* [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order
@ 2025-09-02 2:58 Wei Yang
2025-09-02 8:04 ` Vlastimil Babka
` (2 more replies)
0 siblings, 3 replies; 6+ messages in thread
From: Wei Yang @ 2025-09-02 2:58 UTC (permalink / raw)
To: akpm, vbabka, hannes, ziy
Cc: linux-mm, vishal.moola, Wei Yang, David Hildenbrand
We iterate pfn from order 0 to MAX_PAGE_ORDER aligned to find large buddy.
While if the order is less than start_pfn aligned order, we would get the
same pfn and do the same check again.
Iterate from start_pfn aligned order to reduce duplicated work.
Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
---
v2: add comment on assignment of order
---
mm/page_alloc.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 07d79ae557f8..5d9ceca869e5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2033,7 +2033,13 @@ static int move_freepages_block(struct zone *zone, struct page *page,
/* Look for a buddy that straddles start_pfn */
static unsigned long find_large_buddy(unsigned long start_pfn)
{
- int order = 0;
+ /*
+ * If start_pfn is not an order-0 PageBuddy, next PageBuddy containing
+ * start_pfn has minimal order of __ffs(start_pfn) + 1. Start checking
+ * the order with __ffs(start_pfn). If start_pfn is order-0 PageBuddy,
+ * the starting order does not matter.
+ */
+ int order = start_pfn ? __ffs(start_pfn) : MAX_PAGE_ORDER;
struct page *page;
unsigned long pfn = start_pfn;
--
2.34.1
^ permalink raw reply related [flat|nested] 6+ messages in thread* Re: [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order
2025-09-02 2:58 [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order Wei Yang
@ 2025-09-02 8:04 ` Vlastimil Babka
2025-09-02 14:33 ` Johannes Weiner
2025-09-02 16:25 ` Vishal Moola (Oracle)
2 siblings, 0 replies; 6+ messages in thread
From: Vlastimil Babka @ 2025-09-02 8:04 UTC (permalink / raw)
To: Wei Yang, akpm, hannes, ziy; +Cc: linux-mm, vishal.moola, David Hildenbrand
On 9/2/25 04:58, Wei Yang wrote:
> We iterate pfn from order 0 to MAX_PAGE_ORDER aligned to find large buddy.
> While if the order is less than start_pfn aligned order, we would get the
> same pfn and do the same check again.
>
> Iterate from start_pfn aligned order to reduce duplicated work.
>
> Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@gmail.com
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: David Hildenbrand <david@redhat.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
>
> ---
> v2: add comment on assignment of order
> ---
> mm/page_alloc.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 07d79ae557f8..5d9ceca869e5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2033,7 +2033,13 @@ static int move_freepages_block(struct zone *zone, struct page *page,
> /* Look for a buddy that straddles start_pfn */
> static unsigned long find_large_buddy(unsigned long start_pfn)
> {
> - int order = 0;
> + /*
> + * If start_pfn is not an order-0 PageBuddy, next PageBuddy containing
> + * start_pfn has minimal order of __ffs(start_pfn) + 1. Start checking
> + * the order with __ffs(start_pfn). If start_pfn is order-0 PageBuddy,
> + * the starting order does not matter.
> + */
> + int order = start_pfn ? __ffs(start_pfn) : MAX_PAGE_ORDER;
> struct page *page;
> unsigned long pfn = start_pfn;
>
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order
2025-09-02 2:58 [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order Wei Yang
2025-09-02 8:04 ` Vlastimil Babka
@ 2025-09-02 14:33 ` Johannes Weiner
2025-09-02 14:36 ` Zi Yan
2025-09-02 16:25 ` Vishal Moola (Oracle)
2 siblings, 1 reply; 6+ messages in thread
From: Johannes Weiner @ 2025-09-02 14:33 UTC (permalink / raw)
To: Wei Yang; +Cc: akpm, vbabka, ziy, linux-mm, vishal.moola, David Hildenbrand
On Tue, Sep 02, 2025 at 02:58:07AM +0000, Wei Yang wrote:
> We iterate pfn from order 0 to MAX_PAGE_ORDER aligned to find large buddy.
> While if the order is less than start_pfn aligned order, we would get the
> same pfn and do the same check again.
>
> Iterate from start_pfn aligned order to reduce duplicated work.
>
> Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@gmail.com
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: David Hildenbrand <david@redhat.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
>
> ---
> v2: add comment on assignment of order
> ---
> mm/page_alloc.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 07d79ae557f8..5d9ceca869e5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2033,7 +2033,13 @@ static int move_freepages_block(struct zone *zone, struct page *page,
> /* Look for a buddy that straddles start_pfn */
> static unsigned long find_large_buddy(unsigned long start_pfn)
> {
> - int order = 0;
> + /*
> + * If start_pfn is not an order-0 PageBuddy, next PageBuddy containing
> + * start_pfn has minimal order of __ffs(start_pfn) + 1. Start checking
> + * the order with __ffs(start_pfn). If start_pfn is order-0 PageBuddy,
> + * the starting order does not matter.
> + */
> + int order = start_pfn ? __ffs(start_pfn) : MAX_PAGE_ORDER;
This should be __ffs(start_pfn) - 1, no?
If you have the lowest bit set in the pfn, you should check the
order-1 buddy to the left first. But ffs(1) is already 1, which means
the loop will check order-2 next.
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order
2025-09-02 14:33 ` Johannes Weiner
@ 2025-09-02 14:36 ` Zi Yan
2025-09-02 15:12 ` Johannes Weiner
0 siblings, 1 reply; 6+ messages in thread
From: Zi Yan @ 2025-09-02 14:36 UTC (permalink / raw)
To: Johannes Weiner
Cc: Wei Yang, akpm, vbabka, linux-mm, vishal.moola, David Hildenbrand
On 2 Sep 2025, at 10:33, Johannes Weiner wrote:
> On Tue, Sep 02, 2025 at 02:58:07AM +0000, Wei Yang wrote:
>> We iterate pfn from order 0 to MAX_PAGE_ORDER aligned to find large buddy.
>> While if the order is less than start_pfn aligned order, we would get the
>> same pfn and do the same check again.
>>
>> Iterate from start_pfn aligned order to reduce duplicated work.
>>
>> Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@gmail.com
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>> Reviewed-by: Zi Yan <ziy@nvidia.com>
>>
>> ---
>> v2: add comment on assignment of order
>> ---
>> mm/page_alloc.c | 8 +++++++-
>> 1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 07d79ae557f8..5d9ceca869e5 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2033,7 +2033,13 @@ static int move_freepages_block(struct zone *zone, struct page *page,
>> /* Look for a buddy that straddles start_pfn */
>> static unsigned long find_large_buddy(unsigned long start_pfn)
>> {
>> - int order = 0;
>> + /*
>> + * If start_pfn is not an order-0 PageBuddy, next PageBuddy containing
>> + * start_pfn has minimal order of __ffs(start_pfn) + 1. Start checking
>> + * the order with __ffs(start_pfn). If start_pfn is order-0 PageBuddy,
>> + * the starting order does not matter.
>> + */
>> + int order = start_pfn ? __ffs(start_pfn) : MAX_PAGE_ORDER;
>
> This should be __ffs(start_pfn) - 1, no?
>
> If you have the lowest bit set in the pfn, you should check the
> order-1 buddy to the left first. But ffs(1) is already 1, which means
> the loop will check order-2 next.
__ffs() seems different from usespace ffs() and is 0-index, so __ffs() is
what you mean ffs() - 1.
Best Regards,
Yan, Zi
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order
2025-09-02 14:36 ` Zi Yan
@ 2025-09-02 15:12 ` Johannes Weiner
0 siblings, 0 replies; 6+ messages in thread
From: Johannes Weiner @ 2025-09-02 15:12 UTC (permalink / raw)
To: Zi Yan; +Cc: Wei Yang, akpm, vbabka, linux-mm, vishal.moola, David Hildenbrand
On Tue, Sep 02, 2025 at 10:36:14AM -0400, Zi Yan wrote:
> On 2 Sep 2025, at 10:33, Johannes Weiner wrote:
>
> > On Tue, Sep 02, 2025 at 02:58:07AM +0000, Wei Yang wrote:
> >> We iterate pfn from order 0 to MAX_PAGE_ORDER aligned to find large buddy.
> >> While if the order is less than start_pfn aligned order, we would get the
> >> same pfn and do the same check again.
> >>
> >> Iterate from start_pfn aligned order to reduce duplicated work.
> >>
> >> Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@gmail.com
> >> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> >> Cc: Johannes Weiner <hannes@cmpxchg.org>
> >> Cc: Zi Yan <ziy@nvidia.com>
> >> Cc: Vlastimil Babka <vbabka@suse.cz>
> >> Cc: David Hildenbrand <david@redhat.com>
> >> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> >> Reviewed-by: Zi Yan <ziy@nvidia.com>
> >>
> >> ---
> >> v2: add comment on assignment of order
> >> ---
> >> mm/page_alloc.c | 8 +++++++-
> >> 1 file changed, 7 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >> index 07d79ae557f8..5d9ceca869e5 100644
> >> --- a/mm/page_alloc.c
> >> +++ b/mm/page_alloc.c
> >> @@ -2033,7 +2033,13 @@ static int move_freepages_block(struct zone *zone, struct page *page,
> >> /* Look for a buddy that straddles start_pfn */
> >> static unsigned long find_large_buddy(unsigned long start_pfn)
> >> {
> >> - int order = 0;
> >> + /*
> >> + * If start_pfn is not an order-0 PageBuddy, next PageBuddy containing
> >> + * start_pfn has minimal order of __ffs(start_pfn) + 1. Start checking
> >> + * the order with __ffs(start_pfn). If start_pfn is order-0 PageBuddy,
> >> + * the starting order does not matter.
> >> + */
> >> + int order = start_pfn ? __ffs(start_pfn) : MAX_PAGE_ORDER;
> >
> > This should be __ffs(start_pfn) - 1, no?
> >
> > If you have the lowest bit set in the pfn, you should check the
> > order-1 buddy to the left first. But ffs(1) is already 1, which means
> > the loop will check order-2 next.
>
> __ffs() seems different from usespace ffs() and is 0-index, so __ffs() is
> what you mean ffs() - 1.
I'm going back to reading school.
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order
2025-09-02 2:58 [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order Wei Yang
2025-09-02 8:04 ` Vlastimil Babka
2025-09-02 14:33 ` Johannes Weiner
@ 2025-09-02 16:25 ` Vishal Moola (Oracle)
2 siblings, 0 replies; 6+ messages in thread
From: Vishal Moola (Oracle) @ 2025-09-02 16:25 UTC (permalink / raw)
To: Wei Yang; +Cc: akpm, vbabka, hannes, ziy, linux-mm, David Hildenbrand
On Tue, Sep 02, 2025 at 02:58:07AM +0000, Wei Yang wrote:
> We iterate pfn from order 0 to MAX_PAGE_ORDER aligned to find large buddy.
> While if the order is less than start_pfn aligned order, we would get the
> same pfn and do the same check again.
>
> Iterate from start_pfn aligned order to reduce duplicated work.
>
> Link: https://lkml.kernel.org/r/20250828091618.7869-1-richard.weiyang@gmail.com
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: David Hildenbrand <david@redhat.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
^ permalink raw reply [flat|nested] 6+ messages in thread
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2025-09-02 2:58 [Patch v2] mm/page_alloc: find_large_buddy() from start_pfn aligned order Wei Yang
2025-09-02 8:04 ` Vlastimil Babka
2025-09-02 14:33 ` Johannes Weiner
2025-09-02 14:36 ` Zi Yan
2025-09-02 15:12 ` Johannes Weiner
2025-09-02 16:25 ` Vishal Moola (Oracle)
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