if we release
> both locks here it may not tremedously on the VM migration use case because
> of the long-standing mmu notifier invalidation procedure.

Oh... indeed. Thanks for pointing that out.

>
> To summarize.. I think a simpler start version of hugetlb MADV_COLLAPSE can
> drop this "if" block, and let the userapp decide the step size of COLLAPSE?

I'll drop this resched logic. Thanks Peter.