Re: [PATCH v2 1/4] mm/page_isolation: make page isolation a standalone bit.

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Feb 14, 2025 at 10:42:12AM -0500, Zi Yan wrote:
> During page isolation, the original migratetype is overwritten, since
> MIGRATE_* are enums. Change MIGRATE_ISOLATE to be a standalone bit like
> PB_migrate_skip. pageblock bits needs to be word aligned, so expand
> the number of pageblock bits from 4 to 8 and make migrate isolate bit 7.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> ---
>  include/linux/mmzone.h          | 18 +++++++++++++-----
>  include/linux/page-isolation.h  |  2 +-
>  include/linux/pageblock-flags.h | 33 ++++++++++++++++++++++++++++++++-
>  mm/page_alloc.c                 | 21 +++++++++++++++++++--
>  4 files changed, 65 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 8aecbbb0b685..3c7d3f22ccb2 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -106,14 +106,22 @@ static inline bool migratetype_is_mergeable(int mt)
>  
>  extern int page_group_by_mobility_disabled;
>  
> -#define MIGRATETYPE_MASK ((1UL << PB_migratetype_bits) - 1)
> +#ifdef CONFIG_MEMORY_ISOLATION
> +#define MIGRATETYPE_NO_ISO_MASK (BIT(PB_migratetype_bits) - 1)
> +#define MIGRATETYPE_MASK (MIGRATETYPE_NO_ISO_MASK | PB_migrate_isolate_bit)
> +#else
> +#define MIGRATETYPE_NO_ISO_MASK (BIT(PB_migratetype_bits) - 1)
> +#define MIGRATETYPE_MASK (BIT(PB_migratetype_bits) - 1)
> +#endif

There is no user of the NO_ISO_MASK until the last patch. Can you
please defer introduction until then?

> -#define get_pageblock_migratetype(page)					\
> -	get_pfnblock_flags_mask(page, page_to_pfn(page), MIGRATETYPE_MASK)
> +#define get_pageblock_migratetype(page) \
> +		get_pfnblock_flags_mask(page, page_to_pfn(page), \
> +			MIGRATETYPE_MASK)
>  
> -#define folio_migratetype(folio)				\
> -	get_pfnblock_flags_mask(&folio->page, folio_pfn(folio),		\
> +#define folio_migratetype(folio) \
> +		get_pfnblock_flags_mask(&folio->page, folio_pfn(folio), \
>  			MIGRATETYPE_MASK)

That's a spurious change currently. I assume you tweaked the
MIGRATETYPE_MASK parameter during development, but I can't see a
functional difference now.

> @@ -373,7 +374,13 @@ unsigned long get_pfnblock_flags_mask(const struct page *page,
>  	 * racy, are not corrupted.
>  	 */
>  	word = READ_ONCE(bitmap[word_bitidx]);
> -	return (word >> bitidx) & mask;
> +	flags = (word >> bitidx) & mask;
> +
> +#ifdef CONFIG_MEMORY_ISOLATION
> +	if (flags & PB_migrate_isolate_bit)
> +		return MIGRATE_ISOLATE;
> +#endif
> +	return flags;
>  }
>  
>  static __always_inline int get_pfnblock_migratetype(const struct page *page,
> @@ -397,8 +404,18 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
>  	unsigned long bitidx, word_bitidx;
>  	unsigned long word;
>  
> +#ifdef CONFIG_MEMORY_ISOLATION
> +	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 8);
> +	/* keep other migratetype bits if MIGRATE_ISOLATE is set */
> +	if (flags == MIGRATE_ISOLATE) {
> +		mask &= ~((1UL << PB_migratetype_bits) - 1);
> +		flags = PB_migrate_isolate_bit;
> +	}

Please change the callers in both cases to pass the appropriate masks
of interest instead.

That's likely a bit of churn in the allocator code, but adding caller
specifics to this function violates abstraction layering rules.