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

[Peter Xu <peterx@redhat.com>]

        On Tue, Jan 17, 2023 at 01:38:24PM -0800, James Houghton wrote:
> > > +             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.

Oops, yes, I somehow missed that one.  No worry - it's reported by
lockdep. :)

> 
> >
> > 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.

Sounds good, thanks.

-- 
Peter Xu