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Message-Id: <20240814230533.54938-1-21cnbao@gmail.com>
Date: Thu, 15 Aug 2024 11:05:33 +1200
From: Barry Song <21cnbao@...il.com>
To: usamaarif642@...il.com
Cc: akpm@...ux-foundation.org,
baohua@...nel.org,
cerasuolodomenico@...il.com,
corbet@....net,
david@...hat.com,
hannes@...xchg.org,
kernel-team@...a.com,
linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org,
linux-mm@...ck.org,
riel@...riel.com,
roman.gushchin@...ux.dev,
rppt@...nel.org,
ryan.roberts@....com,
shakeel.butt@...ux.dev,
willy@...radead.org,
yuzhao@...gle.com
Subject: Re: [PATCH v3 4/6] mm: Introduce a pageflag for partially mapped folios
On Thu, Aug 15, 2024 at 12:37 AM Usama Arif <usamaarif642@...il.com> wrote:
[snip]
> >>>>
> >>>> -void deferred_split_folio(struct folio *folio)
> >>>> +void deferred_split_folio(struct folio *folio, bool partially_mapped)
> >>>> {
> >>>> struct deferred_split *ds_queue = get_deferred_split_queue(folio);
> >>>> #ifdef CONFIG_MEMCG
> >>>> @@ -3485,14 +3487,17 @@ void deferred_split_folio(struct folio *folio)
> >>>> if (folio_test_swapcache(folio))
> >>>> return;
> >>>>
> >>>> - if (!list_empty(&folio->_deferred_list))
> >>>> - return;
> >>>> -
> >>>> spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> >>>> + if (partially_mapped)
> >>>> + folio_set_partially_mapped(folio);
> >>>> + else
> >>>> + folio_clear_partially_mapped(folio);
> >>>> if (list_empty(&folio->_deferred_list)) {
> >>>> - if (folio_test_pmd_mappable(folio))
> >>>> - count_vm_event(THP_DEFERRED_SPLIT_PAGE);
> >>>> - count_mthp_stat(folio_order(folio), MTHP_STAT_SPLIT_DEFERRED);
> >>>> + if (partially_mapped) {
> >>>> + if (folio_test_pmd_mappable(folio))
> >>>> + count_vm_event(THP_DEFERRED_SPLIT_PAGE);
> >>>> + count_mthp_stat(folio_order(folio), MTHP_STAT_SPLIT_DEFERRED);
> >>>
> >>> This code completely broke MTHP_STAT_SPLIT_DEFERRED for PMD_ORDER. It
> >>> added the folio to the deferred_list as entirely_mapped
> >>> (partially_mapped == false).
> >>> However, when partially_mapped becomes true, there's no opportunity to
> >>> add it again
> >>> as it has been there on the list. Are you consistently seeing the counter for
> >>> PMD_ORDER as 0?
> >>>
> >>
> >> Ah I see it, this should fix it?
> >>
> >> -void deferred_split_folio(struct folio *folio)
> >> +/* partially_mapped=false won't clear PG_partially_mapped folio flag */
> >> +void deferred_split_folio(struct folio *folio, bool partially_mapped)
> >> {
> >> struct deferred_split *ds_queue = get_deferred_split_queue(folio);
> >> #ifdef CONFIG_MEMCG
> >> @@ -3485,14 +3488,14 @@ void deferred_split_folio(struct folio *folio)
> >> if (folio_test_swapcache(folio))
> >> return;
> >>
> >> - if (!list_empty(&folio->_deferred_list))
> >> - return;
> >> -
> >> spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> >> - if (list_empty(&folio->_deferred_list)) {
> >> + if (partially_mapped) {
> >> + 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);
> >> + }
> >> + if (list_empty(&folio->_deferred_list)) {
> >> list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
> >> ds_queue->split_queue_len++;
> >> #ifdef CONFIG_MEMCG
> >>
> >
> > not enough. as deferred_split_folio(true) won't be called if folio has been
> > deferred_list in __folio_remove_rmap():
> >
> > if (partially_mapped && folio_test_anon(folio) &&
> > list_empty(&folio->_deferred_list))
> > deferred_split_folio(folio, true);
> >
> > so you will still see 0.
> >
>
> ah yes, Thanks.
>
> So below diff over the current v3 series should work for all cases:
>
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index b4d72479330d..482e3ab60911 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3483,6 +3483,7 @@ void __folio_undo_large_rmappable(struct folio *folio)
> spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> }
>
> +/* partially_mapped=false won't clear PG_partially_mapped folio flag */
> void deferred_split_folio(struct folio *folio, bool partially_mapped)
> {
> struct deferred_split *ds_queue = get_deferred_split_queue(folio);
> @@ -3515,16 +3516,16 @@ void deferred_split_folio(struct folio *folio, bool partially_mapped)
> return;
>
> spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> - if (partially_mapped)
> + if (partially_mapped) {
> folio_set_partially_mapped(folio);
> - else
> - folio_clear_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);
> + } else {
> + /* partially mapped folios cannont become partially unmapped */
> + VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio);
> + }
> if (list_empty(&folio->_deferred_list)) {
> - if (partially_mapped) {
> - if (folio_test_pmd_mappable(folio))
> - count_vm_event(THP_DEFERRED_SPLIT_PAGE);
> - count_mthp_stat(folio_order(folio), MTHP_STAT_SPLIT_DEFERRED);
> - }
> list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
> ds_queue->split_queue_len++;
> #ifdef CONFIG_MEMCG
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 9ad558c2bad0..4c330635aa4e 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1578,7 +1578,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> * Check partially_mapped first to ensure it is a large folio.
> */
> if (partially_mapped && folio_test_anon(folio) &&
> - list_empty(&folio->_deferred_list))
> + !folio_test_partially_mapped(folio))
> deferred_split_folio(folio, true);
>
> __folio_mod_stat(folio, -nr, -nr_pmdmapped);
>
This is an improvement, but there's still an issue. Two or more threads can
execute deferred_split_folio() simultaneously, which could lead to
DEFERRED_SPLIT being added multiple times. We should double-check
the status after acquiring the spinlock.
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index f401ceded697..3d247826fb95 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3607,10 +3607,12 @@ void deferred_split_folio(struct folio *folio, bool partially_mapped)
spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
if (partially_mapped) {
- 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);
+ 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);
+ }
} else {
/* partially mapped folios cannont become partially unmapped */
VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio);
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