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

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Date:   Mon, 25 Sep 2023 15:35:36 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <muchun.song@...ux.dev>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>,
        Joao Martins <joao.m.martins@...cle.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Rientjes <rientjes@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Barry Song <21cnbao@...il.com>, Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun Duan <duanxiongchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v5 4/8] hugetlb: perform vmemmap restoration on a list of
 pages

On 09/25/23 10:12, Mike Kravetz wrote:
> 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@...cle.com>
> > > ---
> > >   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().

Correction.  My testing with simulated errors took the else branch here.

-- 
Mike Kravetz