Re: [PATCH] mm/rmap: do not add fully unmapped large folio to deferred split list

[Yang Shi]

On Fri, Apr 12, 2024 at 2:06 PM Zi Yan <ziy@xxxxxxxxxx> 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@xxxxxxxxxx>
> >>>>
> >>>> 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@xxxxxxxxxx>
> >>>> ---
> >>>>    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.

>
>
>
> [1] https://lore.kernel.org/linux-mm/e3e14098-eade-483e-a459-e43200b87941@xxxxxxx/
>
> --
> Best Regards,
> Yan, Zi