[PATCH] mm: hugetlb_vmemmap: use PageCompound() instead of PageReserved()

[PATCH] mm: hugetlb_vmemmap: use PageCompound() instead of PageReserved()

[Muchun Song]

The ckeck of PageReserved() is easy to be broken in the future, PageCompound()
is more stable to check if the page should be split.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
 mm/hugetlb_vmemmap.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
index 4b9734777f69..8068fe890f52 100644
--- a/mm/hugetlb_vmemmap.c
+++ b/mm/hugetlb_vmemmap.c
@@ -73,8 +73,10 @@ static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
 		 * be treated as indepdenent small pages (as they can be freed
 		 * individually).
 		 */
-		if (!PageReserved(head))
+		if (PageCompound(head)) {
+			VM_BUG_ON(compound_order(head) != get_order(PMD_SIZE));
 			split_page(head, get_order(PMD_SIZE));
+		}
 
 		/* Make pte visible before pmd. See comment in pmd_install(). */
 		smp_wmb();
-- 
2.11.0

Re: [PATCH] mm: hugetlb_vmemmap: use PageCompound() instead of PageReserved()

[Mika Penttilä]

Hi,


On 19.7.2023 9.31, Muchun Song wrote:
> The ckeck of PageReserved() is easy to be broken in the future, PageCompound()
> is more stable to check if the page should be split.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>   mm/hugetlb_vmemmap.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> index 4b9734777f69..8068fe890f52 100644
> --- a/mm/hugetlb_vmemmap.c
> +++ b/mm/hugetlb_vmemmap.c
> @@ -73,8 +73,10 @@ static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
>   		 * be treated as indepdenent small pages (as they can be freed
>   		 * individually).
>   		 */
> -		if (!PageReserved(head))
> +		if (PageCompound(head)) {
> +			VM_BUG_ON(compound_order(head) != get_order(PMD_SIZE));
>   			split_page(head, get_order(PMD_SIZE));

I think vmemmap pages are not compound pages (even order > 0).
They would bug on here trying to split_page() :


void split_page(struct page *page, unsigned int order)
{
         int i;

         VM_BUG_ON_PAGE(PageCompound(page), page);


> +		}
>   
>   		/* Make pte visible before pmd. See comment in pmd_install(). */
>   		smp_wmb();

--Mika

Re: [PATCH] mm: hugetlb_vmemmap: use PageCompound() instead of PageReserved()

[Muchun Song]

> On Jul 19, 2023, at 14:51, Mika Penttilä <mpenttil@redhat.com> wrote:
> 
> Hi,
> 
> 
> On 19.7.2023 9.31, Muchun Song wrote:
>> The ckeck of PageReserved() is easy to be broken in the future, PageCompound()
>> is more stable to check if the page should be split.
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>>  mm/hugetlb_vmemmap.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>> index 4b9734777f69..8068fe890f52 100644
>> --- a/mm/hugetlb_vmemmap.c
>> +++ b/mm/hugetlb_vmemmap.c
>> @@ -73,8 +73,10 @@ static int split_vmemmap_huge_pmd(pmd_t *pmd, unsigned long start)
>>   * be treated as indepdenent small pages (as they can be freed
>>   * individually).
>>   */
>> - if (!PageReserved(head))
>> + if (PageCompound(head)) {
>> + 	VM_BUG_ON(compound_order(head) != get_order(PMD_SIZE));
>>   	split_page(head, get_order(PMD_SIZE));
> 
> I think vmemmap pages are not compound pages (even order > 0).
> They would bug on here trying to split_page() :

You are right. I have missed this. Now I remember why I use
PageReserved() instead of PageCompound() when I fist submit
the commit 39d35edee453. Thanks for your reminder.

Sorry for the noise. Please ignore this patch.

Thanks.

> 
> 
> void split_page(struct page *page, unsigned int order)
> {
>        int i;
> 
>        VM_BUG_ON_PAGE(PageCompound(page), page);
> 
> 
>> + }
>>     /* Make pte visible before pmd. See comment in pmd_install(). */
>>   smp_wmb();
> 
> --Mika
>