linux-kernel - Re: [PATCH v5] mm: fix is_pinnable

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Date:   Thu, 12 May 2022 13:51:47 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        John Dias <joaodias@...gle.com>,
        "David Hildenbrand" <david@...hat.com>
Subject: Re: [PATCH v5] mm: fix is_pinnable_page against on cma page

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@...nel.org>
> Cc: John Hubbard <jhubbard@...dia.com>
> Cc: David Hildenbrand <david@...hat.com>
> Signed-off-by: Minchan Kim <minchan@...nel.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@...dia.com>

thanks,
-- 
John Hubbard
NVIDIA

> +	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)