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

[James Houghton]

On Wed, Jul 19, 2023 at 5:19 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 07/19/23 17:02, James Houghton wrote:
> > 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?
>
> add_hugetlb_folio() will call enqueue_hugetlb_folio() which will move
> the  folio from the existing list we are processing to the hugetlb free
> list.  Therefore, the page for which we could not restore vmemmap is not
> on the list for that 'if (clear_dtor)' block of code.

Oh, I see. Thanks! Unless you think it's pretty obvious, perhaps a
comment would be good to add here, to explain that folios are removed
from 'list' if their vmemmap isn't restored.

Unrelated nit: I think you mean to use
folio_test_hugetlb_vmemmap_optimized instead of HPageVmemmapOptimized
in this patch.

Feel free to add:

Acked-by: James Houghton <jthoughton@xxxxxxxxxx>


>
> --
> Mike Kravetz