* [PATCH] mm/page_alloc: check the correct buddy if it is a starting block
@ 2025-09-04 2:06 Wei Yang
2025-09-05 2:01 ` Zi Yan
2025-09-05 9:40 ` David Hildenbrand
0 siblings, 2 replies; 6+ messages in thread
From: Wei Yang @ 2025-09-04 2:06 UTC (permalink / raw)
To: akpm
Cc: linux-mm, Wei Yang, Johannes Weiner, Zi Yan, David Hildenbrand,
Baolin Wang, Vlastimil Babka
find_large_buddy() search buddy based on start_pfn, which maybe
different from page's pfn, e.g. when page is not pageblock aligned,
because prep_move_freepages_block() always align start_pfn to pageblock.
This means when we found a starting block at start_pfn, it may check
on the wrong page theoretically.
The good news is the page passed to __move_freepages_block_isolate() has
only two possible cases:
* page is pageblock aligned
* page is __first_valid_page() of this block
So it is safe for the first case, and it won't get a buddy larger than
pageblock for the second case.
To eliminate the ambiguity, unify the handling for starting/tail block.
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
---
Tried memory online/offine, which looks good.
---
mm/page_alloc.c | 17 ++++-------------
1 file changed, 4 insertions(+), 13 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5183a646fa09..453cd361386c 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2093,6 +2093,7 @@ static bool __move_freepages_block_isolate(struct zone *zone,
unsigned long start_pfn, pfn;
int from_mt;
int to_mt;
+ struct page *buddy;
if (isolate == get_pageblock_isolate(page)) {
VM_WARN_ONCE(1, "%s a pageblock that is already in that state",
@@ -2107,10 +2108,10 @@ static bool __move_freepages_block_isolate(struct zone *zone,
if (pageblock_order == MAX_PAGE_ORDER)
goto move;
- /* We're a tail block in a larger buddy */
pfn = find_large_buddy(start_pfn);
- if (pfn != start_pfn) {
- struct page *buddy = pfn_to_page(pfn);
+ buddy = pfn_to_page(pfn);
+ /* We're a part of a larger buddy */
+ if (PageBuddy(buddy) && buddy_order(buddy) > pageblock_order) {
int order = buddy_order(buddy);
del_page_from_free_list(buddy, zone, order,
@@ -2120,16 +2121,6 @@ static bool __move_freepages_block_isolate(struct zone *zone,
return true;
}
- /* We're the starting block of a larger buddy */
- if (PageBuddy(page) && buddy_order(page) > pageblock_order) {
- int order = buddy_order(page);
-
- del_page_from_free_list(page, zone, order,
- get_pfnblock_migratetype(page, pfn));
- toggle_pageblock_isolate(page, isolate);
- split_large_buddy(zone, page, pfn, order, FPI_NONE);
- return true;
- }
move:
/* Use MIGRATETYPE_MASK to get non-isolate migratetype */
if (isolate) {
--
2.34.1
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] mm/page_alloc: check the correct buddy if it is a starting block
2025-09-04 2:06 [PATCH] mm/page_alloc: check the correct buddy if it is a starting block Wei Yang
@ 2025-09-05 2:01 ` Zi Yan
2025-09-05 3:11 ` Wei Yang
2025-09-05 9:40 ` David Hildenbrand
1 sibling, 1 reply; 6+ messages in thread
From: Zi Yan @ 2025-09-05 2:01 UTC (permalink / raw)
To: Wei Yang
Cc: akpm, linux-mm, Johannes Weiner, David Hildenbrand, Baolin Wang,
Vlastimil Babka
On 3 Sep 2025, at 22:06, Wei Yang wrote:
> find_large_buddy() search buddy based on start_pfn, which maybe
> different from page's pfn, e.g. when page is not pageblock aligned,
> because prep_move_freepages_block() always align start_pfn to pageblock.
>
> This means when we found a starting block at start_pfn, it may check
> on the wrong page theoretically.
and not split the free page as it is supposed to, causing a freelist migratetype
mismatch.
>
> The good news is the page passed to __move_freepages_block_isolate() has
> only two possible cases:
>
> * page is pageblock aligned
> * page is __first_valid_page() of this block
>
> So it is safe for the first case, and it won't get a buddy larger than
> pageblock for the second case.
>
> To eliminate the ambiguity, unify the handling for starting/tail block.
To fix the issue, check the returned pfn of find_large_buddy() to decide
whether to split the free page:
1. if it is not a PageBuddy pfn, no split;
2. if it is a PageBuddy pfn but order <= pageblock_order, no split;
3. if it is a PageBuddy pfn with order > pageblock_order,
start_pfn is either in the starting block or tail block, split the PageBuddy
at pageblock_order level.
Otherwise, LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>
>
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
>
> ---
> Tried memory online/offine, which looks good.
> ---
> mm/page_alloc.c | 17 ++++-------------
> 1 file changed, 4 insertions(+), 13 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5183a646fa09..453cd361386c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2093,6 +2093,7 @@ static bool __move_freepages_block_isolate(struct zone *zone,
> unsigned long start_pfn, pfn;
> int from_mt;
> int to_mt;
> + struct page *buddy;
>
> if (isolate == get_pageblock_isolate(page)) {
> VM_WARN_ONCE(1, "%s a pageblock that is already in that state",
> @@ -2107,10 +2108,10 @@ static bool __move_freepages_block_isolate(struct zone *zone,
> if (pageblock_order == MAX_PAGE_ORDER)
> goto move;
>
> - /* We're a tail block in a larger buddy */
> pfn = find_large_buddy(start_pfn);
> - if (pfn != start_pfn) {
> - struct page *buddy = pfn_to_page(pfn);
> + buddy = pfn_to_page(pfn);
> + /* We're a part of a larger buddy */
> + if (PageBuddy(buddy) && buddy_order(buddy) > pageblock_order) {
> int order = buddy_order(buddy);
>
> del_page_from_free_list(buddy, zone, order,
> @@ -2120,16 +2121,6 @@ static bool __move_freepages_block_isolate(struct zone *zone,
> return true;
> }
>
> - /* We're the starting block of a larger buddy */
> - if (PageBuddy(page) && buddy_order(page) > pageblock_order) {
> - int order = buddy_order(page);
> -
> - del_page_from_free_list(page, zone, order,
> - get_pfnblock_migratetype(page, pfn));
> - toggle_pageblock_isolate(page, isolate);
> - split_large_buddy(zone, page, pfn, order, FPI_NONE);
> - return true;
> - }
> move:
> /* Use MIGRATETYPE_MASK to get non-isolate migratetype */
> if (isolate) {
> --
> 2.34.1
--
Best Regards,
Yan, Zi
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] mm/page_alloc: check the correct buddy if it is a starting block
2025-09-05 2:01 ` Zi Yan
@ 2025-09-05 3:11 ` Wei Yang
2025-09-05 3:25 ` Andrew Morton
0 siblings, 1 reply; 6+ messages in thread
From: Wei Yang @ 2025-09-05 3:11 UTC (permalink / raw)
To: Zi Yan
Cc: Wei Yang, akpm, linux-mm, Johannes Weiner, David Hildenbrand,
Baolin Wang, Vlastimil Babka
On Thu, Sep 04, 2025 at 10:01:01PM -0400, Zi Yan wrote:
>On 3 Sep 2025, at 22:06, Wei Yang wrote:
>
>> find_large_buddy() search buddy based on start_pfn, which maybe
>> different from page's pfn, e.g. when page is not pageblock aligned,
>> because prep_move_freepages_block() always align start_pfn to pageblock.
>>
>> This means when we found a starting block at start_pfn, it may check
>> on the wrong page theoretically.
>
>and not split the free page as it is supposed to, causing a freelist migratetype
>mismatch.
>
Thanks, this is important.
>>
>> The good news is the page passed to __move_freepages_block_isolate() has
>> only two possible cases:
>>
>> * page is pageblock aligned
>> * page is __first_valid_page() of this block
>>
>> So it is safe for the first case, and it won't get a buddy larger than
>> pageblock for the second case.
>>
>> To eliminate the ambiguity, unify the handling for starting/tail block.
>
>To fix the issue, check the returned pfn of find_large_buddy() to decide
>whether to split the free page:
>1. if it is not a PageBuddy pfn, no split;
>2. if it is a PageBuddy pfn but order <= pageblock_order, no split;
>3. if it is a PageBuddy pfn with order > pageblock_order,
> start_pfn is either in the starting block or tail block, split the PageBuddy
> at pageblock_order level.
>
>
>Otherwise, LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>
Thanks.
@Andrew, if not bother, would you mind adjust the changelog?
--
Wei Yang
Help you, Help me
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] mm/page_alloc: check the correct buddy if it is a starting block
2025-09-05 3:11 ` Wei Yang
@ 2025-09-05 3:25 ` Andrew Morton
0 siblings, 0 replies; 6+ messages in thread
From: Andrew Morton @ 2025-09-05 3:25 UTC (permalink / raw)
To: Wei Yang
Cc: Zi Yan, linux-mm, Johannes Weiner, David Hildenbrand,
Baolin Wang, Vlastimil Babka
On Fri, 5 Sep 2025 03:11:30 +0000 Wei Yang <richard.weiyang@gmail.com> wrote:
> >>
> >> The good news is the page passed to __move_freepages_block_isolate() has
> >> only two possible cases:
> >>
> >> * page is pageblock aligned
> >> * page is __first_valid_page() of this block
> >>
> >> So it is safe for the first case, and it won't get a buddy larger than
> >> pageblock for the second case.
> >>
> >> To eliminate the ambiguity, unify the handling for starting/tail block.
> >
> >To fix the issue, check the returned pfn of find_large_buddy() to decide
> >whether to split the free page:
> >1. if it is not a PageBuddy pfn, no split;
> >2. if it is a PageBuddy pfn but order <= pageblock_order, no split;
> >3. if it is a PageBuddy pfn with order > pageblock_order,
> > start_pfn is either in the starting block or tail block, split the PageBuddy
> > at pageblock_order level.
> >
> >
> >Otherwise, LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>
>
> Thanks.
>
> @Andrew, if not bother, would you mind adjust the changelog?
This?
From: Wei Yang <richard.weiyang@gmail.com>
Subject: mm/page_alloc: check the correct buddy if it is a starting block
Date: Thu, 4 Sep 2025 02:06:54 +0000
find_large_buddy() search buddy based on start_pfn, which may be different
from page's pfn, e.g. when page is not pageblock aligned, because
prep_move_freepages_block() always aligns start_pfn to pageblock.
This means when we found a starting block at start_pfn, it may check on
the wrong page theoretically.
The good news is the page passed to __move_freepages_block_isolate() has
only two possible cases:
* page is pageblock aligned
* page is __first_valid_page() of this block
So it is safe for the first case, and it won't get a buddy larger than
pageblock for the second case.
To fix the issue, check the returned pfn of find_large_buddy() to
decide whether to split the free page:
1. if it is not a PageBuddy pfn, no split;
2. if it is a PageBuddy pfn but order <= pageblock_order, no split;
3. if it is a PageBuddy pfn with order > pageblock_order,
start_pfn is either in the starting block or tail block, split the PageBuddy
at pageblock_order level.
Link: https://lkml.kernel.org/r/20250904020654.28689-1-richard.weiyang@gmail.com
Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] mm/page_alloc: check the correct buddy if it is a starting block
2025-09-04 2:06 [PATCH] mm/page_alloc: check the correct buddy if it is a starting block Wei Yang
2025-09-05 2:01 ` Zi Yan
@ 2025-09-05 9:40 ` David Hildenbrand
2025-09-05 13:35 ` Wei Yang
1 sibling, 1 reply; 6+ messages in thread
From: David Hildenbrand @ 2025-09-05 9:40 UTC (permalink / raw)
To: Wei Yang, akpm
Cc: linux-mm, Johannes Weiner, Zi Yan, Baolin Wang, Vlastimil Babka
On 04.09.25 04:06, Wei Yang wrote:
> find_large_buddy() search buddy based on start_pfn, which maybe
> different from page's pfn, e.g. when page is not pageblock aligned,
> because prep_move_freepages_block() always align start_pfn to pageblock.
>
> This means when we found a starting block at start_pfn, it may check
> on the wrong page theoretically.
>
> The good news is the page passed to __move_freepages_block_isolate() has
> only two possible cases:
>
> * page is pageblock aligned
> * page is __first_valid_page() of this block
>
> So it is safe for the first case, and it won't get a buddy larger than
> pageblock for the second case.
>
> To eliminate the ambiguity, unify the handling for starting/tail block.
>
> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
>
> ---
> Tried memory online/offine, which looks good.
> ---
> mm/page_alloc.c | 17 ++++-------------
> 1 file changed, 4 insertions(+), 13 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5183a646fa09..453cd361386c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2093,6 +2093,7 @@ static bool __move_freepages_block_isolate(struct zone *zone,
> unsigned long start_pfn, pfn;
> int from_mt;
> int to_mt;
> + struct page *buddy;
>
> if (isolate == get_pageblock_isolate(page)) {
> VM_WARN_ONCE(1, "%s a pageblock that is already in that state",
> @@ -2107,10 +2108,10 @@ static bool __move_freepages_block_isolate(struct zone *zone,
> if (pageblock_order == MAX_PAGE_ORDER)
> goto move;
>
> - /* We're a tail block in a larger buddy */
> pfn = find_large_buddy(start_pfn);
> - if (pfn != start_pfn) {
> - struct page *buddy = pfn_to_page(pfn);
> + buddy = pfn_to_page(pfn);
While at it, can you please rename "pfn" to "buddy_pfn" ?
> + /* We're a part of a larger buddy */
> + if (PageBuddy(buddy) && buddy_order(buddy) > pageblock_order) {
> int order = buddy_order(buddy);
Yes, I think this should work.
--
Cheers
David / dhildenb
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] mm/page_alloc: check the correct buddy if it is a starting block
2025-09-05 9:40 ` David Hildenbrand
@ 2025-09-05 13:35 ` Wei Yang
0 siblings, 0 replies; 6+ messages in thread
From: Wei Yang @ 2025-09-05 13:35 UTC (permalink / raw)
To: David Hildenbrand
Cc: Wei Yang, akpm, linux-mm, Johannes Weiner, Zi Yan, Baolin Wang,
Vlastimil Babka
On Fri, Sep 05, 2025 at 11:40:42AM +0200, David Hildenbrand wrote:
>On 04.09.25 04:06, Wei Yang wrote:
>> find_large_buddy() search buddy based on start_pfn, which maybe
>> different from page's pfn, e.g. when page is not pageblock aligned,
>> because prep_move_freepages_block() always align start_pfn to pageblock.
>>
>> This means when we found a starting block at start_pfn, it may check
>> on the wrong page theoretically.
>>
>> The good news is the page passed to __move_freepages_block_isolate() has
>> only two possible cases:
>>
>> * page is pageblock aligned
>> * page is __first_valid_page() of this block
>>
>> So it is safe for the first case, and it won't get a buddy larger than
>> pageblock for the second case.
>>
>> To eliminate the ambiguity, unify the handling for starting/tail block.
>>
>> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>> Cc: Zi Yan <ziy@nvidia.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>>
>> ---
>> Tried memory online/offine, which looks good.
>> ---
>> mm/page_alloc.c | 17 ++++-------------
>> 1 file changed, 4 insertions(+), 13 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 5183a646fa09..453cd361386c 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -2093,6 +2093,7 @@ static bool __move_freepages_block_isolate(struct zone *zone,
>> unsigned long start_pfn, pfn;
>> int from_mt;
>> int to_mt;
>> + struct page *buddy;
>> if (isolate == get_pageblock_isolate(page)) {
>> VM_WARN_ONCE(1, "%s a pageblock that is already in that state",
>> @@ -2107,10 +2108,10 @@ static bool __move_freepages_block_isolate(struct zone *zone,
>> if (pageblock_order == MAX_PAGE_ORDER)
>> goto move;
>> - /* We're a tail block in a larger buddy */
>> pfn = find_large_buddy(start_pfn);
>> - if (pfn != start_pfn) {
>> - struct page *buddy = pfn_to_page(pfn);
>> + buddy = pfn_to_page(pfn);
>
>While at it, can you please rename "pfn" to "buddy_pfn" ?
>
Ok.
>> + /* We're a part of a larger buddy */
>> + if (PageBuddy(buddy) && buddy_order(buddy) > pageblock_order) {
>> int order = buddy_order(buddy);
>
>Yes, I think this should work.
>
@Andrew, looks I will send a v2 with updated changelog. Thanks
--
Wei Yang
Help you, Help me
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2025-09-05 13:35 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2025-09-04 2:06 [PATCH] mm/page_alloc: check the correct buddy if it is a starting block Wei Yang
2025-09-05 2:01 ` Zi Yan
2025-09-05 3:11 ` Wei Yang
2025-09-05 3:25 ` Andrew Morton
2025-09-05 9:40 ` David Hildenbrand
2025-09-05 13:35 ` Wei Yang
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