x86/mm: Pre-allocate P4D/PUD pages for vmalloc area

Pre-allocate the page-table pages for the vmalloc area at the level
which needs synchronization on x86-64, which is P4D for 5-level and
PUD for 4-level paging.

Doing this at boot makes sure no synchronization of that area is
necessary at runtime. The synchronization takes the pgd_lock and
iterates over all page-tables in the system, so it can take quite long
and is better avoided.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>
Link: https://lore.kernel.org/r/20200721095953.6218-2-joro@8bytes.org

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index dbae185511cdf3f642518850e80c5b075728f13b..e76bdb00146002f559a2e8bf4d1fb4e52c793831 100644 (file)
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1238,6 +1238,56 @@ static void __init register_page_bootmem_info(void)
 #endif
 }
 
+/*
+ * Pre-allocates page-table pages for the vmalloc area in the kernel page-table.
+ * Only the level which needs to be synchronized between all page-tables is
+ * allocated because the synchronization can be expensive.
+ */
+static void __init preallocate_vmalloc_pages(void)
+{
+       unsigned long addr;
+       const char *lvl;
+
+       for (addr = VMALLOC_START; addr <= VMALLOC_END; addr = ALIGN(addr + 1, PGDIR_SIZE)) {
+               pgd_t *pgd = pgd_offset_k(addr);
+               p4d_t *p4d;
+               pud_t *pud;
+
+               p4d = p4d_offset(pgd, addr);
+               if (p4d_none(*p4d)) {
+                       /* Can only happen with 5-level paging */
+                       p4d = p4d_alloc(&init_mm, pgd, addr);
+                       if (!p4d) {
+                               lvl = "p4d";
+                               goto failed;
+                       }
+               }
+
+               if (pgtable_l5_enabled())
+                       continue;
+
+               pud = pud_offset(p4d, addr);
+               if (pud_none(*pud)) {
+                       /* Ends up here only with 4-level paging */
+                       pud = pud_alloc(&init_mm, p4d, addr);
+                       if (!pud) {
+                               lvl = "pud";
+                               goto failed;
+                       }
+               }
+       }
+
+       return;
+
+failed:
+
+       /*
+        * The pages have to be there now or they will be missing in
+        * process page-tables later.
+        */
+       panic("Failed to pre-allocate %s pages for vmalloc area\n", lvl);
+}
+
 void __init mem_init(void)
 {
        pci_iommu_alloc();
@@ -1261,6 +1311,8 @@ void __init mem_init(void)
        if (get_gate_vma(&init_mm))
                kclist_add(&kcore_vsyscall, (void *)VSYSCALL_ADDR, PAGE_SIZE, KCORE_USER);
 
+       preallocate_vmalloc_pages();
+
        mem_init_print_info(NULL);
 }