[PATCH 13/22] mm: account page table pages when allocated

[Kevin Brodsky]

As a step towards removing pagetable ctor/dtor calls for kernel
mm's, move the memory accounting from the ctor/dtor to
pagetable_{alloc,free}.

The ctor is normally called after pagetable_alloc() (and dtor before
pagetable_free()) so this patch should have no functional effect in
most cases. Where calls to ctor/dtor are currently missing (e.g.
x86's split_large_page()), this patch should improve the accounting.

This patch is based on Matthew's earlier implementation linked
below, with the difference that when freeing the accounting is
updated in __pagetable_free(), meaning that the update is delayed
until the pages are actually freed in the
CONFIG_ASYNC_KERNEL_PGTABLE_FREE case. This shouldn't matter in
practice.

Suggested-by: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20251113140448.1814860-3-willy@xxxxxxxxxxxxx
Signed-off-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
---
 include/linux/mm.h | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b336eb32faa0..361a0d7b74c5 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3681,6 +3681,11 @@ static inline struct ptdesc *pagetable_alloc_noprof(gfp_t gfp, unsigned int orde
 {
 	struct page *page = alloc_pages_noprof(gfp | __GFP_COMP, order);
 
+	if (!page)
+		return NULL;
+
+	lruvec_stat_add_folio(page_folio(page), NR_PAGETABLE);
+
 	return page_ptdesc(page);
 }
 #define pagetable_alloc(...)	alloc_hooks(pagetable_alloc_noprof(__VA_ARGS__))
@@ -3689,6 +3694,8 @@ static inline void __pagetable_free(struct ptdesc *pt)
 {
 	struct page *page = ptdesc_page(pt);
 
+	lruvec_stat_sub_folio(page_folio(page), NR_PAGETABLE);
+
 	__free_pages(page, compound_order(page));
 }
 
@@ -3797,7 +3804,6 @@ static inline void __pagetable_ctor(struct ptdesc *ptdesc)
 	struct folio *folio = ptdesc_folio(ptdesc);
 
 	__folio_set_pgtable(folio);
-	lruvec_stat_add_folio(folio, NR_PAGETABLE);
 }
 
 static inline void pagetable_dtor(struct ptdesc *ptdesc)
@@ -3806,7 +3812,6 @@ static inline void pagetable_dtor(struct ptdesc *ptdesc)
 
 	ptlock_free(ptdesc);
 	__folio_clear_pgtable(folio);
-	lruvec_stat_sub_folio(folio, NR_PAGETABLE);
 }
 
 static inline void pagetable_dtor_free(struct ptdesc *ptdesc)

-- 
2.51.2