x86/mm: Reduce PAE-mode per task pgd allocation overhead from 4K to 32 bytes

With more embedded systems emerging using Quark, among other
things, 32-bit kernel matters again. 32-bit machine and kernel
uses PAE paging, which currently wastes at least 4K of memory
per process on Linux where we have to reserve an entire page to
support a single 32-byte PGD structure. It would be a very good
thing if we could eliminate that wastage.

PAE paging is used to access more than 4GB memory on x86-32. And
it is required for NX.

In this patch, we still allocate one page for pgd for a Xen
domain and 64-bit kernel because one page pgd is assumed in
these cases. But we can save memory space by only allocating
32-byte pgd for 32-bit PAE kernel when it is not running as a
Xen domain.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Glenn Williamson <glenn.p.williamson@intel.com>
Cc: H. Peter Anvin <hpa@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1421382601-46912-1-git-send-email-fenghua.yu@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 6fb6927..d223e1f 100644 (file)
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -271,12 +271,87 @@ static void pgd_prepopulate_pmd(struct mm_struct *mm, pgd_t *pgd, pmd_t *pmds[])
        }
 }
 
+/*
+ * 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;
+
+static int __init pgd_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 0;
+
+       /*
+        * 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);
+       if (!pgd_cache)
+               return -ENOMEM;
+
+       return 0;
+}
+core_initcall(pgd_cache_init);
+
+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.
+        */
+       if (!SHARED_KERNEL_PMD)
+               return (pgd_t *)__get_free_page(PGALLOC_GFP);
+
+       /*
+        * 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, PGALLOC_GFP);
+}
+
+static inline void _pgd_free(pgd_t *pgd)
+{
+       if (!SHARED_KERNEL_PMD)
+               free_page((unsigned long)pgd);
+       else
+               kmem_cache_free(pgd_cache, pgd);
+}
+#else
+static inline pgd_t *_pgd_alloc(void)
+{
+       return (pgd_t *)__get_free_page(PGALLOC_GFP);
+}
+
+static inline void _pgd_free(pgd_t *pgd)
+{
+       free_page((unsigned long)pgd);
+}
+#endif /* CONFIG_X86_PAE */
+
 pgd_t *pgd_alloc(struct mm_struct *mm)
 {
        pgd_t *pgd;
        pmd_t *pmds[PREALLOCATED_PMDS];
 
-       pgd = (pgd_t *)__get_free_page(PGALLOC_GFP);
+       pgd = _pgd_alloc();
 
        if (pgd == NULL)
                goto out;
@@ -306,7 +381,7 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
 out_free_pmds:
        free_pmds(pmds);
 out_free_pgd:
-       free_page((unsigned long)pgd);
+       _pgd_free(pgd);
 out:
        return NULL;
 }
@@ -316,7 +391,7 @@ void pgd_free(struct mm_struct *mm, pgd_t *pgd)
        pgd_mop_up_pmds(mm, pgd);
        pgd_dtor(pgd);
        paravirt_pgd_free(mm, pgd);
-       free_page((unsigned long)pgd);
+       _pgd_free(pgd);
 }
 
 /*