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SubjectRe: [PATCH v5] mm: fix is_pinnable_page against on cma page
On Thu, May 12, 2022 at 01:51:47PM -0700, John Hubbard wrote:
> On 5/12/22 13:41, Minchan Kim wrote:
> > Pages on CMA area could have MIGRATE_ISOLATE as well as MIGRATE_CMA
> > so current is_pinnable_page could miss CMA pages which has MIGRATE_
> > ISOLATE. It ends up pinning CMA pages as longterm at pin_user_pages
> > APIs so CMA allocation keep failed until the pin is released.
> >
> > CPU 0 CPU 1 - Task B
> >
> > cma_alloc
> > alloc_contig_range
> > pin_user_pages_fast(FOLL_LONGTERM)
> > change pageblock as MIGRATE_ISOLATE
> > internal_get_user_pages_fast
> > lockless_pages_from_mm
> > gup_pte_range
> > try_grab_folio
> > is_pinnable_page
> > return true;
> > So, pinned the page successfully.
> > page migration failure with pinned page
> > ..
> > .. After 30 sec
> > unpin_user_page(page)
> >
> > CMA allocation succeeded after 30 sec.
> >
> > The CMA allocation path protects the migration type change race
> > using zone->lock but what GUP path need to know is just whether the
> > page is on CMA area or not rather than exact migration type.
> > Thus, we don't need zone->lock but just checks migration type in
> > either of (MIGRATE_ISOLATE and MIGRATE_CMA).
> >
> > Adding the MIGRATE_ISOLATE check in is_pinnable_page could cause
> > rejecting of pinning pages on MIGRATE_ISOLATE pageblocks even
> > though it's neither CMA nor movable zone if the page is temporarily
> > unmovable. However, such a migration failure by unexpected temporal
> > refcount holding is general issue, not only come from MIGRATE_ISOLATE
> > and the MIGRATE_ISOLATE is also transient state like other temporal
> > elevated refcount problem.
> >
> > Cc: "Paul E . McKenney" <paulmck@kernel.org>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> > * from v4 - https://lore.kernel.org/all/20220510211743.95831-1-minchan@kernel.org/
> > * clarification why we need READ_ONCE - Paul
> > * Adding a comment about READ_ONCE - John
> >
> > * from v3 - https://lore.kernel.org/all/20220509153430.4125710-1-minchan@kernel.org/
> > * Fix typo and adding more description - akpm
> >
> > * from v2 - https://lore.kernel.org/all/20220505064429.2818496-1-minchan@kernel.org/
> > * Use __READ_ONCE instead of volatile - akpm
> >
> > * from v1 - https://lore.kernel.org/all/20220502173558.2510641-1-minchan@kernel.org/
> > * fix build warning - lkp
> > * fix refetching issue of migration type
> > * add side effect on !ZONE_MOVABLE and !MIGRATE_CMA in description - david
> >
> > include/linux/mm.h | 16 ++++++++++++++--
> > 1 file changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 6acca5cecbc5..2d7a5d87decd 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1625,8 +1625,20 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
> > #ifdef CONFIG_MIGRATION
> > static inline bool is_pinnable_page(struct page *page)
> > {
> > - return !(is_zone_movable_page(page) || is_migrate_cma_page(page)) ||
> > - is_zero_pfn(page_to_pfn(page));
> > +#ifdef CONFIG_CMA
> > + /*
> > + * Defend against future compiler LTO features, or code refactoring
> > + * that inlines the above function, by forcing a single read. Because,
> > + * this routine races with set_pageblock_migratetype(), and we want to
> > + * avoid reading zero, when actually one or the other flags was set.
> > + */
>
> The most interesting line got dropped in this version. :)
>
> This is missing:
>
> int __mt = get_pageblock_migratetype(page);
>
> Assuming that that is restored, please feel free to add:
>
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>

Just caught after clicked the button with my fat finger :(

Thanks, John!

Andrew, Could you pick this up?

From 90ad049d48f5c36075f17ac996dfe3c33127aeb6 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@kernel.org>
Date: Mon, 2 May 2022 10:03:48 -0700
Subject: [PATCH v5] mm: fix is_pinnable_page against on cma page

Pages on CMA area could have MIGRATE_ISOLATE as well as MIGRATE_CMA
so current is_pinnable_page could miss CMA pages which has MIGRATE_
ISOLATE. It ends up pinning CMA pages as longterm at pin_user_pages
APIs so CMA allocation keep failed until the pin is released.

CPU 0 CPU 1 - Task B

cma_alloc
alloc_contig_range
pin_user_pages_fast(FOLL_LONGTERM)
change pageblock as MIGRATE_ISOLATE
internal_get_user_pages_fast
lockless_pages_from_mm
gup_pte_range
try_grab_folio
is_pinnable_page
return true;
So, pinned the page successfully.
page migration failure with pinned page
..
.. After 30 sec
unpin_user_page(page)

CMA allocation succeeded after 30 sec.

The CMA allocation path protects the migration type change race
using zone->lock but what GUP path need to know is just whether the
page is on CMA area or not rather than exact migration type.
Thus, we don't need zone->lock but just checks migration type in
either of (MIGRATE_ISOLATE and MIGRATE_CMA).

Adding the MIGRATE_ISOLATE check in is_pinnable_page could cause
rejecting of pinning pages on MIGRATE_ISOLATE pageblocks even
though it's neither CMA nor movable zone if the page is temporarily
unmovable. However, such a migration failure by unexpected temporal
refcount holding is general issue, not only come from MIGRATE_ISOLATE
and the MIGRATE_ISOLATE is also transient state like other temporal
elevated refcount problem.

Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
* from v4 - https://lore.kernel.org/all/20220510211743.95831-1-minchan@kernel.org/
* clarification why we need READ_ONCE - Paul
* Adding a comment about READ_ONCE - John

* from v3 - https://lore.kernel.org/all/20220509153430.4125710-1-minchan@kernel.org/
* Fix typo and adding more description - akpm

* from v2 - https://lore.kernel.org/all/20220505064429.2818496-1-minchan@kernel.org/
* Use __READ_ONCE instead of volatile - akpm

* from v1 - https://lore.kernel.org/all/20220502173558.2510641-1-minchan@kernel.org/
* fix build warning - lkp
* fix refetching issue of migration type
* add side effect on !ZONE_MOVABLE and !MIGRATE_CMA in description - david

include/linux/mm.h | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6acca5cecbc5..b23c6f1b90b5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1625,8 +1625,21 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
#ifdef CONFIG_MIGRATION
static inline bool is_pinnable_page(struct page *page)
{
- return !(is_zone_movable_page(page) || is_migrate_cma_page(page)) ||
- is_zero_pfn(page_to_pfn(page));
+#ifdef CONFIG_CMA
+ /*
+ * Defend against future compiler LTO features, or code refactoring
+ * that inlines the above function, by forcing a single read. Because,
+ * this routine races with set_pageblock_migratetype(), and we want to
+ * avoid reading zero, when actually one or the other flags was set.
+ */
+ int __mt = get_pageblock_migratetype(page);
+ int mt = __READ_ONCE(__mt);
+
+ if (mt & (MIGRATE_CMA | MIGRATE_ISOLATE))
+ return false;
+#endif
+
+ return !(is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page)));
}
#else
static inline bool is_pinnable_page(struct page *page)
--
2.36.0.550.gb090851708-goog
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