Re: [PATCH hotfix 2/2] mm/thp: fix deferred split unqueue naming and locking

[Yang Shi]

On Thu, Oct 24, 2024 at 1:00 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Wed, Oct 23, 2024 at 9:13 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> >
> > Recent changes are putting more pressure on THP deferred split queues:
> > under load revealing long-standing races, causing list_del corruptions,
> > "Bad page state"s and worse (I keep BUGs in both of those, so usually
> > don't get to see how badly they end up without).  The relevant recent
> > changes being 6.8's mTHP, 6.10's mTHP swapout, and 6.12's mTHP swapin,
> > improved swap allocation, and underused THP splitting.
> >
> > Before fixing locking: rename misleading folio_undo_large_rmappable(),
> > which does not undo large_rmappable, to folio_unqueue_deferred_split(),
> > which is what it does.  But that and its out-of-line __callee are mm
> > internals of very limited usability: add comment and WARN_ON_ONCEs to
> > check usage; and return a bool to say if a deferred split was unqueued,
> > which can then be used in WARN_ON_ONCEs around safety checks (sparing
> > callers the arcane conditionals in __folio_unqueue_deferred_split()).
> >
> > Swapout: mem_cgroup_swapout() has been resetting folio->memcg_data 0
> > without checking and unqueueing a THP folio from deferred split list;
> > which is unfortunate, since the split_queue_lock depends on the memcg
> > (when memcg is enabled); so swapout has been unqueueing such THPs later,
> > when freeing the folio, using the pgdat's lock instead: potentially
> > corrupting the memcg's list.  __remove_mapping() has frozen refcount to
> > 0 here, so no problem with calling folio_unqueue_deferred_split() before
> > resetting memcg_data.
> >
> > That goes back to 5.4 commit 87eaceb3faa5 ("mm: thp: make deferred split
> > shrinker memcg aware"): which included a check on swapcache before adding
> > to deferred queue (which can now be removed), but no check on deferred
> > queue before adding THP to swapcache (maybe other circumstances prevented
> > it at that time, but not now).
>
> If I remember correctly, THP just can be added to deferred list when
> there is no PMD map before mTHP swapout, so shrink_page_list() did
> check THP's compound_mapcount (called _entire_mapcount now) before
> adding it to swap cache.

But as David pointed out there may be PMD mapped THP on deferred list.
We should just not hit it by luck. The migration failure may be very
rare because migration may split the THP or retry a few times, and
migration is called with holding extra refcount and having THP off
lru.

>
> Now the code just checked whether the large folio is on deferred list or not.
>
> >
> > Memcg-v1 move (deprecated): mem_cgroup_move_account() has been changing
> > folio->memcg_data without checking and unqueueing a THP folio from the
> > deferred list, sometimes corrupting "from" memcg's list, like swapout.
> > Refcount is non-zero here, so folio_unqueue_deferred_split() can only be
> > used in a WARN_ON_ONCE to validate the fix, which must be done earlier:
> > mem_cgroup_move_charge_pte_range() first try to split the THP (splitting
> > of course unqueues), or skip it if that fails.  Not ideal, but moving
> > charge has been requested, and khugepaged should repair the THP later:
> > nobody wants new custom unqueueing code just for this deprecated case.
> >
> > The 87eaceb3faa5 commit did have the code to move from one deferred list
> > to another (but was not conscious of its unsafety while refcount non-0);
> > but that was removed by 5.6 commit fac0516b5534 ("mm: thp: don't need
> > care deferred split queue in memcg charge move path"), which argued that
> > the existence of a PMD mapping guarantees that the THP cannot be on a
> > deferred list.  I'm not sure if that was true at the time (swapcache
> > remapped?), but it's clearly not true since 6.12 commit dafff3f4c850
> > ("mm: split underused THPs").
>
> Same reason as above.
>
> >
> > [Note in passing: mem_cgroup_move_charge_pte_range() just skips mTHPs,
> > large but not PMD-mapped: that's safe, but perhaps not intended: it's
> > arguable whether the deprecated feature should be updated to work
> > better with the new feature; but certainly not in this patch.]
> >
> > Backport to 6.11 should be straightforward. Earlier backports must take
> > care that other _deferred_list fixes and dependencies are included.  It
> > is unclear whether these fixes are realistically needed before 6.12.
> >
> > Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
> > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > ---
> >  mm/huge_memory.c   | 35 +++++++++++++++++++++--------------
> >  mm/internal.h      | 10 +++++-----
> >  mm/memcontrol-v1.c | 25 +++++++++++++++++++++++++
> >  mm/memcontrol.c    |  8 +++++---
> >  mm/migrate.c       |  4 ++--
> >  mm/page_alloc.c    |  4 +++-
> >  mm/swap.c          |  4 ++--
> >  mm/vmscan.c        |  4 ++--
> >  8 files changed, 65 insertions(+), 29 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index a1d345f1680c..dc7d5bb76495 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -3588,10 +3588,27 @@ int split_folio_to_list(struct folio *folio, struct list_head *list)
> >         return split_huge_page_to_list_to_order(&folio->page, list, ret);
> >  }
> >
> > -void __folio_undo_large_rmappable(struct folio *folio)
> > +/*
> > + * __folio_unqueue_deferred_split() is not to be called directly:
> > + * the folio_unqueue_deferred_split() inline wrapper in mm/internal.h
> > + * limits its calls to those folios which may have a _deferred_list for
> > + * queueing THP splits, and that list is (racily observed to be) non-empty.
> > + *
> > + * It is unsafe to call folio_unqueue_deferred_split() until folio refcount is
> > + * zero: because even when split_queue_lock is held, a non-empty _deferred_list
> > + * might be in use on deferred_split_scan()'s unlocked on-stack list.
> > + *
> > + * If memory cgroups are enabled, split_queue_lock is in the mem_cgroup: it is
> > + * therefore important to unqueue deferred split before changing folio memcg.
> > + */
> > +bool __folio_unqueue_deferred_split(struct folio *folio)
> >  {
> >         struct deferred_split *ds_queue;
> >         unsigned long flags;
> > +       bool unqueued = false;
> > +
> > +       WARN_ON_ONCE(folio_ref_count(folio));
> > +       WARN_ON_ONCE(!mem_cgroup_disabled() && !folio_memcg(folio));
> >
> >         ds_queue = get_deferred_split_queue(folio);
> >         spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> > @@ -3603,8 +3620,11 @@ void __folio_undo_large_rmappable(struct folio *folio)
> >                                       MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, -1);
> >                 }
> >                 list_del_init(&folio->_deferred_list);
> > +               unqueued = true;
> >         }
> >         spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> > +
> > +       return unqueued;        /* useful for debug warnings */
> >  }
> >
> >  /* partially_mapped=false won't clear PG_partially_mapped folio flag */
> > @@ -3626,19 +3646,6 @@ void deferred_split_folio(struct folio *folio, bool partially_mapped)
> >         if (!partially_mapped && !split_underused_thp)
> >                 return;
> >
> > -       /*
> > -        * The try_to_unmap() in page reclaim path might reach here too,
> > -        * this may cause a race condition to corrupt deferred split queue.
> > -        * And, if page reclaim is already handling the same folio, it is
> > -        * unnecessary to handle it again in shrinker.
> > -        *
> > -        * Check the swapcache flag to determine if the folio is being
> > -        * handled by page reclaim since THP swap would add the folio into
> > -        * swap cache before calling try_to_unmap().
> > -        */
> > -       if (folio_test_swapcache(folio))
> > -               return;
> > -
> >         spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> >         if (partially_mapped) {
> >                 if (!folio_test_partially_mapped(folio)) {
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 93083bbeeefa..16c1f3cd599e 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -639,11 +639,11 @@ static inline void folio_set_order(struct folio *folio, unsigned int order)
> >  #endif
> >  }
> >
> > -void __folio_undo_large_rmappable(struct folio *folio);
> > -static inline void folio_undo_large_rmappable(struct folio *folio)
> > +bool __folio_unqueue_deferred_split(struct folio *folio);
> > +static inline bool folio_unqueue_deferred_split(struct folio *folio)
> >  {
> >         if (folio_order(folio) <= 1 || !folio_test_large_rmappable(folio))
> > -               return;
> > +               return false;
> >
> >         /*
> >          * At this point, there is no one trying to add the folio to
> > @@ -651,9 +651,9 @@ static inline void folio_undo_large_rmappable(struct folio *folio)
> >          * to check without acquiring the split_queue_lock.
> >          */
> >         if (data_race(list_empty(&folio->_deferred_list)))
> > -               return;
> > +               return false;
> >
> > -       __folio_undo_large_rmappable(folio);
> > +       return __folio_unqueue_deferred_split(folio);
> >  }
> >
> >  static inline struct folio *page_rmappable_folio(struct page *page)
> > diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c
> > index 81d8819f13cd..f8744f5630bb 100644
> > --- a/mm/memcontrol-v1.c
> > +++ b/mm/memcontrol-v1.c
> > @@ -848,6 +848,8 @@ static int mem_cgroup_move_account(struct folio *folio,
> >         css_get(&to->css);
> >         css_put(&from->css);
> >
> > +       /* Warning should never happen, so don't worry about refcount non-0 */
> > +       WARN_ON_ONCE(folio_unqueue_deferred_split(folio));
> >         folio->memcg_data = (unsigned long)to;
> >
> >         __folio_memcg_unlock(from);
> > @@ -1217,7 +1219,9 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,
> >         enum mc_target_type target_type;
> >         union mc_target target;
> >         struct folio *folio;
> > +       bool tried_split_before = false;
> >
> > +retry_pmd:
> >         ptl = pmd_trans_huge_lock(pmd, vma);
> >         if (ptl) {
> >                 if (mc.precharge < HPAGE_PMD_NR) {
> > @@ -1227,6 +1231,27 @@ static int mem_cgroup_move_charge_pte_range(pmd_t *pmd,
> >                 target_type = get_mctgt_type_thp(vma, addr, *pmd, &target);
> >                 if (target_type == MC_TARGET_PAGE) {
> >                         folio = target.folio;
> > +                       /*
> > +                        * Deferred split queue locking depends on memcg,
> > +                        * and unqueue is unsafe unless folio refcount is 0:
> > +                        * split or skip if on the queue? first try to split.
> > +                        */
> > +                       if (!list_empty(&folio->_deferred_list)) {
> > +                               spin_unlock(ptl);
> > +                               if (!tried_split_before)
> > +                                       split_folio(folio);
> > +                               folio_unlock(folio);
> > +                               folio_put(folio);
> > +                               if (tried_split_before)
> > +                                       return 0;
> > +                               tried_split_before = true;
> > +                               goto retry_pmd;
> > +                       }
> > +                       /*
> > +                        * So long as that pmd lock is held, the folio cannot
> > +                        * be racily added to the _deferred_list, because
> > +                        * __folio_remove_rmap() will find !partially_mapped.
> > +                        */
> >                         if (folio_isolate_lru(folio)) {
> >                                 if (!mem_cgroup_move_account(folio, true,
> >                                                              mc.from, mc.to)) {
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 2703227cce88..06df2af97415 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -4629,9 +4629,6 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
> >         struct obj_cgroup *objcg;
> >
> >         VM_BUG_ON_FOLIO(folio_test_lru(folio), folio);
> > -       VM_BUG_ON_FOLIO(folio_order(folio) > 1 &&
> > -                       !folio_test_hugetlb(folio) &&
> > -                       !list_empty(&folio->_deferred_list), folio);
> >
> >         /*
> >          * Nobody should be changing or seriously looking at
> > @@ -4678,6 +4675,7 @@ static void uncharge_folio(struct folio *folio, struct uncharge_gather *ug)
> >                         ug->nr_memory += nr_pages;
> >                 ug->pgpgout++;
> >
> > +               WARN_ON_ONCE(folio_unqueue_deferred_split(folio));
> >                 folio->memcg_data = 0;
> >         }
> >
> > @@ -4789,6 +4787,9 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
> >
> >         /* Transfer the charge and the css ref */
> >         commit_charge(new, memcg);
> > +
> > +       /* Warning should never happen, so don't worry about refcount non-0 */
> > +       WARN_ON_ONCE(folio_unqueue_deferred_split(old));
> >         old->memcg_data = 0;
> >  }
> >
> > @@ -4975,6 +4976,7 @@ void mem_cgroup_swapout(struct folio *folio, swp_entry_t entry)
> >         VM_BUG_ON_FOLIO(oldid, folio);
> >         mod_memcg_state(swap_memcg, MEMCG_SWAP, nr_entries);
> >
> > +       folio_unqueue_deferred_split(folio);
> >         folio->memcg_data = 0;
> >
> >         if (!mem_cgroup_is_root(memcg))
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index df91248755e4..691f25ee2489 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -489,7 +489,7 @@ static int __folio_migrate_mapping(struct address_space *mapping,
> >                     folio_test_large_rmappable(folio)) {
> >                         if (!folio_ref_freeze(folio, expected_count))
> >                                 return -EAGAIN;
> > -                       folio_undo_large_rmappable(folio);
> > +                       folio_unqueue_deferred_split(folio);
> >                         folio_ref_unfreeze(folio, expected_count);
> >                 }
> >
> > @@ -514,7 +514,7 @@ static int __folio_migrate_mapping(struct address_space *mapping,
> >         }
> >
> >         /* Take off deferred split queue while frozen and memcg set */
> > -       folio_undo_large_rmappable(folio);
> > +       folio_unqueue_deferred_split(folio);
> >
> >         /*
> >          * Now we know that no one else is looking at the folio:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 4b21a368b4e2..57f64b5d0004 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2681,7 +2681,9 @@ void free_unref_folios(struct folio_batch *folios)
> >                 unsigned long pfn = folio_pfn(folio);
> >                 unsigned int order = folio_order(folio);
> >
> > -               folio_undo_large_rmappable(folio);
> > +               if (mem_cgroup_disabled())
> > +                       folio_unqueue_deferred_split(folio);
>
> This looks confusing. It looks all callsites of free_unref_folios()
> have folio_unqueue_deferred_split() and memcg uncharge called before
> it. If there is any problem, memcg uncharge should catch it. Did I
> miss something?
>
> > +
> >                 if (!free_pages_prepare(&folio->page, order))
> >                         continue;
> >                 /*
> > diff --git a/mm/swap.c b/mm/swap.c
> > index 835bdf324b76..b8e3259ea2c4 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -121,7 +121,7 @@ void __folio_put(struct folio *folio)
> >         }
> >
> >         page_cache_release(folio);
> > -       folio_undo_large_rmappable(folio);
> > +       folio_unqueue_deferred_split(folio);
> >         mem_cgroup_uncharge(folio);
> >         free_unref_page(&folio->page, folio_order(folio));
> >  }
> > @@ -988,7 +988,7 @@ void folios_put_refs(struct folio_batch *folios, unsigned int *refs)
> >                         free_huge_folio(folio);
> >                         continue;
> >                 }
> > -               folio_undo_large_rmappable(folio);
> > +               folio_unqueue_deferred_split(folio);
> >                 __page_cache_release(folio, &lruvec, &flags);
> >
> >                 if (j != i)
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index eb4e8440c507..635d45745b73 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -1475,7 +1475,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >                  */
> >                 nr_reclaimed += nr_pages;
> >
> > -               folio_undo_large_rmappable(folio);
> > +               folio_unqueue_deferred_split(folio);
> >                 if (folio_batch_add(&free_folios, folio) == 0) {
> >                         mem_cgroup_uncharge_folios(&free_folios);
> >                         try_to_unmap_flush();
> > @@ -1863,7 +1863,7 @@ static unsigned int move_folios_to_lru(struct lruvec *lruvec,
> >                 if (unlikely(folio_put_testzero(folio))) {
> >                         __folio_clear_lru_flags(folio);
> >
> > -                       folio_undo_large_rmappable(folio);
> > +                       folio_unqueue_deferred_split(folio);
> >                         if (folio_batch_add(&free_folios, folio) == 0) {
> >                                 spin_unlock_irq(&lruvec->lru_lock);
> >                                 mem_cgroup_uncharge_folios(&free_folios);
> > --
> > 2.35.3