[PATCH] i386: PARAVIRT: Allow paravirt backend to choose kernel PMD sharing

Normally when running in PAE mode, the 4th PMD maps the kernel address space,
which can be shared among all processes (since they all need the same kernel
mappings).

Xen, however, does not allow guests to have the kernel pmd shared between page
tables, so parameterize pgtable.c to allow both modes of operation.

There are several side-effects of this.  One is that vmalloc will update the
kernel address space mappings, and those updates need to be propagated into
all processes if the kernel mappings are not intrinsically shared.  In the
non-PAE case, this is done by maintaining a pgd_list of all processes; this
list is used when all process pagetables must be updated.  pgd_list is
threaded via otherwise unused entries in the page structure for the pgd, which
means that the pgd must be page-sized for this to work.

Normally the PAE pgd is only 4x64 byte entries large, but Xen requires the PAE
pgd to page aligned anyway, so this patch forces the pgd to be page
aligned+sized when the kernel pmd is unshared, to accomodate both these
requirements.

Also, since there may be several distinct kernel pmds (if the user/kernel
split is below 3G), there's no point in allocating them from a slab cache;
they're just allocated with get_free_page and initialized appropriately.  (Of
course the could be cached if there is just a single kernel pmd - which is the
default with a 3G user/kernel split - but it doesn't seem worthwhile to add
yet another case into this code).

[ Many thanks to wli for review comments. ]

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: William Lee Irwin III <wli@holomorphy.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Zachary Amsden <zach@vmware.com>
Cc: Christoph Lameter <clameter@sgi.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 70 insertions, 18 deletions

diff --git a/arch/i386/mm/pgtable.c b/arch/i386/mm/pgtable.c
index 99c09edc3dbb..9a96c1647428 100644
--- a/arch/i386/mm/pgtable.c
+++ b/arch/i386/mm/pgtable.c
@@ -232,42 +232,92 @@ static inline void pgd_list_del(pgd_t *pgd)
                 set_page_private(next, (unsigned long)pprev);
 }
 
+#if (PTRS_PER_PMD == 1)
+/* Non-PAE pgd constructor */
 void pgd_ctor(void *pgd, struct kmem_cache *cache, unsigned long unused)
 {
         unsigned long flags;
 
-        if (PTRS_PER_PMD == 1) {
-                memset(pgd, 0, USER_PTRS_PER_PGD*sizeof(pgd_t));
-                spin_lock_irqsave(&pgd_lock, flags);
-        }
+        /* !PAE, no pagetable sharing */
+        memset(pgd, 0, USER_PTRS_PER_PGD*sizeof(pgd_t));
+
+        spin_lock_irqsave(&pgd_lock, flags);
 
+        /* must happen under lock */
         clone_pgd_range((pgd_t *)pgd + USER_PTRS_PER_PGD,
                         swapper_pg_dir + USER_PTRS_PER_PGD,
                         KERNEL_PGD_PTRS);
-
-        if (PTRS_PER_PMD > 1)
-                return;
-
-        /* must happen under lock */
         paravirt_alloc_pd_clone(__pa(pgd) >> PAGE_SHIFT,
-                        __pa(swapper_pg_dir) >> PAGE_SHIFT,
-                        USER_PTRS_PER_PGD, PTRS_PER_PGD - USER_PTRS_PER_PGD);
-
+                                __pa(swapper_pg_dir) >> PAGE_SHIFT,
+                                USER_PTRS_PER_PGD,
+                                KERNEL_PGD_PTRS);
         pgd_list_add(pgd);
         spin_unlock_irqrestore(&pgd_lock, flags);
 }
+#else  /* PTRS_PER_PMD > 1 */
+/* PAE pgd constructor */
+void pgd_ctor(void *pgd, struct kmem_cache *cache, unsigned long unused)
+{
+        /* PAE, kernel PMD may be shared */
+
+        if (SHARED_KERNEL_PMD) {
+                clone_pgd_range((pgd_t *)pgd + USER_PTRS_PER_PGD,
+                                swapper_pg_dir + USER_PTRS_PER_PGD,
+                                KERNEL_PGD_PTRS);
+        } else {
+                unsigned long flags;
+
+                memset(pgd, 0, USER_PTRS_PER_PGD*sizeof(pgd_t));
+                spin_lock_irqsave(&pgd_lock, flags);
+                pgd_list_add(pgd);
+                spin_unlock_irqrestore(&pgd_lock, flags);
+        }
+}
+#endif  /* PTRS_PER_PMD */
 
-/* never called when PTRS_PER_PMD > 1 */
 void pgd_dtor(void *pgd, struct kmem_cache *cache, unsigned long unused)
 {
         unsigned long flags; /* can be called from interrupt context */
 
+        BUG_ON(SHARED_KERNEL_PMD);
+
         paravirt_release_pd(__pa(pgd) >> PAGE_SHIFT);
         spin_lock_irqsave(&pgd_lock, flags);
         pgd_list_del(pgd);
         spin_unlock_irqrestore(&pgd_lock, flags);
 }
 
+#define UNSHARED_PTRS_PER_PGD                           \
+        (SHARED_KERNEL_PMD ? USER_PTRS_PER_PGD : PTRS_PER_PGD)
+
+/* If we allocate a pmd for part of the kernel address space, then
+   make sure its initialized with the appropriate kernel mappings.
+   Otherwise use a cached zeroed pmd.  */
+static pmd_t *pmd_cache_alloc(int idx)
+{
+        pmd_t *pmd;
+
+        if (idx >= USER_PTRS_PER_PGD) {
+                pmd = (pmd_t *)__get_free_page(GFP_KERNEL);
+
+                if (pmd)
+                        memcpy(pmd,
+                               (void *)pgd_page_vaddr(swapper_pg_dir[idx]),
+                               sizeof(pmd_t) * PTRS_PER_PMD);
+        } else
+                pmd = kmem_cache_alloc(pmd_cache, GFP_KERNEL);
+
+        return pmd;
+}
+
+static void pmd_cache_free(pmd_t *pmd, int idx)
+{
+        if (idx >= USER_PTRS_PER_PGD)
+                free_page((unsigned long)pmd);
+        else
+                kmem_cache_free(pmd_cache, pmd);
+}
+
 pgd_t *pgd_alloc(struct mm_struct *mm)
 {
         int i;
@@ -276,10 +326,12 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
         if (PTRS_PER_PMD == 1 || !pgd)
                 return pgd;
 
-        for (i = 0; i < USER_PTRS_PER_PGD; ++i) {
-                pmd_t *pmd = kmem_cache_alloc(pmd_cache, GFP_KERNEL);
+        for (i = 0; i < UNSHARED_PTRS_PER_PGD; ++i) {
+                pmd_t *pmd = pmd_cache_alloc(i);
+
                 if (!pmd)
                         goto out_oom;
+
                 paravirt_alloc_pd(__pa(pmd) >> PAGE_SHIFT);
                 set_pgd(&pgd[i], __pgd(1 + __pa(pmd)));
         }
@@ -290,7 +342,7 @@ out_oom:
                 pgd_t pgdent = pgd[i];
                 void* pmd = (void *)__va(pgd_val(pgdent)-1);
                 paravirt_release_pd(__pa(pmd) >> PAGE_SHIFT);
-                kmem_cache_free(pmd_cache, pmd);
+                pmd_cache_free(pmd, i);
         }
         kmem_cache_free(pgd_cache, pgd);
         return NULL;
@@ -302,11 +354,11 @@ void pgd_free(pgd_t *pgd)
 
         /* in the PAE case user pgd entries are overwritten before usage */
         if (PTRS_PER_PMD > 1)
-                for (i = 0; i < USER_PTRS_PER_PGD; ++i) {
+                for (i = 0; i < UNSHARED_PTRS_PER_PGD; ++i) {
                         pgd_t pgdent = pgd[i];
                         void* pmd = (void *)__va(pgd_val(pgdent)-1);
                         paravirt_release_pd(__pa(pmd) >> PAGE_SHIFT);
-                        kmem_cache_free(pmd_cache, pmd);
+                        pmd_cache_free(pmd, i);
                 }
         /* in the non-PAE case, free_pgtables() clears user pgd entries */
         kmem_cache_free(pgd_cache, pgd);