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Message-ID: <CAGsJ_4z2YL01wvVgsFsbzGAQ5KowXgxLcj=Y7DSX7ODOF5MUvw@mail.gmail.com>
Date: Tue, 20 Aug 2024 09:55:49 +1200
From: Barry Song <baohua@...nel.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: 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, david@...hat.com, ryan.roberts@....com, rppt@...nel.org,
willy@...radead.org, cerasuolodomenico@...il.com, ryncsn@...il.com,
corbet@....net, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
kernel-team@...a.com
Subject: Re: [PATCH v4 4/6] mm: Introduce a pageflag for partially mapped folios
On Tue, Aug 20, 2024 at 9:34 AM Barry Song <baohua@...nel.org> wrote:
>
> On Tue, Aug 20, 2024 at 8:16 AM Usama Arif <usamaarif642@...il.com> wrote:
> >
> >
> >
> > On 19/08/2024 20:00, Barry Song wrote:
> > > On Tue, Aug 20, 2024 at 2:17 AM Usama Arif <usamaarif642@...il.com> wrote:
> > >>
> > >>
> > >>
> > >> On 19/08/2024 09:29, Barry Song wrote:
> > >>> Hi Usama,
> > >>>
> > >>> I feel it is much better now! thanks!
> > >>>
> > >>> On Mon, Aug 19, 2024 at 2:31 PM Usama Arif <usamaarif642@...il.com> wrote:
> > >>>>
> > >>>> Currently folio->_deferred_list is used to keep track of
> > >>>> partially_mapped folios that are going to be split under memory
> > >>>> pressure. In the next patch, all THPs that are faulted in and collapsed
> > >>>> by khugepaged are also going to be tracked using _deferred_list.
> > >>>>
> > >>>> This patch introduces a pageflag to be able to distinguish between
> > >>>> partially mapped folios and others in the deferred_list at split time in
> > >>>> deferred_split_scan. Its needed as __folio_remove_rmap decrements
> > >>>> _mapcount, _large_mapcount and _entire_mapcount, hence it won't be
> > >>>> possible to distinguish between partially mapped folios and others in
> > >>>> deferred_split_scan.
> > >>>>
> > >>>> Eventhough it introduces an extra flag to track if the folio is
> > >>>> partially mapped, there is no functional change intended with this
> > >>>> patch and the flag is not useful in this patch itself, it will
> > >>>> become useful in the next patch when _deferred_list has non partially
> > >>>> mapped folios.
> > >>>>
> > >>>> Signed-off-by: Usama Arif <usamaarif642@...il.com>
> > >>>> ---
> > >>>> include/linux/huge_mm.h | 4 ++--
> > >>>> include/linux/page-flags.h | 11 +++++++++++
> > >>>> mm/huge_memory.c | 23 ++++++++++++++++-------
> > >>>> mm/internal.h | 4 +++-
> > >>>> mm/memcontrol.c | 3 ++-
> > >>>> mm/migrate.c | 3 ++-
> > >>>> mm/page_alloc.c | 5 +++--
> > >>>> mm/rmap.c | 5 +++--
> > >>>> mm/vmscan.c | 3 ++-
> > >>>> 9 files changed, 44 insertions(+), 17 deletions(-)
> > >>>>
> > >>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> > >>>> index 4c32058cacfe..969f11f360d2 100644
> > >>>> --- a/include/linux/huge_mm.h
> > >>>> +++ b/include/linux/huge_mm.h
> > >>>> @@ -321,7 +321,7 @@ static inline int split_huge_page(struct page *page)
> > >>>> {
> > >>>> return split_huge_page_to_list_to_order(page, NULL, 0);
> > >>>> }
> > >>>> -void deferred_split_folio(struct folio *folio);
> > >>>> +void deferred_split_folio(struct folio *folio, bool partially_mapped);
> > >>>>
> > >>>> void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> > >>>> unsigned long address, bool freeze, struct folio *folio);
> > >>>> @@ -495,7 +495,7 @@ static inline int split_huge_page(struct page *page)
> > >>>> {
> > >>>> return 0;
> > >>>> }
> > >>>> -static inline void deferred_split_folio(struct folio *folio) {}
> > >>>> +static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {}
> > >>>> #define split_huge_pmd(__vma, __pmd, __address) \
> > >>>> do { } while (0)
> > >>>>
> > >>>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > >>>> index a0a29bd092f8..c3bb0e0da581 100644
> > >>>> --- a/include/linux/page-flags.h
> > >>>> +++ b/include/linux/page-flags.h
> > >>>> @@ -182,6 +182,7 @@ enum pageflags {
> > >>>> /* At least one page in this folio has the hwpoison flag set */
> > >>>> PG_has_hwpoisoned = PG_active,
> > >>>> PG_large_rmappable = PG_workingset, /* anon or file-backed */
> > >>>> + PG_partially_mapped = PG_reclaim, /* was identified to be partially mapped */
> > >>>> };
> > >>>>
> > >>>> #define PAGEFLAGS_MASK ((1UL << NR_PAGEFLAGS) - 1)
> > >>>> @@ -861,8 +862,18 @@ static inline void ClearPageCompound(struct page *page)
> > >>>> ClearPageHead(page);
> > >>>> }
> > >>>> FOLIO_FLAG(large_rmappable, FOLIO_SECOND_PAGE)
> > >>>> +FOLIO_TEST_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
> > >>>> +/*
> > >>>> + * PG_partially_mapped is protected by deferred_split split_queue_lock,
> > >>>> + * so its safe to use non-atomic set/clear.
> > >>>> + */
> > >>>> +__FOLIO_SET_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
> > >>>> +__FOLIO_CLEAR_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
> > >>>> #else
> > >>>> FOLIO_FLAG_FALSE(large_rmappable)
> > >>>> +FOLIO_TEST_FLAG_FALSE(partially_mapped)
> > >>>> +__FOLIO_SET_FLAG_NOOP(partially_mapped)
> > >>>> +__FOLIO_CLEAR_FLAG_NOOP(partially_mapped)
> > >>>> #endif
> > >>>>
> > >>>> #define PG_head_mask ((1UL << PG_head))
> > >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > >>>> index 2d77b5d2291e..70ee49dfeaad 100644
> > >>>> --- a/mm/huge_memory.c
> > >>>> +++ b/mm/huge_memory.c
> > >>>> @@ -3398,6 +3398,7 @@ 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) {
> > >>>> @@ -3454,11 +3455,13 @@ 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);
> > >>>
> > >>> is it possible to make things clearer by
> > >>>
> > >>> if (folio_clear_partially_mapped)
> > >>> __folio_clear_partially_mapped(folio);
> > >>>
> > >>> While writing without conditions isn't necessarily wrong, adding a condition
> > >>> will improve the readability of the code and enhance the clarity of my mTHP
> > >>> counters series. also help decrease smp cache sync if we can avoid
> > >>> unnecessary writing?
> > >>>
> > >>
> > >> Do you mean if(folio_test_partially_mapped(folio))?
> > >>
> > >> I don't like this idea. I think it makes the readability worse? If I was looking at if (test) -> clear for the first time, I would become confused why its being tested if its going to be clear at the end anyways?
> > >
> > > In the pmd-order case, the majority of folios are not partially mapped.
> > > Unconditional writes will trigger cache synchronization across all
> > > CPUs (related to the MESI protocol), making them more costly. By
> > > using conditional writes, such as "if(test) write," we can avoid
> > > most unnecessary writes, which is much more efficient. Additionally,
> > > we only need to manage nr_split_deferred when the condition
> > > is met. We are carefully evaluating all scenarios to determine
> > > if modifications to the partially_mapped flag are necessary.
> > >
> >
> >
> > Hmm okay, as you said its needed for nr_split_deferred anyways. Something like below is ok to fold in?
> >
> > commit 4ae9e2067346effd902b342296987b97dee29018 (HEAD)
> > Author: Usama Arif <usamaarif642@...il.com>
> > Date: Mon Aug 19 21:07:16 2024 +0100
> >
> > mm: Introduce a pageflag for partially mapped folios fix
> >
> > Test partially_mapped flag before clearing it. This should
> > avoid unnecessary writes and will be needed in the nr_split_deferred
> > series.
> >
> > Signed-off-by: Usama Arif <usamaarif642@...il.com>
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 5d67d3b3c1b2..ccde60aaaa0f 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3479,7 +3479,8 @@ 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);
> > + if (folio_test_partially_mapped(folio))
> > + __folio_clear_partially_mapped(folio);
> > }
> > spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> > }
> > @@ -3610,7 +3611,8 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> > } else {
> > /* We lost race with folio_put() */
> > list_del_init(&folio->_deferred_list);
> > - __folio_clear_partially_mapped(folio);
> > + if (folio_test_partially_mapped(folio))
> > + __folio_clear_partially_mapped(folio);
> > ds_queue->split_queue_len--;
> > }
> > if (!--sc->nr_to_scan)
> >
>
> Do we also need if (folio_test_partially_mapped(folio)) in
> split_huge_page_to_list_to_order()?
>
> I recall that in Yu Zhao's TAO, there’s a chance of splitting (shattering)
> non-partially-mapped folios. To be future-proof, we might want to handle
> both cases equally.
we recall we also have a real case which can split entirely_mapped
folio:
mm: huge_memory: enable debugfs to split huge pages to any order
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fc4d182316bd5309b4066fd9ef21529ea397a7d4
>
> By the way, we might not need to clear the flag for a new folio. This differs
> from the init_list, which is necessary. If a new folio has the partially_mapped
> flag, it indicates that we failed to clear it when freeing the folio to
> the buddy system, which is a bug we need to fix in the free path.
>
> Thanks
> Barry
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