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

[Zi Yan <ziy@nvidia.com>]

On 27 May 2025, at 5:46, Vlastimil Babka wrote:

> 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?

Sure.

>
>> +
>> +	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
Will rename it.

> 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.

Got it. Thank you for the explanation. Will fix all *_pfnblock_bit() functions
and add a comment about why atomic variants are used.

>
>> +}
>> +
>> +/**
>> + * 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.

Ack.

Best Regards,
Yan, Zi