[PATCH v2] mm: fix is_pinnable_page against on cma page

[PATCH v2] mm: fix is_pinnable_page against on cma page

[Minchan Kim]

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 putting CMA pages longterm pinning possible on
pin_user_pages APIs so CMA allocation fails.

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 type. Thus, we don't
need zone->lock but just checks the migratype in either of
(MIGRATE_ISOLATE and MIGRATE_CMA).

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

Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
* 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 | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6acca5cecbc5..e77758e2035e 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1625,8 +1625,18 @@ 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
+	/*
+	 * use volatile to use local variable mt instead of
+	 * refetching mt value.
+	 */
+	volatile int mt = get_pageblock_migratetype(page);
+
+	if (mt == MIGRATE_CMA || mt == 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)
-- 
2.36.0.512.ge40c2bad7a-goog

Re: [PATCH v2] mm: fix is_pinnable_page against on cma page

[Andrew Morton]

On Wed,  4 May 2022 23:44:29 -0700 Minchan Kim <minchan@kernel.org> 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 putting CMA pages longterm pinning possible on
> pin_user_pages APIs so CMA allocation fails.
> 
> 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 type. Thus, we don't
> need zone->lock but just checks the migratype in either of
> (MIGRATE_ISOLATE and MIGRATE_CMA).
> 
> Adding the MIGRATE_ISOLATE check in is_pinnable_page could cause
> rejecting of pinning the page on MIGRATE_ISOLATE pageblock even
> thouth it's neither CMA nor movable zone if the page is temporarily

"though"

> unmovable. However, the migration failure is general issue, not
> only come from MIGRATE_ISOLATE and the MIGRATE_ISOLATE is also
> transient state like other temporal refcount holding of pages.
> 
> ...
>
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1625,8 +1625,18 @@ 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
> +	/*
> +	 * use volatile to use local variable mt instead of
> +	 * refetching mt value.
> +	 */
> +	volatile int mt = get_pageblock_migratetype(page);
> +
> +	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
> +		return false;
> +#endif

Open-coded use of `volatile' draws unwelcome attention.

What are we trying to do here?  Prevent the compiler from rerunning all
of get_pageblock_migratetype() (really __get_pfnblock_flags_mask())
twice?  That would be pretty dumb of it?

Would a suitably-commented something like

	int __mt = get_pageblock_migratetype(page);
	int mt = __READ_ONCE(__mt);

express this better?
	
> +
> +	return !(is_zone_movable_page(page) || is_zero_pfn(page_to_pfn(page)));
>  }
>  #else
>  static inline bool is_pinnable_page(struct page *page)

Re: [PATCH v2] mm: fix is_pinnable_page against on cma page

[David Hildenbrand]

On 07.05.22 21:23, Andrew Morton wrote:
> On Wed,  4 May 2022 23:44:29 -0700 Minchan Kim <minchan@kernel.org> 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 putting CMA pages longterm pinning possible on
>> pin_user_pages APIs so CMA allocation fails.
>>
>> 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 type. Thus, we don't
>> need zone->lock but just checks the migratype in either of
>> (MIGRATE_ISOLATE and MIGRATE_CMA).
>>
>> Adding the MIGRATE_ISOLATE check in is_pinnable_page could cause
>> rejecting of pinning the page on MIGRATE_ISOLATE pageblock even
>> thouth it's neither CMA nor movable zone if the page is temporarily
> 
> "though"
> 
>> unmovable. However, the migration failure is general issue, not
>> only come from MIGRATE_ISOLATE and the MIGRATE_ISOLATE is also
>> transient state like other temporal refcount holding of pages.
>>
>> ...
>>
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1625,8 +1625,18 @@ 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
>> +	/*
>> +	 * use volatile to use local variable mt instead of
>> +	 * refetching mt value.
>> +	 */
>> +	volatile int mt = get_pageblock_migratetype(page);
>> +
>> +	if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
>> +		return false;
>> +#endif
> 
> Open-coded use of `volatile' draws unwelcome attention.
> 
> What are we trying to do here?  Prevent the compiler from rerunning all
> of get_pageblock_migratetype() (really __get_pfnblock_flags_mask())
> twice?  That would be pretty dumb of it?
> 
> Would a suitably-commented something like
> 
> 	int __mt = get_pageblock_migratetype(page);
> 	int mt = __READ_ONCE(__mt);
> 
> express this better?

Yes, we want READ_ONCE I think. Apart from that LGTM.


-- 
Thanks,

David / dhildenb