[PATCH] X86-32: Allocate 256 bytes for pgd in PAE paging

[Fenghua Yu]

From: Fenghua Yu <fenghua.yu@xxxxxxxxx>

X86 32-bit machine and kernel use PAE paging, which currently wastes about
4K of memory per process on Linux where we have to reserve an entire page to
support a single 256-byte PGD structure.  It would be a very good thing if
we could eliminate that wastage.

Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---
 arch/x86/mm/pgtable.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 6fb6927..e75f923 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -1,5 +1,6 @@
 #include <linux/mm.h>
 #include <linux/gfp.h>
+#include <linux/slab.h>
 #include <asm/pgalloc.h>
 #include <asm/pgtable.h>
 #include <asm/tlb.h>
@@ -276,7 +277,27 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
 	pgd_t *pgd;
 	pmd_t *pmds[PREALLOCATED_PMDS];
 
+	/*
+	 * Xen paravirt assumes pgd table should be in one page. pgd in
+	 * 64 bit also needs to be in one page.
+	 *
+	 * But PAE without Xen only needs to allocate 256 bytes for pgd.
+	 *
+	 * So if kernel is compiled as PAE model without Xen, we allocate
+	 * 256 bytes for pgd entries to save memory space.
+	 *
+	 * In other cases, one page is allocated for pgd. In theory, a kernel
+	 * in PAE mode not running in Xen could allocate 256 bytes for pgd
+	 * as well. But that will make the allocation and free more complex
+	 * but not useful in reality. To simplify the code and testing, we just
+	 * allocate one page when CONFIG_XEN is enabled regardelss kernel
+	 * is running in Xen or not.
+	 */
+#if defined(CONFIG_X86_PAE) && !defined(CONFIG_XEN)
+	pgd = kmalloc(sizeof(pgdval_t) * PTRS_PER_PGD, PGALLOC_GFP);
+#else
 	pgd = (pgd_t *)__get_free_page(PGALLOC_GFP);
+#endif
 
 	if (pgd == NULL)
 		goto out;
@@ -306,7 +327,11 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
 out_free_pmds:
 	free_pmds(pmds);
 out_free_pgd:
+#if defined(CONFIG_X86_PAE) && !defined(CONFIG_XEN)
+	kfree(pgd);
+#else
 	free_page((unsigned long)pgd);
+#endif
 out:
 	return NULL;
 }
@@ -316,7 +341,12 @@ void pgd_free(struct mm_struct *mm, pgd_t *pgd)
 	pgd_mop_up_pmds(mm, pgd);
 	pgd_dtor(pgd);
 	paravirt_pgd_free(mm, pgd);
+#if defined(CONFIG_X86_PAE) && !defined(CONFIG_XEN)
+	kfree(pgd);
+#else
 	free_page((unsigned long)pgd);
+#endif
+
 }
 
 /*
-- 
1.8.1.2

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