Date:   Mon, 21 Mar 2022 14:23:07 -0400
From:   Zi Yan <ziy@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Eric Ren <renzhengeek@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        Oscar Salvador <osalvador@...e.de>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Subject: Re: [PATCH v8 2/5] mm: page_isolation: check specified range for unmovable pages

On 21 Mar 2022, at 13:30, David Hildenbrand wrote:

> On 17.03.22 16:37, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>>
>> Enable set_migratetype_isolate() to check specified sub-range for
>> unmovable pages during isolation. Page isolation is done
>> at max(MAX_ORDER_NR_PAEGS, pageblock_nr_pages) granularity, but not all
>> pages within that granularity are intended to be isolated. For example,
>> alloc_contig_range(), which uses page isolation, allows ranges without
>> alignment. This commit makes unmovable page check only look for
>> interesting pages, so that page isolation can succeed for any
>> non-overlapping ranges.
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>>  include/linux/page-isolation.h | 10 +++++
>>  mm/page_alloc.c                | 13 +------
>>  mm/page_isolation.c            | 69 ++++++++++++++++++++--------------
>>  3 files changed, 51 insertions(+), 41 deletions(-)
>>
>> diff --git a/include/linux/page-isolation.h b/include/linux/page-isolation.h
>> index e14eddf6741a..eb4a208fe907 100644
>> --- a/include/linux/page-isolation.h
>> +++ b/include/linux/page-isolation.h
>> @@ -15,6 +15,16 @@ static inline bool is_migrate_isolate(int migratetype)
>>  {
>>  	return migratetype == MIGRATE_ISOLATE;
>>  }
>> +static inline unsigned long pfn_max_align_down(unsigned long pfn)
>> +{
>> +	return ALIGN_DOWN(pfn, MAX_ORDER_NR_PAGES);
>> +}
>> +
>> +static inline unsigned long pfn_max_align_up(unsigned long pfn)
>> +{
>> +	return ALIGN(pfn, MAX_ORDER_NR_PAGES);
>> +}
>> +
>>  #else
>>  static inline bool has_isolate_pageblock(struct zone *zone)
>>  {
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 6de57d058d3d..680580a40a35 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -8937,16 +8937,6 @@ void *__init alloc_large_system_hash(const char *tablename,
>>  }
>>
>>  #ifdef CONFIG_CONTIG_ALLOC
>> -static unsigned long pfn_max_align_down(unsigned long pfn)
>> -{
>> -	return ALIGN_DOWN(pfn, MAX_ORDER_NR_PAGES);
>> -}
>> -
>> -static unsigned long pfn_max_align_up(unsigned long pfn)
>> -{
>> -	return ALIGN(pfn, MAX_ORDER_NR_PAGES);
>> -}
>> -
>>  #if defined(CONFIG_DYNAMIC_DEBUG) || \
>>  	(defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))
>>  /* Usage: See admin-guide/dynamic-debug-howto.rst */
>> @@ -9091,8 +9081,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
>>  	 * put back to page allocator so that buddy can use them.
>>  	 */
>>
>> -	ret = start_isolate_page_range(pfn_max_align_down(start),
>> -				       pfn_max_align_up(end), migratetype, 0);
>> +	ret = start_isolate_page_range(start, end, migratetype, 0);
>>  	if (ret)
>>  		return ret;
>
> Shouldn't we similarly adjust undo_isolate_page_range()? IOW, all users
> of pfn_max_align_down()/pfn_max_align_up(). would be gone from that file
> and you can move these defines into mm/page_isolation.c instead of
> include/linux/page-isolation.h?

undo_isolate_page_range() faces much simpler situation, just needing
to unset migratetype. We can just pass pageblock_nr_pages aligned range
to it. For start_isolate_page_range(), start and end are also used for
has_unmovable_pages() for precise unmovable page identification, so
they cannot be pageblock_nr_pages aligned. But for readability and symmetry,
yes, I can change undo_isolate_page_range() too.

>
> Maybe perform this change in a separate patch for
> start_isolate_page_range() and undo_isolate_page_range() ?

The change is trivial enough to be folded into this one.

>
>>
>> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
>> index b34f1310aeaa..419c805dbdcd 100644
>> --- a/mm/page_isolation.c
>> +++ b/mm/page_isolation.c
>> @@ -16,7 +16,8 @@
>>  #include <trace/events/page_isolation.h>
>>
>>  /*
>> - * This function checks whether pageblock includes unmovable pages or not.
>> + * This function checks whether pageblock within [start_pfn, end_pfn) includes
>> + * unmovable pages or not.
>
> I think we still want to limit that to a single pageblock (see below),
> as we're going to isolate individual pageblocks. Then an updated
> description could be:
>
> "This function checks whether the range [start_pfn, end_pfn) includes
> unmovable pages or not. The range must fall into a single pageblock and
> consequently belong to a single zone."
>

Sure.

>>   *
>>   * PageLRU check without isolation or lru_lock could race so that
>>   * MIGRATE_MOVABLE block might include unmovable pages. And __PageMovable
>> @@ -28,27 +29,26 @@
>>   * cannot get removed (e.g., via memory unplug) concurrently.
>>   *
>>   */
>> -static struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>> -				 int migratetype, int flags)
>> +static struct page *has_unmovable_pages(unsigned long start_pfn, unsigned long end_pfn,
>> +				int migratetype, int flags)
>>  {
>> -	unsigned long iter = 0;
>> -	unsigned long pfn = page_to_pfn(page);
>> -	unsigned long offset = pfn % pageblock_nr_pages;
>> +	unsigned long pfn = start_pfn;
>>
>> -	if (is_migrate_cma_page(page)) {
>> -		/*
>> -		 * CMA allocations (alloc_contig_range) really need to mark
>> -		 * isolate CMA pageblocks even when they are not movable in fact
>> -		 * so consider them movable here.
>> -		 */
>> -		if (is_migrate_cma(migratetype))
>> -			return NULL;
>
> If we're really dealing with a range that falls into a single pageblock,
> then you can leave the is_migrate_cma_page() in place and also lookup
> the zone only once. That should speed up things and minimize the
> required changes.
>
> You can then further add VM_BUG_ON()s that make sure that start_pfn and
> end_pfn-1 belong to a single pageblock.

Sure.

>
>> +	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
>> +		struct page *page = pfn_to_page(pfn);
>> +		struct zone *zone = page_zone(page);
>>
>> -		return page;
>> -	}
>> +		if (is_migrate_cma_page(page)) {
>> +			/*
>> +			 * CMA allocations (alloc_contig_range) really need to mark
>> +			 * isolate CMA pageblocks even when they are not movable in fact
>> +			 * so consider them movable here.
>> +			 */
>> +			if (is_migrate_cma(migratetype))
>> +				return NULL;
>>
>> -	for (; iter < pageblock_nr_pages - offset; iter++) {
>> -		page = pfn_to_page(pfn + iter);
>> +			return page;
>> +		}
>>
>>  		/*
>>  		 * Both, bootmem allocations and memory holes are marked
>> @@ -85,7 +85,7 @@ static struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>  			}
>>
>>  			skip_pages = compound_nr(head) - (page - head);
>> -			iter += skip_pages - 1;
>> +			pfn += skip_pages - 1;
>>  			continue;
>>  		}
>>
>> @@ -97,7 +97,7 @@ static struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>  		 */
>>  		if (!page_ref_count(page)) {
>>  			if (PageBuddy(page))
>> -				iter += (1 << buddy_order(page)) - 1;
>> +				pfn += (1 << buddy_order(page)) - 1;
>>  			continue;
>>  		}
>>
>> @@ -134,7 +134,13 @@ static struct page *has_unmovable_pages(struct zone *zone, struct page *page,
>>  	return NULL;
>>  }
>>
>> -static int set_migratetype_isolate(struct page *page, int migratetype, int isol_flags)
>> +/*
>> + * This function set pageblock migratetype to isolate if no unmovable page is
>> + * present in [start_pfn, end_pfn). The pageblock must intersect with
>> + * [start_pfn, end_pfn).
>> + */
>> +static int set_migratetype_isolate(struct page *page, int migratetype, int isol_flags,
>> +			unsigned long start_pfn, unsigned long end_pfn)
>>  {
>>  	struct zone *zone = page_zone(page);
>>  	struct page *unmovable;
>> @@ -155,8 +161,13 @@ static int set_migratetype_isolate(struct page *page, int migratetype, int isol_
>>  	/*
>>  	 * FIXME: Now, memory hotplug doesn't call shrink_slab() by itself.
>>  	 * We just check MOVABLE pages.
>> +	 *
>> +	 * Pass the intersection of [start_pfn, end_pfn) and the page's pageblock
>> +	 * to avoid redundant checks.
>>  	 */
>
> I think I'd prefer some helper variables for readability.

Will do.

>
>> -	unmovable = has_unmovable_pages(zone, page, migratetype, isol_flags);
>> +	unmovable = has_unmovable_pages(max(page_to_pfn(page), start_pfn),
>> +			min(ALIGN(page_to_pfn(page) + 1, pageblock_nr_pages), end_pfn),
>> +			migratetype, isol_flags);
>>  	if (!unmovable) {
>>  		unsigned long nr_pages;
>>  		int mt = get_pageblock_migratetype(page);
>> @@ -267,7 +278,6 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
>>   * be MIGRATE_ISOLATE.
>>   * @start_pfn:		The lower PFN of the range to be isolated.
>>   * @end_pfn:		The upper PFN of the range to be isolated.
>> - *			start_pfn/end_pfn must be aligned to pageblock_order.
>>   * @migratetype:	Migrate type to set in error recovery.
>>   * @flags:		The following flags are allowed (they can be combined in
>>   *			a bit mask)
>> @@ -309,15 +319,16 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
>>  	unsigned long pfn;
>>  	struct page *page;
>>
>> -	BUG_ON(!IS_ALIGNED(start_pfn, pageblock_nr_pages));
>> -	BUG_ON(!IS_ALIGNED(end_pfn, pageblock_nr_pages));
>> +	unsigned long isolate_start = pfn_max_align_down(start_pfn);
>> +	unsigned long isolate_end = pfn_max_align_up(end_pfn);
>>
>> -	for (pfn = start_pfn;
>> -	     pfn < end_pfn;
>> +	for (pfn = isolate_start;
>> +	     pfn < isolate_end;
>>  	     pfn += pageblock_nr_pages) {
>>  		page = __first_valid_page(pfn, pageblock_nr_pages);
>> -		if (page && set_migratetype_isolate(page, migratetype, flags)) {
>> -			undo_isolate_page_range(start_pfn, pfn, migratetype);
>> +		if (page && set_migratetype_isolate(page, migratetype, flags,
>> +					start_pfn, end_pfn)) {
>> +			undo_isolate_page_range(isolate_start, pfn, migratetype);
>>  			return -EBUSY;
>>  		}
>>  	}
>
>
> -- 
> Thanks,
>
> David / dhildenb

Thanks for your review.


--
Best Regards,
Yan, Zi

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