[PATCH 1/1] x86/vmemmap: Use direct-mapped VA instead of vmemmap-based VA

[Gwan-gyeong Mun]

When a process leads the addition of a struct page to vmemmap
(e.g. hot-plug), the page table update for the newly added vmemmap-based
virtual address is updated first in init_mm's page table and then
synchronized later.
If the vmemmap-based virtual address is accessed through the process's
page table before this sync, a page fault will occur.

This translates vmemmap-based virtual address to direct-mapped virtual
address and use it, if the current top-level page table is not init_mm's
page table when accessing a vmemmap-based virtual address before this sync.

Fixes: faf1c0008a33 ("x86/vmemmap: optimize for consecutive sections in partial populated PMDs")
Signed-off-by: Gwan-gyeong Mun <gwan-gyeong.mun@xxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
Cc: Byungchul Park <byungchul@xxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
 arch/x86/mm/init_64.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 01ea7c6df303..1cb26f692831 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -844,6 +844,17 @@ void __init paging_init(void)
  */
 static unsigned long unused_pmd_start __meminitdata;
 
+static void * __meminit vmemmap_va_to_kaddr(unsigned long vmemmap_va)
+{
+	void *kaddr = (void *)vmemmap_va;
+	pgd_t *pgd = __va(read_cr3_pa());
+
+	if (init_mm.pgd != pgd)
+		kaddr = __va(slow_virt_to_phys(kaddr));
+
+	return kaddr;
+}
+
 static void __meminit vmemmap_flush_unused_pmd(void)
 {
 	if (!unused_pmd_start)
@@ -851,7 +862,7 @@ static void __meminit vmemmap_flush_unused_pmd(void)
 	/*
 	 * Clears (unused_pmd_start, PMD_END]
 	 */
-	memset((void *)unused_pmd_start, PAGE_UNUSED,
+	memset(vmemmap_va_to_kaddr(unused_pmd_start), PAGE_UNUSED,
 	       ALIGN(unused_pmd_start, PMD_SIZE) - unused_pmd_start);
 	unused_pmd_start = 0;
 }
@@ -882,7 +893,7 @@ static void __meminit __vmemmap_use_sub_pmd(unsigned long start)
 	 * case the first memmap never gets initialized e.g., because the memory
 	 * block never gets onlined).
 	 */
-	memset((void *)start, 0, sizeof(struct page));
+	memset(vmemmap_va_to_kaddr(start), 0, sizeof(struct page));
 }
 
 static void __meminit vmemmap_use_sub_pmd(unsigned long start, unsigned long end)
@@ -924,7 +935,7 @@ static void __meminit vmemmap_use_new_sub_pmd(unsigned long start, unsigned long
 	 * Mark with PAGE_UNUSED the unused parts of the new memmap range
 	 */
 	if (!IS_ALIGNED(start, PMD_SIZE))
-		memset((void *)page, PAGE_UNUSED, start - page);
+		memset(vmemmap_va_to_kaddr(page), PAGE_UNUSED, start - page);
 
 	/*
 	 * We want to avoid memset(PAGE_UNUSED) when populating the vmemmap of
-- 
2.48.1