[PATCH] mm/ptdesc: Derive from the compound head in page_ptdesc()

[PATCH] mm/ptdesc: Derive from the compound head in page_ptdesc()

[Anshuman Khandual]

struct ptdesc (including all relevant helpers) support multi order compound
pages. But page_ptdesc() coverts given page into its own ptdesc rather than
deriving from its compound head as would have been expected otherwise. Just
change the macro to fetch the struct ptdesc from the compound head instead,
so that the same struct ptdesc is reached from all tail pages.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
---
This applies on v6.18-rc5

Found via code inspection. Apparently struct ptdesc could represent a page
table page which is multi order looking into helpers as ptdesc_nr_pages(),
__pagetable_ctor/dtor() and pagetable_free() etc. Am I missing something ?

 include/linux/mm_types.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 90e5790c318f..f7107bc55d1e 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -604,9 +604,9 @@ static_assert(sizeof(struct ptdesc) <= sizeof(struct page));
 	const struct ptdesc *:		(const struct folio *)(pt),	\
 	struct ptdesc *:		(struct folio *)(pt)))
 
-#define page_ptdesc(p)			(_Generic((p),			\
-	const struct page *:		(const struct ptdesc *)(p),	\
-	struct page *:			(struct ptdesc *)(p)))
+#define page_ptdesc(p)			(_Generic((p),					\
+	const struct page *:		(const struct ptdesc *)_compound_head(p),	\
+	struct page *:			(struct ptdesc *)_compound_head(p)))
 
 #ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
 static inline void ptdesc_pmd_pts_init(struct ptdesc *ptdesc)
-- 
2.30.2

Re: [PATCH] mm/ptdesc: Derive from the compound head in page_ptdesc()

[David Hildenbrand (Red Hat)]

On 10.11.25 07:37, Anshuman Khandual wrote:
> struct ptdesc (including all relevant helpers) support multi order compound
> pages. But page_ptdesc() coverts given page into its own ptdesc rather than
> deriving from its compound head as would have been expected otherwise. Just
> change the macro to fetch the struct ptdesc from the compound head instead,
> so that the same struct ptdesc is reached from all tail pages.
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> ---
> This applies on v6.18-rc5
> 
> Found via code inspection. Apparently struct ptdesc could represent a page
> table page which is multi order looking into helpers as ptdesc_nr_pages(),
> __pagetable_ctor/dtor() and pagetable_free() etc. Am I missing something ?
> 
>   include/linux/mm_types.h | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 90e5790c318f..f7107bc55d1e 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -604,9 +604,9 @@ static_assert(sizeof(struct ptdesc) <= sizeof(struct page));
>   	const struct ptdesc *:		(const struct folio *)(pt),	\
>   	struct ptdesc *:		(struct folio *)(pt)))
>   
> -#define page_ptdesc(p)			(_Generic((p),			\
> -	const struct page *:		(const struct ptdesc *)(p),	\
> -	struct page *:			(struct ptdesc *)(p)))
> +#define page_ptdesc(p)			(_Generic((p),					\
> +	const struct page *:		(const struct ptdesc *)_compound_head(p),	\
> +	struct page *:			(struct ptdesc *)_compound_head(p)))

Well, this adds overhead :)

The real question is when we would be converting from a tail page to a 
ptdesc.

Take a look at pmd_ptdesc()->pmd_pgtable_page() where we avoid looking 
up a tail page in the first place.

Re: [PATCH] mm/ptdesc: Derive from the compound head in page_ptdesc()

[Matthew Wilcox]

On Mon, Nov 10, 2025 at 06:37:25AM +0000, Anshuman Khandual wrote:
> struct ptdesc (including all relevant helpers) support multi order compound
> pages. But page_ptdesc() coverts given page into its own ptdesc rather than
> deriving from its compound head as would have been expected otherwise. Just
> change the macro to fetch the struct ptdesc from the compound head instead,
> so that the same struct ptdesc is reached from all tail pages.

Please review
https://lore.kernel.org/linux-mm/20251020001652.2116669-1-willy@infradead.org/
instead.

Re: [PATCH] mm/ptdesc: Derive from the compound head in page_ptdesc()

[Anshuman Khandual]

On 10/11/25 2:23 PM, David Hildenbrand (Red Hat) wrote:
> On 10.11.25 07:37, Anshuman Khandual wrote:
>> struct ptdesc (including all relevant helpers) support multi order compound
>> pages. But page_ptdesc() coverts given page into its own ptdesc rather than
>> deriving from its compound head as would have been expected otherwise. Just
>> change the macro to fetch the struct ptdesc from the compound head instead,
>> so that the same struct ptdesc is reached from all tail pages.
>>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>> Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
>> Cc: Vlastimil Babka <vbabka@suse.cz>
>> Cc: Mike Rapoport <rppt@kernel.org>
>> Cc: Suren Baghdasaryan <surenb@google.com>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: linux-mm@kvack.org
>> Cc: linux-kernel@vger.kernel.org
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
>> ---
>> This applies on v6.18-rc5
>>
>> Found via code inspection. Apparently struct ptdesc could represent a page
>> table page which is multi order looking into helpers as ptdesc_nr_pages(),
>> __pagetable_ctor/dtor() and pagetable_free() etc. Am I missing something ?
>>
>>   include/linux/mm_types.h | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>> index 90e5790c318f..f7107bc55d1e 100644
>> --- a/include/linux/mm_types.h
>> +++ b/include/linux/mm_types.h
>> @@ -604,9 +604,9 @@ static_assert(sizeof(struct ptdesc) <= sizeof(struct page));
>>       const struct ptdesc *:        (const struct folio *)(pt),    \
>>       struct ptdesc *:        (struct folio *)(pt)))
>>   -#define page_ptdesc(p)            (_Generic((p),            \
>> -    const struct page *:        (const struct ptdesc *)(p),    \
>> -    struct page *:            (struct ptdesc *)(p)))
>> +#define page_ptdesc(p)            (_Generic((p),                    \
>> +    const struct page *:        (const struct ptdesc *)_compound_head(p),    \
>> +    struct page *:            (struct ptdesc *)_compound_head(p)))
> 
> Well, this adds overhead :)
> 
> The real question is when we would be converting from a tail page to a ptdesc.
> 
> Take a look at pmd_ptdesc()->pmd_pgtable_page() where we avoid looking up a tail page in the first place.
> 

Agreed - it does avoid looking into the tail pages. Currently there
are no instances where tail pages are converted into struct ptdesc.
But on its own page_ptdesc() does not look right and would not work
when applied on a tail page.

static inline struct page *pmd_pgtable_page(pmd_t *pmd)
{
        unsigned long mask = ~(PTRS_PER_PMD * sizeof(pmd_t) - 1);
        return virt_to_page((void *)((unsigned long) pmd & mask));
}

static inline struct ptdesc *pmd_ptdesc(pmd_t *pmd)
{
        return page_ptdesc(pmd_pgtable_page(pmd));
}

Re: [PATCH] mm/ptdesc: Derive from the compound head in page_ptdesc()

[Anshuman Khandual]

On 10/11/25 7:08 PM, Matthew Wilcox wrote:
> On Mon, Nov 10, 2025 at 06:37:25AM +0000, Anshuman Khandual wrote:
>> struct ptdesc (including all relevant helpers) support multi order compound
>> pages. But page_ptdesc() coverts given page into its own ptdesc rather than
>> deriving from its compound head as would have been expected otherwise. Just
>> change the macro to fetch the struct ptdesc from the compound head instead,
>> so that the same struct ptdesc is reached from all tail pages.
> 
> Please review
> https://lore.kernel.org/linux-mm/20251020001652.2116669-1-willy@infradead.org/
> instead.

Sure, will do. Thanks for the pointer.