Message-ID: <CAHbLzkpKBzRUv+QVwsfSxMw7kuSkZhyYo9yjWS5x28fk8j_hRA@mail.gmail.com>
Date: Fri, 12 Apr 2024 17:50:56 -0700
From: Yang Shi <shy828301@...il.com>
To: Zi Yan <ziy@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, linux-mm@...ck.org, 
	Andrew Morton <akpm@...ux-foundation.org>, Matthew Wilcox <willy@...radead.org>, 
	Ryan Roberts <ryan.roberts@....com>, Barry Song <21cnbao@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On Fri, Apr 12, 2024 at 3:59 PM Zi Yan <ziy@...dia.com> wrote:
>
> On 12 Apr 2024, at 18:29, Yang Shi wrote:
>
> > On Fri, Apr 12, 2024 at 2:06 PM Zi Yan <ziy@...dia.com> wrote:
> >>
> >> On 12 Apr 2024, at 15:32, David Hildenbrand wrote:
> >>
> >>> On 12.04.24 16:35, Zi Yan wrote:
> >>>> On 11 Apr 2024, at 11:46, David Hildenbrand wrote:
> >>>>
> >>>>> On 11.04.24 17:32, Zi Yan wrote:
> >>>>>> From: Zi Yan <ziy@...dia.com>
> >>>>>>
> >>>>>> In __folio_remove_rmap(), a large folio is added to deferred split list
> >>>>>> if any page in a folio loses its final mapping. It is possible that
> >>>>>> the folio is unmapped fully, but it is unnecessary to add the folio
> >>>>>> to deferred split list at all. Fix it by checking folio mapcount before
> >>>>>> adding a folio to deferred split list.
> >>>>>>
> >>>>>> Signed-off-by: Zi Yan <ziy@...dia.com>
> >>>>>> ---
> >>>>>>    mm/rmap.c | 9 ++++++---
> >>>>>>    1 file changed, 6 insertions(+), 3 deletions(-)
> >>>>>>
> >>>>>> diff --git a/mm/rmap.c b/mm/rmap.c
> >>>>>> index 2608c40dffad..d599a772e282 100644
> >>>>>> --- a/mm/rmap.c
> >>>>>> +++ b/mm/rmap.c
> >>>>>> @@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >>>>>>                    enum rmap_level level)
> >>>>>>    {
> >>>>>>            atomic_t *mapped = &folio->_nr_pages_mapped;
> >>>>>> -  int last, nr = 0, nr_pmdmapped = 0;
> >>>>>> +  int last, nr = 0, nr_pmdmapped = 0, mapcount = 0;
> >>>>>>            enum node_stat_item idx;
> >>>>>>            __folio_rmap_sanity_checks(folio, page, nr_pages, level);
> >>>>>> @@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >>>>>>                            break;
> >>>>>>                    }
> >>>>>>   -                atomic_sub(nr_pages, &folio->_large_mapcount);
> >>>>>> +          mapcount = atomic_sub_return(nr_pages,
> >>>>>> +                                       &folio->_large_mapcount) + 1;
> >>>>>
> >>>>> That becomes a new memory barrier on some archs. Rather just re-read it below. Re-reading should be fine here.
> >>>>
> >>>> Would atomic_sub_return_relaxed() work? Originally I was using atomic_read(mapped)
> >>>> below, but to save an atomic op, I chose to read mapcount here.
> >>>
> >>> Some points:
> >>>
> >>> (1) I suggest reading about atomic get/set vs. atomic RMW vs. atomic
> >>> RMW that return a value -- and how they interact with memory barriers.
> >>> Further, how relaxed variants are only optimized on some architectures.
> >>>
> >>> atomic_read() is usually READ_ONCE(), which is just an "ordinary" memory
> >>> access that should not be refetched. Usually cheaper than most other stuff
> >>> that involves atomics.
> >>
> >> I should have checked the actual implementation instead of being fooled
> >> by the name. Will read about it. Thanks.
> >>
> >>>
> >>> (2) We can either use folio_large_mapcount() == 0 or !atomic_read(mapped)
> >>> to figure out if the folio is now completely unmapped.
> >>>
> >>> (3) There is one fundamental issue: if we are not batch-unmapping the whole
> >>> thing, we will still add the folios to the deferred split queue. Migration
> >>> would still do that, or if there are multiple VMAs covering a folio.
> >>>
> >>> (4) We should really avoid making common operations slower only to make
> >>> some unreliable stats less unreliable.
> >>>
> >>>
> >>> We should likely do something like the following, which might even be a bit
> >>> faster in some cases because we avoid a function call in case we unmap
> >>> individual PTEs by checking _deferred_list ahead of time
> >>>
> >>> diff --git a/mm/rmap.c b/mm/rmap.c
> >>> index 2608c40dffad..356598b3dc3c 100644
> >>> --- a/mm/rmap.c
> >>> +++ b/mm/rmap.c
> >>> @@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >>>                  * page of the folio is unmapped and at least one page
> >>>                  * is still mapped.
> >>>                  */
> >>> -               if (folio_test_large(folio) && folio_test_anon(folio))
> >>> -                       if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
> >>> -                               deferred_split_folio(folio);
> >>> +               if (folio_test_large(folio) && folio_test_anon(folio) &&
> >>> +                   (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped) &&
> >>> +                   atomic_read(mapped) &&
> >>> +                   data_race(list_empty(&folio->_deferred_list)))
> >>
> >> data_race() might not be needed, as Ryan pointed out[1]
> >>
> >>> +                       deferred_split_folio(folio);
> >>>         }
> >>>
> >>> I also thought about handling the scenario where we unmap the whole
> >>> think in smaller chunks. We could detect "!atomic_read(mapped)" and
> >>> detect that it is on the deferred split list, and simply remove it
> >>> from that list incrementing an THP_UNDO_DEFERRED_SPLIT_PAGE event.
> >>>
> >>> But it would be racy with concurrent remapping of the folio (might happen with
> >>> anon folios in corner cases I guess).
> >>>
> >>> What we can do is the following, though:
> >>>
> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >>> index dc30139590e6..f05cba1807f2 100644
> >>> --- a/mm/huge_memory.c
> >>> +++ b/mm/huge_memory.c
> >>> @@ -3133,6 +3133,8 @@ void folio_undo_large_rmappable(struct folio *folio)
> >>>         ds_queue = get_deferred_split_queue(folio);
> >>>         spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> >>>         if (!list_empty(&folio->_deferred_list)) {
> >>> +               if (folio_test_pmd_mappable(folio))
> >>> +                       count_vm_event(THP_UNDO_DEFERRED_SPLIT_PAGE);
> >>>                 ds_queue->split_queue_len--;
> >>>                 list_del_init(&folio->_deferred_list);
> >>>         }
> >>>
> >>> Adding the right event of course.
> >>>
> >>>
> >>> Then it's easy to filter out these "temporarily added to the list, but never split
> >>> before the folio was freed" cases.
> >>
> >> So instead of making THP_DEFERRED_SPLIT_PAGE precise, use
> >> THP_DEFERRED_SPLIT_PAGE - THP_UNDO_DEFERRED_SPLIT_PAGE instead? That should work.
> >
> > It is definitely possible that the THP on the deferred split queue are
> > freed instead of split. For example, 1M is unmapped for a 2M THP, then
> > later the remaining 1M is unmapped, or the process exits before memory
> > pressure happens. So how come we can tell it is "temporarily added to
> > list"? Then THP_DEFERRED_SPLIT_PAGE - THP_UNDO_DEFERRED_SPLIT_PAGE
> > actually just counts how many pages are still on deferred split queue.
> > It may be useful. However the counter is typically used to estimate
> > how many THP are partially unmapped during a period of time. So we
> > just need to know the initial value and the value when we read it
> > again.
> >
> >>
> >> I wonder what THP_DEFERRED_SPLIT_PAGE counts. If it counts THP deferred
> >> splits, why not just move the counter to deferred_split_scan(), where the actual
> >> split happens. Or the counter has a different meaning?
> >
> > The deferred_split_scan() / deferred_split_count() just can return the
> > number of pages on a specific queue (a specific node with a specific
> > memcg). But THP_DEFERRED_SPLIT_PAGE is a global counter. Did I miss
> > something? Or you mean traverse all memcgs and all nodes? It sounds
> > too overkilling.
>
> I mean instead of increasing THP_DEFERRED_SPLIT_PAGE when a folio is added
> to the split list, increase it when a folio is split in deferred_split_scan(),
> regardless which list the folio is on.

It will have overlap with thp_split_page. And what if memory pressure
doesn't happen? The counter will be 0 even though thousands THP have
been partially unmapped.

>
> --
> Best Regards,
> Yan, Zi

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