Re: [PATCH v5 4/8] hugetlb: perform vmemmap restoration on a list of pages

[Mike Kravetz]

On 09/25/23 21:54, Muchun Song wrote:
> 
> 
> On 2023/9/25 08:39, Mike Kravetz wrote:
> > The routine update_and_free_pages_bulk already performs vmemmap
> > restoration on the list of hugetlb pages in a separate step.  In
> > preparation for more functionality to be added in this step, create a
> > new routine hugetlb_vmemmap_restore_folios() that will restore
> > vmemmap for a list of folios.
> > 
> > This new routine must provide sufficient feedback about errors and
> > actual restoration performed so that update_and_free_pages_bulk can
> > perform optimally.
> > 
> > Special care must be taken when encountering an error from
> > hugetlb_vmemmap_restore_folios.  We want to continue making as much
> > forward progress as possible.  A new routine bulk_vmemmap_restore_error
> > handles this specific situation.
> > 
> > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > ---
> >   mm/hugetlb.c         | 98 +++++++++++++++++++++++++++++++-------------
> >   mm/hugetlb_vmemmap.c | 38 +++++++++++++++++
> >   mm/hugetlb_vmemmap.h | 10 +++++
> >   3 files changed, 118 insertions(+), 28 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index da0ebd370b5f..53df35fbc3f2 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1834,50 +1834,92 @@ static void update_and_free_hugetlb_folio(struct hstate *h, struct folio *folio,
> >   		schedule_work(&free_hpage_work);
> >   }
> > -static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
> > +static void bulk_vmemmap_restore_error(struct hstate *h,
> > +					struct list_head *folio_list,
> > +					struct list_head *non_hvo_folios)
> >   {
> >   	struct folio *folio, *t_folio;
> > -	bool clear_dtor = false;
> > -	/*
> > -	 * First allocate required vmemmmap (if necessary) for all folios on
> > -	 * list.  If vmemmap can not be allocated, we can not free folio to
> > -	 * lower level allocator, so add back as hugetlb surplus page.
> > -	 * add_hugetlb_folio() removes the page from THIS list.
> > -	 * Use clear_dtor to note if vmemmap was successfully allocated for
> > -	 * ANY page on the list.
> > -	 */
> > -	list_for_each_entry_safe(folio, t_folio, list, lru) {
> > -		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
> > +	if (!list_empty(non_hvo_folios)) {
> > +		/*
> > +		 * Free any restored hugetlb pages so that restore of the
> > +		 * entire list can be retried.
> > +		 * The idea is that in the common case of ENOMEM errors freeing
> > +		 * hugetlb pages with vmemmap we will free up memory so that we
> > +		 * can allocate vmemmap for more hugetlb pages.
> > +		 */
> > +		list_for_each_entry_safe(folio, t_folio, non_hvo_folios, lru) {
> > +			list_del(&folio->lru);
> > +			spin_lock_irq(&hugetlb_lock);
> > +			__clear_hugetlb_destructor(h, folio);
> > +			spin_unlock_irq(&hugetlb_lock);
> > +			update_and_free_hugetlb_folio(h, folio, false);
> > +			cond_resched();
> > +		}
> > +	} else {
> > +		/*
> > +		 * In the case where there are no folios which can be
> > +		 * immediately freed, we loop through the list trying to restore
> > +		 * vmemmap individually in the hope that someone elsewhere may
> > +		 * have done something to cause success (such as freeing some
> > +		 * memory).  If unable to restore a hugetlb page, the hugetlb
> > +		 * page is made a surplus page and removed from the list.
> > +		 * If are able to restore vmemmap and free one hugetlb page, we
> > +		 * quit processing the list to retry the bulk operation.
> > +		 */
> > +		list_for_each_entry_safe(folio, t_folio, folio_list, lru)
> >   			if (hugetlb_vmemmap_restore(h, &folio->page)) {
> 
> IIUC, the folio should be deleted from the folio list since this
> huge page will be added to the free list (the list is corrupted),
> right?

Good catch!  Yes, we should remove from the list here.

I did exercise this specific code path and there was no list corruption.  In
any case, I will add the list_del().

Thank you!
-- 
Mike Kravetz