Re: [PATCH v4 4/8] hugetlb: perform vmemmap restoration on a list of pages

[Mike Kravetz]

On 09/19/23 17:52, Muchun Song wrote:
> 
> 
> On 2023/9/19 07:01, 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.
> > 
> > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > ---
> >   mm/hugetlb.c         | 36 ++++++++++++++++++------------------
> >   mm/hugetlb_vmemmap.c | 37 +++++++++++++++++++++++++++++++++++++
> >   mm/hugetlb_vmemmap.h | 11 +++++++++++
> >   3 files changed, 66 insertions(+), 18 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index d6f3db3c1313..814bb1982274 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1836,36 +1836,36 @@ static void update_and_free_hugetlb_folio(struct hstate *h, struct folio *folio,
> >   static void update_and_free_pages_bulk(struct hstate *h, struct list_head *list)
> >   {
> > +	int ret;
> > +	unsigned long restored;
> >   	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.
> > +	 * First allocate required vmemmmap (if necessary) for all folios.
> >   	 */
> > -	list_for_each_entry_safe(folio, t_folio, list, lru) {
> > -		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
> > -			if (hugetlb_vmemmap_restore(h, &folio->page)) {
> > -				spin_lock_irq(&hugetlb_lock);
> > +	ret = hugetlb_vmemmap_restore_folios(h, list, &restored);
> > +
> > +	/*
> > +	 * If there was an error restoring vmemmap for ANY folios on the list,
> > +	 * add them back as surplus hugetlb pages.  add_hugetlb_folio() removes
> > +	 * the folio from THIS list.
> > +	 */
> > +	if (ret < 0) {
> > +		spin_lock_irq(&hugetlb_lock);
> > +		list_for_each_entry_safe(folio, t_folio, list, lru)
> > +			if (folio_test_hugetlb_vmemmap_optimized(folio))
> >   				add_hugetlb_folio(h, folio, true);
> > -				spin_unlock_irq(&hugetlb_lock);
> > -			} else
> > -				clear_dtor = true;
> > -		}
> > +		spin_unlock_irq(&hugetlb_lock);
> >   	}
> >   	/*
> > -	 * If vmemmmap allocation was performed on any folio above, take lock
> > -	 * to clear destructor of all folios on list.  This avoids the need to
> > +	 * If vmemmmap allocation was performed on ANY folio , take lock to
> > +	 * clear destructor of all folios on list.  This avoids the need to
> >   	 * lock/unlock for each individual folio.
> >   	 * The assumption is vmemmap allocation was performed on all or none
> >   	 * of the folios on the list.  This is true expect in VERY rare cases.
> >   	 */
> > -	if (clear_dtor) {
> > +	if (restored) {
> >   		spin_lock_irq(&hugetlb_lock);
> >   		list_for_each_entry(folio, list, lru)
> >   			__clear_hugetlb_destructor(h, folio);
> > diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> > index 4558b814ffab..463a4037ec6e 100644
> > --- a/mm/hugetlb_vmemmap.c
> > +++ b/mm/hugetlb_vmemmap.c
> > @@ -480,6 +480,43 @@ int hugetlb_vmemmap_restore(const struct hstate *h, struct page *head)
> >   	return ret;
> >   }
> > +/**
> > + * hugetlb_vmemmap_restore_folios - restore vmemmap for every folio on the list.
> > + * @h:		struct hstate.
> > + * @folio_list:	list of folios.
> > + * @restored:	Set to number of folios for which vmemmap was restored
> > + *		successfully if caller passes a non-NULL pointer.
> > + *
> > + * Return: %0 if vmemmap exists for all folios on the list.  If an error is
> > + *		encountered restoring vmemmap for ANY folio, an error code
> > + *		will be returned to the caller.  It is then the responsibility
> > + *		of the caller to check the hugetlb vmemmap optimized flag of
> > + *		each folio to determine if vmemmap was actually restored.
> > + */
> > +int hugetlb_vmemmap_restore_folios(const struct hstate *h,
> > +					struct list_head *folio_list,
> > +					unsigned long *restored)
> > +{
> > +	unsigned long num_restored;
> > +	struct folio *folio;
> > +	int ret = 0, t_ret;
> > +
> > +	num_restored = 0;
> > +	list_for_each_entry(folio, folio_list, lru) {
> > +		if (folio_test_hugetlb_vmemmap_optimized(folio)) {
> > +			t_ret = hugetlb_vmemmap_restore(h, &folio->page);
> 
> I still think we should free a non-optimized HugeTLB page if we
> encounter an OOM situation instead of continue to restore
> vemmmap pages. Restoring vmemmmap pages will only aggravate
> the OOM situation. The suitable appraoch is to free a non-optimized
> HugeTLB page to satisfy our allocation of vmemmap pages, what's
> your opinion, Mike?

I agree.

As you mentioned previously, this may complicate this code path a bit.
I will rewrite to make this happen.
-- 
Mike Kravetz