[PATCH v7 08/14] mm/gup: do not migrate zero page

[Pavel Tatashin]

On some platforms ZERO_PAGE(0) might end-up in a movable zone. Do not
migrate zero page in gup during longterm pinning as migration of zero page
is not allowed.

For example, in x86 QEMU with 16G of memory and kernelcore=5G parameter, I
see the following:

Boot#1: zero_pfn  0x48a8d zero_pfn zone: ZONE_DMA32
Boot#2: zero_pfn 0x20168d zero_pfn zone: ZONE_MOVABLE

On x86, empty_zero_page is declared in .bss and depending on the loader
may end up in different physical locations during boots.

Also, move is_zero_pfn() my_zero_pfn() functions under CONFIG_MMU, because
zero_pfn that they are using is declared in memory.c which is compiled
with CONFIG_MMU.

Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 include/linux/mmzone.h  | 4 ++++
 include/linux/pgtable.h | 3 +--
 mm/gup.c                | 2 ++
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index ae588b2f87ef..72b0b6eba854 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -427,6 +427,10 @@ enum zone_type {
 	 *    techniques might use alloc_contig_range() to hide previously
 	 *    exposed pages from the buddy again (e.g., to implement some sort
 	 *    of memory unplug in virtio-mem).
+	 * 6. ZERO_PAGE(0), kernelcore/movablecore setups might create
+	 *    situations where ZERO_PAGE(0) which is allocated differently
+	 *    on different platforms may end up in a movable zone. ZERO_PAGE(0)
+	 *    cannot be migrated.
 	 *
 	 * In general, no unmovable allocations that degrade memory offlining
 	 * should end up in ZONE_MOVABLE. Allocators (like alloc_contig_range())
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index ea5c4102c23e..54a740602618 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1118,6 +1118,7 @@ extern void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
 extern void untrack_pfn_moved(struct vm_area_struct *vma);
 #endif
 
+#ifdef CONFIG_MMU
 #ifdef __HAVE_COLOR_ZERO_PAGE
 static inline int is_zero_pfn(unsigned long pfn)
 {
@@ -1142,8 +1143,6 @@ static inline unsigned long my_zero_pfn(unsigned long addr)
 }
 #endif
 
-#ifdef CONFIG_MMU
-
 #ifndef CONFIG_TRANSPARENT_HUGEPAGE
 static inline int pmd_trans_huge(pmd_t pmd)
 {
diff --git a/mm/gup.c b/mm/gup.c
index 857b273e32ac..fdd5cda30a07 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1580,6 +1580,8 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm,
 		 * of the CMA zone if possible.
 		 */
 		if (is_migrate_cma_page(head)) {
+			if (is_zero_pfn(page_to_pfn(head)))
+				continue;
 			if (PageHuge(head)) {
 				if (!isolate_huge_page(head, &cma_page_list))
 					isolation_error_count++;
-- 
2.25.1