Re: [PATCH v2 2/2] hugetlb: optimize update_and_free_pages_bulk to avoid lock cycles

[James Houghton]

On Tue, Jul 18, 2023 at 9:47 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 07/18/23 09:31, James Houghton wrote:
> > On Mon, Jul 17, 2023 at 5:50 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> > > +        * destructor of all pages on list.
> > > +        */
> > > +       if (clear_dtor) {
> > > +               spin_lock_irq(&hugetlb_lock);
> > > +               list_for_each_entry(page, list, lru)
> > > +                       __clear_hugetlb_destructor(h, page_folio(page));
> > > +               spin_unlock_irq(&hugetlb_lock);
> > >         }
> >
> > I'm not too familiar with this code, but the above block seems weird
> > to me. If we successfully allocated the vmemmap for *any* folio, we
> > clear the hugetlb destructor for all the folios? I feel like we should
> > only be clearing the hugetlb destructor for all folios if the vmemmap
> > allocation succeeded for *all* folios. If the code is functionally
> > correct as is, I'm a little bit confused why we need `clear_dtor`; it
> > seems like this function doesn't really need it. (I could have some
> > huge misunderstanding here.)
> >
>
> Yes, it is a bit strange.
>
> I was thinking this has to also handle the case where hugetlb vmemmap
> optimization is off system wide.  In that case, clear_dtor would never
> be set and there is no sense in ever walking the list and calling
> __clear_hugetlb_destructor() would would be a NOOP in this case.  Think
> of the case where there are TBs of hugetlb pages.
>
> That is one of the reasons I made __clear_hugetlb_destructor() check
> for the need to modify the destructor.  The other reason is in the
> dissolve_free_huge_page() code path where we allocate vmemmap.  I
> suppose, there could be an explicit call to __clear_hugetlb_destructor()
> in dissolve_free_huge_page.  But, I thought it might be better if
> we just handled both cases here.
>
> My thinking is that the clear_dtor boolean would tell us if vmemmap was
> restored for ANY hugetlb page.  I am aware that just because vmemmap was
> allocated for one page, does not mean that it was allocated for others.
> However, in the common case where hugetlb vmemmap optimization is on
> system wide, we would have allocated vmemmap for all pages on the list
> and would need to clear the destructor for them all.
>
> So, clear_dtor is really just an optimization for the
> hugetlb_free_vmemmap=off case.  Perhaps that is just over thinking and
> not a useful miro-optimization.

Ok I think I understand; I think the micro-optimization is fine to
add. But I think there's still a bug here:

If we have two vmemmap-optimized hugetlb pages and restoring the page
structs for one of them fails, that page will end up with the
incorrect dtor (add_hugetlb_folio will set it properly, but then we
clear it afterwards because clear_dtor was set).

What do you think?