linux-kernel - Re: [PATCH v3 4/6] mm: Introduce a pageflag for partially mapped folios

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Message-ID: <CAGsJ_4w13QMqXe8CL280CoHAeVSqHuoSgL0ubNVbGyABuhtGcQ@mail.gmail.com>
Date: Fri, 16 Aug 2024 09:06:25 +1200
From: Barry Song <baohua@...nel.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: David Hildenbrand <david@...hat.com>, akpm@...ux-foundation.org, linux-mm@...ck.org, 
	hannes@...xchg.org, riel@...riel.com, shakeel.butt@...ux.dev, 
	roman.gushchin@...ux.dev, yuzhao@...gle.com, ryan.roberts@....com, 
	rppt@...nel.org, willy@...radead.org, cerasuolodomenico@...il.com, 
	corbet@....net, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
	kernel-team@...a.com
Subject: Re: [PATCH v3 4/6] mm: Introduce a pageflag for partially mapped folios

On Fri, Aug 16, 2024 at 5:10 AM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 15/08/2024 17:33, David Hildenbrand wrote:
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index 6df0e9f4f56c..c024ab0f745c 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -3397,6 +3397,7 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> >>                * page_deferred_list.
> >>                */
> >>               list_del_init(&folio->_deferred_list);
> >> +            folio_clear_partially_mapped(folio);
> >>           }
> >>           spin_unlock(&ds_queue->split_queue_lock);
> >>           if (mapping) {
> >> @@ -3453,11 +3454,12 @@ void __folio_undo_large_rmappable(struct folio *folio)
> >>       if (!list_empty(&folio->_deferred_list)) {
> >>           ds_queue->split_queue_len--;
> >>           list_del_init(&folio->_deferred_list);
> >> +        folio_clear_partially_mapped(folio);
> >>       }
> >>       spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> >>   }
> >>   -void deferred_split_folio(struct folio *folio)
> >> +void deferred_split_folio(struct folio *folio, bool partially_mapped)
> >>   {
> >     /* We lost race with folio_put() */>            list_del_init(&folio->_deferred_list);
>
> Was there some comment here? I just see ">" remove from the start of /* We lost race with folio_put() */
>
> >> +            folio_clear_partially_mapped(folio);
> >>               ds_queue->split_queue_len--;
> >>           }
> >>           if (!--sc->nr_to_scan)
> >> @@ -3558,7 +3564,6 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> >>   next:
> >>           folio_put(folio);
> >>       }
> >> -
> >>       spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> >>       list_splice_tail(&list, &ds_queue->split_queue);
> >>       spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> >> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> >> index 1fdd9eab240c..2ae2d9a18e40 100644
> >> --- a/mm/hugetlb.c
> >> +++ b/mm/hugetlb.c
> >> @@ -1758,6 +1758,7 @@ static void __update_and_free_hugetlb_folio(struct hstate *h,
> >>           free_gigantic_folio(folio, huge_page_order(h));
> >>       } else {
> >>           INIT_LIST_HEAD(&folio->_deferred_list);
> >> +        folio_clear_partially_mapped(folio);
> >>           folio_put(folio);
> >>       }
> >>   }
> >> diff --git a/mm/internal.h b/mm/internal.h
> >> index 52f7fc4e8ac3..d64546b8d377 100644
> >> --- a/mm/internal.h
> >> +++ b/mm/internal.h
> >> @@ -662,8 +662,10 @@ static inline void prep_compound_head(struct page *page, unsigned int order)
> >>       atomic_set(&folio->_entire_mapcount, -1);
> >>       atomic_set(&folio->_nr_pages_mapped, 0);
> >>       atomic_set(&folio->_pincount, 0);
> >> -    if (order > 1)
> >> +    if (order > 1) {
> >>           INIT_LIST_HEAD(&folio->_deferred_list);
> >> +        folio_clear_partially_mapped(folio);
> >
> > Can we use the non-atomic version here?
> >
>
> I believe we can use the non-atomic version in all places where set/clear is done as all set/clear are protected by ds_queue->split_queue_lock. So basically could replace all folio_set/clear_partially_mapped with __folio_set/clear_partially_mapped.
>

right. That is why I thought the below is actually safe.
but i appreciate a test_set of course(and non-atomic):

+               if (!folio_test_partially_mapped(folio)) {
+                       folio_set_partially_mapped(folio);
+                       if (folio_test_pmd_mappable(folio))
+                               count_vm_event(THP_DEFERRED_SPLIT_PAGE);
+                       count_mthp_stat(folio_order(folio),
MTHP_STAT_SPLIT_DEFERRED);
+               }


> But I guess its likely not going to make much difference? I will do it anyways in the next revision, rather than sending a fix patch. There haven't been any reviews for patch 5 so will wait a few days for any comments on that.
>
> Thanks

Thanks
Barry