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SubjectRe: [PATCH v6] mm: fix is_pinnable_page against a cma page
On Tue, May 24, 2022 at 10:15:25AM -0700, 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>

From a memory-ordering viewpoint:

Acked-by: 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 v5 - https://lore.kernel.org/all/20220512204143.3961150-1-minchan@kernel.org/
> * Use READ_ONCE in __get_pfnblock_flags_mask - Jason
> * adding a comment about READ_ONCE - John
>
> * 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 | 9 +++++++--
> mm/page_alloc.c | 8 ++++++--
> 2 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 6acca5cecbc5..ba13411b6dca 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1625,8 +1625,13 @@ 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
> + 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)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 7a2053621e27..348071e5d8d9 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -482,8 +482,12 @@ unsigned long __get_pfnblock_flags_mask(const struct page *page,
> bitidx = pfn_to_bitidx(page, pfn);
> word_bitidx = bitidx / BITS_PER_LONG;
> bitidx &= (BITS_PER_LONG-1);
> -
> - word = bitmap[word_bitidx];
> + /*
> + * This races, without locks, with set_pfnblock_flags_mask(). Ensure
> + * a consistent read of the memory array, so that results, even though
> + * racy, are not corrupted.
> + */
> + word = READ_ONCE(bitmap[word_bitidx]);
> return (word >> bitidx) & mask;
> }
>
> --
> 2.36.1.124.g0e6072fb45-goog
>

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