Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(),
which checks whether the given zone contains holes, and uses pfn_valid()
to check if the end pfn is valid. However pfn_valid() can not make sure
the end pfn is not a hole if the size of a pageblock is larger than the
size of a sub-mem_section, since the struct page getting by pfn_to_page()
may represent a hole or an unusable page frame, which may cause incorrect
zone contiguous is set.

Though another user of pageblock_pfn_to_page() in compaction seems work
well now, it is better to avoid scanning or touching these offline pfns.
So like commit 2d070eab2e82 ("mm: consider zone which is not fully
populated to have holes"), we should also use pfn_to_online_page() for
the end pfn to make sure it is a valid pfn with usable page frame.
Meanwhile the pfn_valid() for end pfn can be dropped now.

Moreover we've already used pfn_to_online_page() for start pfn to make
sure it is online and valid, so the pfn_valid() for the start pfn is
unnecessary, drop it.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 mm/page_alloc.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d0eb280ec7e4..8076f519c572 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1512,9 +1512,6 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
 	/* end_pfn is one past the range we are checking */
 	end_pfn--;
 
-	if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn))
-		return NULL;
-
 	start_page = pfn_to_online_page(start_pfn);
 	if (!start_page)
 		return NULL;
@@ -1522,7 +1519,9 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
 	if (page_zone(start_page) != zone)
 		return NULL;
 
-	end_page = pfn_to_page(end_pfn);
+	end_page = pfn_to_online_page(end_pfn);
+	if (!end_page)
+		return NULL;
 
 	/* This gives a shorter code than deriving page_zone(end_page) */
 	if (page_zone_id(start_page) != page_zone_id(end_page))
-- 
2.27.0

On 12.04.23 12:45, Baolin Wang wrote:
> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(),
> which checks whether the given zone contains holes, and uses pfn_valid()
> to check if the end pfn is valid. However pfn_valid() can not make sure
> the end pfn is not a hole if the size of a pageblock is larger than the
> size of a sub-mem_section, since the struct page getting by pfn_to_page()
> may represent a hole or an unusable page frame, which may cause incorrect
> zone contiguous is set.
> 
> Though another user of pageblock_pfn_to_page() in compaction seems work
> well now, it is better to avoid scanning or touching these offline pfns.
> So like commit 2d070eab2e82 ("mm: consider zone which is not fully
> populated to have holes"), we should also use pfn_to_online_page() for
> the end pfn to make sure it is a valid pfn with usable page frame.
> Meanwhile the pfn_valid() for end pfn can be dropped now.
> 
> Moreover we've already used pfn_to_online_page() for start pfn to make
> sure it is online and valid, so the pfn_valid() for the start pfn is
> unnecessary, drop it.

pageblocks are supposed to fall into a single memory section, so in most 
cases, if the start is online, so is the end.

The exception to this rule is when we have a mixture of ZONE_DEVICE and 
ZONE_* within the same section.

Then, indeed the end might not be online.

BUT, if the end is valid (-> ZONE_DEVICE), then the zone_id will differ. 
[let's ignore any races for now, up to this point they are mostly of 
theoretical nature]

So I don't think this change actually fixes something.


Getting rid of the pfn_valid(start_pfn)  makes sense. Replacing the 
pfn_valid(end_pfn) by a pfn_to_online_page(end_pfn) could make that 
function less efficient.

> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>   mm/page_alloc.c | 7 +++----
>   1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d0eb280ec7e4..8076f519c572 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1512,9 +1512,6 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
>   	/* end_pfn is one past the range we are checking */
>   	end_pfn--;
>   
> -	if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn))
> -		return NULL;
> -
>   	start_page = pfn_to_online_page(start_pfn);
>   	if (!start_page)
>   		return NULL;
> @@ -1522,7 +1519,9 @@ struct page *__pageblock_pfn_to_page(unsigned long start_pfn,
>   	if (page_zone(start_page) != zone)
>   		return NULL;
>   
> -	end_page = pfn_to_page(end_pfn);
> +	end_page = pfn_to_online_page(end_pfn);
> +	if (!end_page)
> +		return NULL;
>   
>   	/* This gives a shorter code than deriving page_zone(end_page) */
>   	if (page_zone_id(start_page) != page_zone_id(end_page))

-- 
Thanks,

David / dhildenb

On 4/12/2023 7:25 PM, David Hildenbrand wrote:
> On 12.04.23 12:45, Baolin Wang wrote:
>> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(),
>> which checks whether the given zone contains holes, and uses pfn_valid()
>> to check if the end pfn is valid. However pfn_valid() can not make sure
>> the end pfn is not a hole if the size of a pageblock is larger than the
>> size of a sub-mem_section, since the struct page getting by pfn_to_page()
>> may represent a hole or an unusable page frame, which may cause incorrect
>> zone contiguous is set.
>>
>> Though another user of pageblock_pfn_to_page() in compaction seems work
>> well now, it is better to avoid scanning or touching these offline pfns.
>> So like commit 2d070eab2e82 ("mm: consider zone which is not fully
>> populated to have holes"), we should also use pfn_to_online_page() for
>> the end pfn to make sure it is a valid pfn with usable page frame.
>> Meanwhile the pfn_valid() for end pfn can be dropped now.
>>
>> Moreover we've already used pfn_to_online_page() for start pfn to make
>> sure it is online and valid, so the pfn_valid() for the start pfn is
>> unnecessary, drop it.
> 
> pageblocks are supposed to fall into a single memory section, so in mos > cases, if the start is online, so is the end.

Yes, the granularity of memory hotplug is a mem_section.

However, suppose the pageblock order is MAX_ORDER-1, and the size of a 
sub-section is 2M, that means a pageblock will fall into 2 sub 
mem-section, and if there is a hole in the zone, that means the 2nd sub 
mem-section can be invalid without setting subsection_map bitmap.

So the start is online can make sure the end pfn of a pageblock is 
online, but a valid start pfn can not make sure the end pfn is valid in 
the bitmap of ms->usage->subsection_map.

> The exception to this rule is when we have a mixture of ZONE_DEVICE and 
> ZONE_* within the same section.
> 
> Then, indeed the end might not be online.
> 
> BUT, if the end is valid (-> ZONE_DEVICE), then the zone_id will differ. 
> [let's ignore any races for now, up to this point they are mostly of 
> theoretical nature]
> 
> So I don't think this change actually fixes something.
> 
> 
> Getting rid of the pfn_valid(start_pfn)  makes sense. Replacing the 

Yes, my motivation is try to optimize the __pageblock_pfn_to_page() 
which is hot when doing compaction, and I saw these pfn_valid() can be 
dropped.

> pfn_valid(end_pfn) by a pfn_to_online_page(end_pfn) could make that 
> function less efficient.
> 
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   mm/page_alloc.c | 7 +++----
>>   1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index d0eb280ec7e4..8076f519c572 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -1512,9 +1512,6 @@ struct page *__pageblock_pfn_to_page(unsigned 
>> long start_pfn,
>>       /* end_pfn is one past the range we are checking */
>>       end_pfn--;
>> -    if (!pfn_valid(start_pfn) || !pfn_valid(end_pfn))
>> -        return NULL;
>> -
>>       start_page = pfn_to_online_page(start_pfn);
>>       if (!start_page)
>>           return NULL;
>> @@ -1522,7 +1519,9 @@ struct page *__pageblock_pfn_to_page(unsigned 
>> long start_pfn,
>>       if (page_zone(start_page) != zone)
>>           return NULL;
>> -    end_page = pfn_to_page(end_pfn);
>> +    end_page = pfn_to_online_page(end_pfn);
>> +    if (!end_page)
>> +        return NULL;
>>       /* This gives a shorter code than deriving page_zone(end_page) */
>>       if (page_zone_id(start_page) != page_zone_id(end_page))
> 

On 4/12/23 14:16, Baolin Wang wrote:
> 
> 
> On 4/12/2023 7:25 PM, David Hildenbrand wrote:
>> On 12.04.23 12:45, Baolin Wang wrote:
>>> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(),
>>> which checks whether the given zone contains holes, and uses pfn_valid()
>>> to check if the end pfn is valid. However pfn_valid() can not make sure
>>> the end pfn is not a hole if the size of a pageblock is larger than the
>>> size of a sub-mem_section, since the struct page getting by pfn_to_page()
>>> may represent a hole or an unusable page frame, which may cause incorrect
>>> zone contiguous is set.
>>>
>>> Though another user of pageblock_pfn_to_page() in compaction seems work
>>> well now, it is better to avoid scanning or touching these offline pfns.
>>> So like commit 2d070eab2e82 ("mm: consider zone which is not fully
>>> populated to have holes"), we should also use pfn_to_online_page() for
>>> the end pfn to make sure it is a valid pfn with usable page frame.
>>> Meanwhile the pfn_valid() for end pfn can be dropped now.
>>>
>>> Moreover we've already used pfn_to_online_page() for start pfn to make
>>> sure it is online and valid, so the pfn_valid() for the start pfn is
>>> unnecessary, drop it.
>> 
>> pageblocks are supposed to fall into a single memory section, so in mos > cases, if the start is online, so is the end.
> 
> Yes, the granularity of memory hotplug is a mem_section.
> 
> However, suppose the pageblock order is MAX_ORDER-1, and the size of a 
> sub-section is 2M, that means a pageblock will fall into 2 sub 
> mem-section, and if there is a hole in the zone, that means the 2nd sub 
> mem-section can be invalid without setting subsection_map bitmap.

Can that really happen? I think the buddy merging in __free_one_page() would
trip on that?

On 4/14/2023 11:07 PM, Vlastimil Babka wrote:
> On 4/12/23 14:16, Baolin Wang wrote:
>>
>>
>> On 4/12/2023 7:25 PM, David Hildenbrand wrote:
>>> On 12.04.23 12:45, Baolin Wang wrote:
>>>> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(),
>>>> which checks whether the given zone contains holes, and uses pfn_valid()
>>>> to check if the end pfn is valid. However pfn_valid() can not make sure
>>>> the end pfn is not a hole if the size of a pageblock is larger than the
>>>> size of a sub-mem_section, since the struct page getting by pfn_to_page()
>>>> may represent a hole or an unusable page frame, which may cause incorrect
>>>> zone contiguous is set.
>>>>
>>>> Though another user of pageblock_pfn_to_page() in compaction seems work
>>>> well now, it is better to avoid scanning or touching these offline pfns.
>>>> So like commit 2d070eab2e82 ("mm: consider zone which is not fully
>>>> populated to have holes"), we should also use pfn_to_online_page() for
>>>> the end pfn to make sure it is a valid pfn with usable page frame.
>>>> Meanwhile the pfn_valid() for end pfn can be dropped now.
>>>>
>>>> Moreover we've already used pfn_to_online_page() for start pfn to make
>>>> sure it is online and valid, so the pfn_valid() for the start pfn is
>>>> unnecessary, drop it.
>>>
>>> pageblocks are supposed to fall into a single memory section, so in mos > cases, if the start is online, so is the end.
>>
>> Yes, the granularity of memory hotplug is a mem_section.
>>
>> However, suppose the pageblock order is MAX_ORDER-1, and the size of a
>> sub-section is 2M, that means a pageblock will fall into 2 sub
>> mem-section, and if there is a hole in the zone, that means the 2nd sub
>> mem-section can be invalid without setting subsection_map bitmap.
> 
> Can that really happen? I think the buddy merging in __free_one_page() would
> trip on that?

I do not think so IIUC. The hole pfns will not free to buddy system, and 
the buddy system did not change the ms->usage->subsection_map of the 
hole pfns, which indicates the hole pfns are invalid.

On Wed 12-04-23 18:45:31, Baolin Wang wrote:
> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(),
> which checks whether the given zone contains holes, and uses pfn_valid()
> to check if the end pfn is valid. However pfn_valid() can not make sure
> the end pfn is not a hole if the size of a pageblock is larger than the
> size of a sub-mem_section, since the struct page getting by pfn_to_page()
> may represent a hole or an unusable page frame, which may cause incorrect
> zone contiguous is set.
> 
> Though another user of pageblock_pfn_to_page() in compaction seems work
> well now, it is better to avoid scanning or touching these offline pfns.
> So like commit 2d070eab2e82 ("mm: consider zone which is not fully
> populated to have holes"), we should also use pfn_to_online_page() for
> the end pfn to make sure it is a valid pfn with usable page frame.
> Meanwhile the pfn_valid() for end pfn can be dropped now.
> 
> Moreover we've already used pfn_to_online_page() for start pfn to make
> sure it is online and valid, so the pfn_valid() for the start pfn is
> unnecessary, drop it.

Is this a theoretical problem or something you have encountered on a
real machine? Could you provide more details please?

-- 
Michal Hocko
SUSE Labs

On 4/12/2023 7:15 PM, Michal Hocko wrote:
> On Wed 12-04-23 18:45:31, Baolin Wang wrote:
>> Now the __pageblock_pfn_to_page() is used by set_zone_contiguous(),
>> which checks whether the given zone contains holes, and uses pfn_valid()
>> to check if the end pfn is valid. However pfn_valid() can not make sure
>> the end pfn is not a hole if the size of a pageblock is larger than the
>> size of a sub-mem_section, since the struct page getting by pfn_to_page()
>> may represent a hole or an unusable page frame, which may cause incorrect
>> zone contiguous is set.
>>
>> Though another user of pageblock_pfn_to_page() in compaction seems work
>> well now, it is better to avoid scanning or touching these offline pfns.
>> So like commit 2d070eab2e82 ("mm: consider zone which is not fully
>> populated to have holes"), we should also use pfn_to_online_page() for
>> the end pfn to make sure it is a valid pfn with usable page frame.
>> Meanwhile the pfn_valid() for end pfn can be dropped now.
>>
>> Moreover we've already used pfn_to_online_page() for start pfn to make
>> sure it is online and valid, so the pfn_valid() for the start pfn is
>> unnecessary, drop it.
> 
> Is this a theoretical problem or something you have encountered on a
> real machine? Could you provide more details please?

As I replied to David, this is just from code inspection when trying to 
remove the unnecessary pfn_valid() in __pageblock_pfn_to_page().