[PATCH v3 0/2] mm: Don't set and reset page count in MEMINIT_EARLY

[PATCH v3 0/2] mm: Don't set and reset page count in MEMINIT_EARLY

[Yajun Deng]

__init_single_page would set page count and __free_pages_core would
reset it. A lot of pages don't need to do this when in MEMINIT_EARLY
context. It's unnecessary and time-consuming.

The 1st patch is pass page count and reserved to __init_single_page.
It's in preparation for the 2nd patch, it didn't change anything.

The 2nd patch only set page count for the reserved region, not all
of the region.

Yajun Deng (2):
  mm: pass page count and reserved to __init_single_page
  mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY

 mm/hugetlb.c    |  2 +-
 mm/internal.h   |  8 +++++++-
 mm/mm_init.c    | 45 ++++++++++++++++++++++++++++-----------------
 mm/page_alloc.c | 20 ++++++++++++--------
 4 files changed, 48 insertions(+), 27 deletions(-)

-- 
2.25.1

[PATCH v3 2/2] mm: Init page count in reserve_bootmem_region when MEMINIT_EARLY

[Yajun Deng]

memmap_init_range() would set page count of all pages, but the free
pages count would be reset in __free_pages_core(). There are opposite
operations. It's unnecessary and time-consuming when it's MEMINIT_EARLY
context.

Init page count in reserve_bootmem_region when in MEMINIT_EARLY context,
and check the page count before reset it.

At the same time, the INIT_LIST_HEAD in reserve_bootmem_region isn't
need, as it already done in __init_single_page.

The following data was tested on an x86 machine with 190GB of RAM.

before:
free_low_memory_core_early()    341ms

after:
free_low_memory_core_early()    285ms

Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
---
v3: same with v2.
v2: check page count instead of check context before reset it.
v1: https://lore.kernel.org/all/20230922070923.355656-1-yajun.deng@linux.dev/
---
 mm/mm_init.c    | 18 +++++++++++++-----
 mm/page_alloc.c | 20 ++++++++++++--------
 2 files changed, 25 insertions(+), 13 deletions(-)

diff --git a/mm/mm_init.c b/mm/mm_init.c
index 07fe7e489769..af1b3e7b0f52 100644
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -718,7 +718,7 @@ static void __meminit init_reserved_page(unsigned long pfn, int nid)
 		if (zone_spans_pfn(zone, pfn))
 			break;
 	}
-	__init_single_page(pfn_to_page(pfn), pfn, zid, nid, INIT_PAGE_COUNT);
+	__init_single_page(pfn_to_page(pfn), pfn, zid, nid, 0);
 }
 #else
 static inline void pgdat_set_deferred_range(pg_data_t *pgdat) {}
@@ -756,8 +756,8 @@ void __meminit reserve_bootmem_region(phys_addr_t start,
 
 			init_reserved_page(start_pfn, nid);
 
-			/* Avoid false-positive PageTail() */
-			INIT_LIST_HEAD(&page->lru);
+			/* Set page count for the reserved region */
+			init_page_count(page);
 
 			/*
 			 * no need for atomic set_bit because the struct
@@ -888,9 +888,17 @@ void __meminit memmap_init_range(unsigned long size, int nid, unsigned long zone
 		}
 
 		page = pfn_to_page(pfn);
-		__init_single_page(page, pfn, zone, nid, INIT_PAGE_COUNT);
-		if (context == MEMINIT_HOTPLUG)
+
+		/* If the context is MEMINIT_EARLY, we will set page count and
+		 * mark page reserved in reserve_bootmem_region, the free region
+		 * wouldn't have page count and we will check the pages count
+		 * in __free_pages_core.
+		 */
+		__init_single_page(page, pfn, zone, nid, 0);
+		if (context == MEMINIT_HOTPLUG) {
+			init_page_count(page);
 			__SetPageReserved(page);
+		}
 
 		/*
 		 * Usually, we want to mark the pageblock MIGRATE_MOVABLE,
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 06be8821d833..b868caabe8dc 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1285,18 +1285,22 @@ void __free_pages_core(struct page *page, unsigned int order)
 	unsigned int loop;
 
 	/*
-	 * When initializing the memmap, __init_single_page() sets the refcount
-	 * of all pages to 1 ("allocated"/"not free"). We have to set the
-	 * refcount of all involved pages to 0.
+	 * When initializing the memmap, memmap_init_range sets the refcount
+	 * of all pages to 1 ("reserved" and "free") in hotplug context. We
+	 * have to set the refcount of all involved pages to 0. Otherwise,
+	 * we don't do it, as reserve_bootmem_region only set the refcount on
+	 * reserve region ("reserved") in early context.
 	 */
-	prefetchw(p);
-	for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
-		prefetchw(p + 1);
+	if (page_count(page)) {
+		prefetchw(p);
+		for (loop = 0; loop < (nr_pages - 1); loop++, p++) {
+			prefetchw(p + 1);
+			__ClearPageReserved(p);
+			set_page_count(p, 0);
+		}
 		__ClearPageReserved(p);
 		set_page_count(p, 0);
 	}
-	__ClearPageReserved(p);
-	set_page_count(p, 0);
 
 	atomic_long_add(nr_pages, &page_zone(page)->managed_pages);
 
-- 
2.25.1

Re: [PATCH v3 1/2] mm: pass page count and reserved to __init_single_page

[David Hildenbrand]

On 26.09.23 04:33, Yajun Deng wrote:
> When we init a single page, we need to mark this page reserved if it
> does. 

I failed to parse the last part of this sentence.

> And some pages may not need to set page count, such as compound
> pages.

Usually, the refcount of all tail pages *must* be zero. Otherwise, 
get_page_unless_zero() would work on tail pages.

Can you elaborate why it should be okay here?


-- 
Cheers,

David / dhildenb

Re: [PATCH v3 1/2] mm: pass page count and reserved to __init_single_page

[Yajun Deng]

On 2023/9/26 15:44, David Hildenbrand wrote:
> On 26.09.23 04:33, Yajun Deng wrote:
>> When we init a single page, we need to mark this page reserved if it
>> does. 
>
> I failed to parse the last part of this sentence.
>
>> And some pages may not need to set page count, such as compound
>> pages.
>
> Usually, the refcount of all tail pages *must* be zero. Otherwise, 
> get_page_unless_zero() would work on tail pages.
>
> Can you elaborate why it should be okay here?
>
>
It means the following code in memmap_init_compound().

-               __init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
+               __init_zone_device_page(page, pfn, zone_idx, nid, pgmap, 0);
                 prep_compound_tail(head, pfn - head_pfn);
                 set_page_count(page, 0);

As we can see, it will reset the page count by 'set_page_count(page, 0)'.

Therefore, we don't need to set page count in __init_zone_device_page(). 
I wasn't clear enough in the commit.

Maybe we can remove the 'set_page_count(page, 0)',  But I didn't do 
that, just to be safe.

Re: [PATCH v3 1/2] mm: pass page count and reserved to __init_single_page

[Mike Rapoport]

On Tue, Sep 26, 2023 at 10:33:40AM +0800, Yajun Deng wrote:
> When we init a single page, we need to mark this page reserved if it
> does. And some pages may not need to set page count, such as compound
> pages.
> 
> Introduce enum init_page_flags, the caller init page count and mark page
> reserved by passing INIT_PAGE_COUNT and INIT_PAGE_RESERVED.
> 
> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
> ---
> v3: Introduce enum init_page_flags.
> v2: Introduce INIT_PAGE_COUNT and INIT_PAGE_RESERVED.
> v1: https://lore.kernel.org/all/20230922070923.355656-1-yajun.deng@linux.dev/
> ---
>  mm/hugetlb.c  |  2 +-
>  mm/internal.h |  8 +++++++-
>  mm/mm_init.c  | 31 +++++++++++++++++--------------
>  3 files changed, 25 insertions(+), 16 deletions(-)
> 
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 06a72c223bce..07fe7e489769 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -1041,7 +1043,7 @@ static void __ref memmap_init_compound(struct page *head,
>  	for (pfn = head_pfn + 1; pfn < end_pfn; pfn++) {
>  		struct page *page = pfn_to_page(pfn);
>  
> -		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap);
> +		__init_zone_device_page(page, pfn, zone_idx, nid, pgmap, 0);

I think the first patch should not contain any functional changes, but only
add the flags parameter to __init_single_page().

>  		prep_compound_tail(head, pfn - head_pfn);
>  		set_page_count(page, 0);
>  

-- 
Sincerely yours,
Mike.