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Message-ID: <CAGsJ_4x4Ha5jDZuhNxXvbYU6WXtR9A-o3QxYUHkcUzrzoNWaMw@mail.gmail.com>
Date: Wed, 14 Aug 2024 23:20:19 +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, corbet@....net,
linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH v3 4/6] mm: Introduce a pageflag for partially mapped folios
On Wed, Aug 14, 2024 at 11:11 PM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 14/08/2024 11:44, Barry Song wrote:
> > On Wed, Aug 14, 2024 at 12:03 AM 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 | 3 +++
> >> mm/huge_memory.c | 21 +++++++++++++--------
> >> mm/hugetlb.c | 1 +
> >> mm/internal.h | 4 +++-
> >> mm/memcontrol.c | 3 ++-
> >> mm/migrate.c | 3 ++-
> >> mm/page_alloc.c | 5 +++--
> >> mm/rmap.c | 3 ++-
> >> mm/vmscan.c | 3 ++-
> >> 10 files changed, 33 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..cecc1bad7910 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, /* was identified to be partially mapped */
> >> };
> >>
> >> #define PAGEFLAGS_MASK ((1UL << NR_PAGEFLAGS) - 1)
> >> @@ -861,8 +862,10 @@ static inline void ClearPageCompound(struct page *page)
> >> ClearPageHead(page);
> >> }
> >> FOLIO_FLAG(large_rmappable, FOLIO_SECOND_PAGE)
> >> +FOLIO_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
> >> #else
> >> FOLIO_FLAG_FALSE(large_rmappable)
> >> +FOLIO_FLAG_FALSE(partially_mapped)
> >> #endif
> >>
> >> #define PG_head_mask ((1UL << PG_head))
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index 6df0e9f4f56c..c024ab0f745c 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -3397,6 +3397,7 @@ 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) {
> >> @@ -3453,11 +3454,12 @@ 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);
> >> }
> >> spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
> >> }
> >>
> >> -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);
> >
> > Hi Usama,
> >
> > Do we need this? When can a partially_mapped folio on deferred_list become
> > non-partially-mapped and need a clear? I understand transferring from
> > entirely_map
> > to partially_mapped is a one way process? partially_mapped folios can't go back
> > to entirely_mapped?
> >
> Hi Barry,
>
> deferred_split_folio function is called in 3 places after this series, at fault, collapse and partial mapping. partial mapping can only happen after fault/collapse, and we have FOLIO_FLAG_FALSE(partially_mapped), i.e. flag initialized to false, so technically its not needed. A partially_mapped folio on deferred list wont become non-partially mapped.
>
> I just did it as a precaution if someone ever changes the kernel and calls deferred_split_folio with partially_mapped set to false after it had been true. The function arguments of deferred_split_folio make it seem that passing partially_mapped=false as an argument would clear it, which is why I cleared it as well. I could change the patch to something like below if it makes things better? i.e. add a comment at the top of the function:
>
to me, it seems a BUG to call with false after a folio has been
partially_mapped. So I'd rather
VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio);
The below should also fix the MTHP_STAT_SPLIT_DEFERRED
counter this patch is breaking?
@@ -3515,16 +3522,18 @@ void deferred_split_folio(struct folio *folio,
bool partially_mapped)
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 (partially_mapped) {
+ if (partially_mapped) {
+ if (!folio_test_set_partially_mapped(folio)) {
+ mod_mthp_stat(folio_order(folio),
+ MTHP_STAT_NR_SPLIT_DEFERRED, 1);
if (folio_test_pmd_mappable(folio))
count_vm_event(THP_DEFERRED_SPLIT_PAGE);
count_mthp_stat(folio_order(folio),
MTHP_STAT_SPLIT_DEFERRED);
}
+ }
+ VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio);
+
+ if (list_empty(&folio->_deferred_list)) {
list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
ds_queue->split_queue_len++;
#ifdef CONFIG_MEMCG
>
> -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 +3488,15 @@ 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);
> 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);
> + }
> list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
> ds_queue->split_queue_len++;
> #ifdef CONFIG_MEMCG
>
>
> > I am trying to rebase my NR_SPLIT_DEFERRED counter on top of your
> > work, but this "clear" makes that job quite tricky. as I am not sure
> > if this patch
> > is going to clear the partially_mapped flag for folios on deferred_list.
> >
> > Otherwise, I can simply do the below whenever folio is leaving deferred_list
> >
> > ds_queue->split_queue_len--;
> > list_del_init(&folio->_deferred_list);
> > if (folio_test_clear_partially_mapped(folio))
> > mod_mthp_stat(folio_order(folio),
> > MTHP_STAT_NR_SPLIT_DEFERRED, -1);
> >
>
Thanks
Barry
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