linux-kernel - [RFC][PATCH 1/8] x86/mm: Always allocate a whole page for PAE PGDs

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Message-Id: <20250123172430.E948520B@davehans-spike.ostc.intel.com>
Date: Thu, 23 Jan 2025 09:24:30 -0800
From: Dave Hansen <dave.hansen@...ux.intel.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org,tglx@...utronix.de,bp@...en8.de,joro@...tes.org,luto@...nel.org,peterz@...radead.org,kirill.shutemov@...ux.intel.com,rick.p.edgecombe@...el.com,jgross@...e.com,Dave Hansen <dave.hansen@...ux.intel.com>
Subject: [RFC][PATCH 1/8] x86/mm: Always allocate a whole page for PAE PGDs


From: Dave Hansen <dave.hansen@...ux.intel.com>

A hardware PAE PGD is only 32 bytes. A PGD is PAGE_SIZE in the other
paging modes. But for reasons*, the kernel _sometimes_ allocates a
whole page even though it only ever uses 32 bytes.

Make PAE less weird. Just allocate a page like the other paging modes.
This was already being done for PTI (and Xen in the past) and nobody
screamed that loudly about it so it can't be that bad.

 * The original reason for PAGE_SIZE allocations for the PAE PGDs was
   Xen's need to detect page table writes. But 32-bit PTI forced it too
   for reasons I'm unclear about.

Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
---

 b/arch/x86/mm/pgtable.c |   63 +++---------------------------------------------
 1 file changed, 4 insertions(+), 59 deletions(-)

diff -puN arch/x86/mm/pgtable.c~no-pae-kmem_cache arch/x86/mm/pgtable.c
--- a/arch/x86/mm/pgtable.c~no-pae-kmem_cache	2025-01-23 09:20:50.202330636 -0800
+++ b/arch/x86/mm/pgtable.c	2025-01-23 09:20:50.206330781 -0800
@@ -357,69 +357,15 @@ static void pgd_prepopulate_user_pmd(str
 {
 }
 #endif
-/*
- * Xen paravirt assumes pgd table should be in one page. 64 bit kernel also
- * assumes that pgd should be in one page.
- *
- * But kernel with PAE paging that is not running as a Xen domain
- * only needs to allocate 32 bytes for pgd instead of one page.
- */
-#ifdef CONFIG_X86_PAE
-
-#include <linux/slab.h>
-
-#define PGD_SIZE	(PTRS_PER_PGD * sizeof(pgd_t))
-#define PGD_ALIGN	32
-
-static struct kmem_cache *pgd_cache;
-
-void __init pgtable_cache_init(void)
-{
-	/*
-	 * When PAE kernel is running as a Xen domain, it does not use
-	 * shared kernel pmd. And this requires a whole page for pgd.
-	 */
-	if (!SHARED_KERNEL_PMD)
-		return;
-
-	/*
-	 * when PAE kernel is not running as a Xen domain, it uses
-	 * shared kernel pmd. Shared kernel pmd does not require a whole
-	 * page for pgd. We are able to just allocate a 32-byte for pgd.
-	 * During boot time, we create a 32-byte slab for pgd table allocation.
-	 */
-	pgd_cache = kmem_cache_create("pgd_cache", PGD_SIZE, PGD_ALIGN,
-				      SLAB_PANIC, NULL);
-}
 
 static inline pgd_t *_pgd_alloc(void)
 {
 	/*
-	 * If no SHARED_KERNEL_PMD, PAE kernel is running as a Xen domain.
-	 * We allocate one page for pgd.
+	 * PTI and Xen need a whole page for the PAE PGD
+	 * even though the hardware only needs 32 bytes.
+	 *
+	 * For simplicity, allocate a page for all users.
 	 */
-	if (!SHARED_KERNEL_PMD)
-		return (pgd_t *)__get_free_pages(GFP_PGTABLE_USER,
-						 PGD_ALLOCATION_ORDER);
-
-	/*
-	 * Now PAE kernel is not running as a Xen domain. We can allocate
-	 * a 32-byte slab for pgd to save memory space.
-	 */
-	return kmem_cache_alloc(pgd_cache, GFP_PGTABLE_USER);
-}
-
-static inline void _pgd_free(pgd_t *pgd)
-{
-	if (!SHARED_KERNEL_PMD)
-		free_pages((unsigned long)pgd, PGD_ALLOCATION_ORDER);
-	else
-		kmem_cache_free(pgd_cache, pgd);
-}
-#else
-
-static inline pgd_t *_pgd_alloc(void)
-{
 	return (pgd_t *)__get_free_pages(GFP_PGTABLE_USER,
 					 PGD_ALLOCATION_ORDER);
 }
@@ -428,7 +374,6 @@ static inline void _pgd_free(pgd_t *pgd)
 {
 	free_pages((unsigned long)pgd, PGD_ALLOCATION_ORDER);
 }
-#endif /* CONFIG_X86_PAE */
 
 pgd_t *pgd_alloc(struct mm_struct *mm)
 {
_