Re: [PATCH v5 1/6] mm/page_alloc: pageblock flags functions clean up.

[Vlastimil Babka <vbabka@suse.cz>]

On 5/23/25 21:12, Zi Yan wrote:
> No functional change is intended.
> 
> 1. Add __NR_PAGEBLOCK_BITS for the number of pageblock flag bits and use
>    roundup_pow_of_two(__NR_PAGEBLOCK_BITS) as NR_PAGEBLOCK_BITS to take
>    right amount of bits for pageblock flags.
> 2. Add {get,set,clear}_pfnblock_bit() to operate one a standalone bit,
>    like PB_migrate_skip.
> 3. Make {get,set}_pfnblock_flags_mask() internal functions and use
>    {get,set}_pfnblock_migratetype() for pageblock migratetype operations.
> 4. Move pageblock flags common code to get_pfnblock_bitmap_bitidx().
> 3. Use MIGRATETYPE_MASK to get the migratetype of a pageblock from its
>    flags.
> 4. Use PB_migrate_end in the definition of MIGRATETYPE_MASK instead of
>    PB_migrate_bits.
> 5. Add a comment on is_migrate_cma_folio() to prevent one from changing it
>    to use get_pageblock_migratetype() and causing issues.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>

<snip>

> +/**
> + * __set_pfnblock_flags_mask - Set the requested group of flags for
> + * a pageblock_nr_pages block of pages
>   * @page: The page within the block of interest
> - * @flags: The flags to set
>   * @pfn: The target page frame number
> + * @flags: The flags to set
>   * @mask: mask of bits that the caller is interested in
>   */
> -void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
> -					unsigned long pfn,
> -					unsigned long mask)
> +static void __set_pfnblock_flags_mask(struct page *page, unsigned long pfn,
> +				      unsigned long flags, unsigned long mask)
>  {
> -	unsigned long *bitmap;
> -	unsigned long bitidx, word_bitidx;
> +	unsigned long *bitmap_word;
> +	unsigned long bitidx;
>  	unsigned long word;
>  
> -	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
> -	BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
> -
> -	bitmap = get_pageblock_bitmap(page, pfn);
> -	bitidx = pfn_to_bitidx(page, pfn);
> -	word_bitidx = bitidx / BITS_PER_LONG;
> -	bitidx &= (BITS_PER_LONG-1);
> -
> -	VM_BUG_ON_PAGE(!zone_spans_pfn(page_zone(page), pfn), page);
> +	get_pfnblock_bitmap_bitidx(page, pfn, &bitmap_word, &bitidx);
>  
>  	mask <<= bitidx;
>  	flags <<= bitidx;
>  
> -	word = READ_ONCE(bitmap[word_bitidx]);
> +	word = READ_ONCE(*bitmap_word);
>  	do {
> -	} while (!try_cmpxchg(&bitmap[word_bitidx], &word, (word & ~mask) | flags));
> +	} while (!try_cmpxchg(bitmap_word, &word, (word & ~mask) | flags));
> +}
> +
> +/**
> + * set_pfnblock_bit - Set a standalone bit of a pageblock
> + * @page: The page within the block of interest
> + * @pfn: The target page frame number
> + * @pb_bit: pageblock bit to set
> + */
> +void set_pfnblock_bit(const struct page *page, unsigned long pfn,
> +		      enum pageblock_bits pb_bit)
> +{
> +	unsigned long *bitmap_word;
> +	unsigned long bitidx;
> +
> +	if (WARN_ON_ONCE(pb_bit <= PB_migrate_end ||
> +			 pb_bit >= __NR_PAGEBLOCK_BITS))
> +		return;

This check appears at 3 places, maybe worth wrapping it in a helper?

> +
> +	get_pfnblock_bitmap_bitidx(page, pfn, &bitmap_word, &bitidx);
> +
> +	__set_bit(bitidx + pb_bit, bitmap_word);

I think it's wrong to use the __set_bit non-atomic variant because e.g.
compaction's PB_migrate_skip (actually a misnomer at this point I think,
e.g. PB_compact_skip would make more sense if you wanted to clean up things
some more) can be modified with no lock. It's why
__set_pfnblock_flags_mask() above uses try_cmpxchg() even though changes to
migratetype are normally done under zone lock.

> +}
> +
> +/**
> + * clear_pfnblock_bit - Clear a standalone bit of a pageblock
> + * @page: The page within the block of interest
> + * @pfn: The target page frame number
> + * @pb_bit: pageblock bit to clear
> + */
> +void clear_pfnblock_bit(const struct page *page, unsigned long pfn,
> +			enum pageblock_bits pb_bit)
> +{
> +	unsigned long *bitmap_word;
> +	unsigned long bitidx;
> +
> +	if (WARN_ON_ONCE(pb_bit <= PB_migrate_end ||
> +			 pb_bit >= __NR_PAGEBLOCK_BITS))
> +		return;
> +
> +	get_pfnblock_bitmap_bitidx(page, pfn, &bitmap_word, &bitidx);
> +
> +	__clear_bit(bitidx + pb_bit, bitmap_word);

Same here.

>  }