Re: [PATCH v3 1/4] mm/page_isolation: make page isolation a standalone bit.

Zi Yan
2025-05-08T15:27:38Z

On 8 May 2025, at 1:24, Johannes Weiner wrote:

> On Wed, May 07, 2025 at 05:10:56PM -0400, Zi Yan wrote:
>> During page isolation, the original migratetype is overwritten, since
>> MIGRATE_* are enums and stored in pageblock bitmaps. Change
>> MIGRATE_ISOLATE to be stored a standalone bit, PB_migrate_isolate, like
>> PB_migrate_skip, so that migratetype is not lost during pageblock
>> isolation. pageblock bits needs to be word aligned, so expand
>> the number of pageblock bits from 4 to 8 and make PB_migrate_isolate bit 7.
>>
>> Signed-off-by: Zi Yan <ziy@nvidia.com>
>> ---
>> include/linux/mmzone.h | 17 ++++++++++----
>> include/linux/page-isolation.h | 2 +-
>> include/linux/pageblock-flags.h | 33 +++++++++++++++++++++++++-
>> mm/page_alloc.c | 41 ++++++++++++++++++++++++++++++++-
>> 4 files changed, 86 insertions(+), 7 deletions(-)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index b19a98c20de8..9ec022a0b826 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -106,14 +106,23 @@ 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_MASK ((BIT(PB_migratetype_bits) - 1) | PB_migrate_isolate_bit)
>> +#else
>> +#define MIGRATETYPE_MASK (BIT(PB_migratetype_bits) - 1)
>> +#endif
>>
>> +#ifdef CONFIG_MEMORY_ISOLATION
>> +unsigned long get_pageblock_migratetype(const struct page *page);
>> +#else
>> #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), \
>> - MIGRATETYPE_MASK)
>> +#endif
>> +
>> +#define folio_migratetype(folio) \
>> + get_pageblock_migratetype(&folio->page)
>> +
>> struct free_area {
>> struct list_head free_list[MIGRATE_TYPES];
>> unsigned long nr_free;
>> diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
>> index 898bb788243b..51797dc39cbc 100644
>> --- a/include/linux/page-isolation.h
>> +++ b/include/linux/page-isolation.h
>> @@ -5,7 +5,7 @@
>> #ifdef CONFIG_MEMORY_ISOLATION
>> static inline bool is_migrate_isolate_page(struct page *page)
>> {
>> - return get_pageblock_migratetype(page) == MIGRATE_ISOLATE;
>> + return get_pageblock_isolate(page);
>
> The old version still works, right?
>
> It would match is_migrate_isolate() a bit better, but no strong
> feelings either way...

Yes and compiler should generate the same code. OK, I will drop this.

>
>> static inline bool is_migrate_isolate(int migratetype)
>> {
>> diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
>> index 0c4963339f0b..9fadae5892b2 100644
>> --- a/include/linux/pageblock-flags.h
>> +++ b/include/linux/pageblock-flags.h
>> @@ -20,7 +20,10 @@ enum pageblock_bits {
>> PB_migrate_end = PB_migrate + PB_migratetype_bits - 1,
>> /* 3 bits required for migrate types */
>> PB_migrate_skip,/* If set the block is skipped by compaction */
>> -
>> +#ifdef CONFIG_MEMORY_ISOLATION
>> + PB_migrate_isolate = 7, /* If set the block is isolated */
>> + /* set it to 7 to make pageblock bit word aligned */
>> +#endif
>> /*
>> * Assume the bits will always align on a word. If this assumption
>> * changes then get/set pageblock needs updating.
>> @@ -28,6 +31,10 @@ enum pageblock_bits {
>> NR_PAGEBLOCK_BITS
>> };
>>
>> +#ifdef CONFIG_MEMORY_ISOLATION
>> +#define PB_migrate_isolate_bit BIT(PB_migrate_isolate)
>> +#endif
>> +
>> #if defined(CONFIG_PAGE_BLOCK_ORDER)
>> #define PAGE_BLOCK_ORDER CONFIG_PAGE_BLOCK_ORDER
>> #else
>> @@ -105,4 +112,28 @@ static inline void set_pageblock_skip(struct page *page)
>> }
>> #endif /* CONFIG_COMPACTION */
>>
>> +#ifdef CONFIG_MEMORY_ISOLATION
>> +#define get_pageblock_isolate(page) \
>> + get_pfnblock_flags_mask(page, page_to_pfn(page), \
>> + PB_migrate_isolate_bit)
>> +#define clear_pageblock_isolate(page) \
>> + set_pfnblock_flags_mask(page, 0, page_to_pfn(page), \
>> + PB_migrate_isolate_bit)
>> +#define set_pageblock_isolate(page) \
>> + set_pfnblock_flags_mask(page, PB_migrate_isolate_bit, \
>> + page_to_pfn(page), \
>> + PB_migrate_isolate_bit)
>
> Would it make sense to move these to page_isolation.c? Then they
> wouldn't have to be macros.

Sure. Although I am using get_pageblock_isolate() in mm/page_alloc.c,
I can change it to use get_pageblock_migratetype() == MIGRATE_ISOLATE.

>
>> +#else
>> +static inline bool get_pageblock_isolate(struct page *page)
>> +{
>> + return false;
>> +}
>> +static inline void clear_pageblock_isolate(struct page *page)
>> +{
>> +}
>> +static inline void set_pageblock_isolate(struct page *page)
>> +{
>> +}
>> +#endif /* CONFIG_MEMORY_ISOLATION */
>> +
>> #endif /* PAGEBLOCK_FLAGS_H */
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index c77592b22256..acf68ef041d8 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -381,12 +381,40 @@ unsigned long get_pfnblock_flags_mask(const struct page *page,
>> return (word >> bitidx) & mask;
>> }
>>
>> +#ifdef CONFIG_MEMORY_ISOLATION
>> +unsigned long get_pageblock_migratetype(const struct page *page)
>> +{
>> + unsigned long flags;
>> +
>> + flags = get_pfnblock_flags_mask(page, page_to_pfn(page),
>> + MIGRATETYPE_MASK);
>> + if (flags & PB_migrate_isolate_bit)
>> + return MIGRATE_ISOLATE;
>> +
>> + return flags;
>> +}
>
> Since MIGRATETYPE_MASK includes the isolate bit if it exists, I think
> you can share the get_pfnblock_flags_mask() part:
>
> static inline get_pageblock_migratetype(const struct page *page)
> {
> unsigned long pfn = page_to_pfn(page);
>
> flags = get_pfnblock_flags_mask(page, pfn, MIGRATETYPE_MASK);
> #ifdef CONFIG_MEMORY_ISOLATEION
> if (flags & PB_migrate_isolate_bit)
> return MIGRATE_ISOLATE;
> #endif
> return flags;
> }
>
>> +static __always_inline int get_pfnblock_migratetype(const struct page *page,
>> + unsigned long pfn)
>> +{
>> + unsigned long flags;
>> +
>> + flags = get_pfnblock_flags_mask(page, pfn,
>> + MIGRATETYPE_MASK);
>> + if (flags & PB_migrate_isolate_bit)
>> + return MIGRATE_ISOLATE;
>> +
>> + return flags;
>> +}
>> +#else
>> static __always_inline int get_pfnblock_migratetype(const struct page *page,
>> unsigned long pfn)
>> {
>> return get_pfnblock_flags_mask(page, pfn, MIGRATETYPE_MASK);
>> }
>
> Same with this one.
>

OK, will change these two.

>>
>> +#endif
>> +
>> /**
>> * 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
>> @@ -402,8 +430,14 @@ 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);
>> + /* extra one for MIGRATE_ISOLATE */
>> + BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits) + 1);
>> +#else
>> BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
>> BUILD_BUG_ON(MIGRATE_TYPES > (1 << PB_migratetype_bits));
>> +#endif
>>
>> bitmap = get_pageblock_bitmap(page, pfn);
>> bitidx = pfn_to_bitidx(page, pfn);
>> @@ -426,7 +460,12 @@ void set_pageblock_migratetype(struct page *page, int migratetype)
>> migratetype < MIGRATE_PCPTYPES))
>> migratetype = MIGRATE_UNMOVABLE;
>>
>> - set_pfnblock_flags_mask(page, (unsigned long)migratetype,
>> +#ifdef CONFIG_MEMORY_ISOLATION
>> + if (migratetype == MIGRATE_ISOLATE)
>> + set_pageblock_isolate(page);
>
> Are there paths actually doing this after the second patch?
>
> There are many instances that want to *read* the migratetype or
> MIGRATE_ISOLATE, but only isolation code should be manipulating that
> bit through the dedicated set/toggle_pageblock_isolate API.
>
> If there isn't one, it might be good to enforce this with a VM_WARN
> instead.

I checked all set_pageblock_migratetype() callers and do not see
one using it for pageblock isolation. Let me replace the code
with a VM_WARN and add a comment to tell users to use dedicated
pageblock isolation APIs.

Thanks for all the reviews. Let me send the fixup patches.

>
>> + else
>> +#endif
>> + set_pfnblock_flags_mask(page, (unsigned long)migratetype,
>> page_to_pfn(page), MIGRATETYPE_MASK);
>
> If the branch stays, you could add a `return' to the MIGRATE_ISOLATE
> leg, drop the else and indent this line normally.

--
Best Regards,
Yan, Zi