From: xu xin <xu.xin16@xxxxxxxxxx>
use_zero_pages may be very useful, not just because of cache colouring
as described in doc, but also because use_zero_pages can accelerate
merging empty pages when there are plenty of empty pages (full of zeros)
as the time of page-by-page comparisons (unstable_tree_search_insert) is
saved.
But when enabling use_zero_pages, madvise(addr, len, MADV_UNMERGEABLE) and
other ways (like write 2 to /sys/kernel/mm/ksm/run) to trigger unsharing
will *not* unshare the shared zeropage as placed by KSM (which may be
against the MADV_UNMERGEABLE documentation at least).
To not blindly unshare all shared zero_pages in applicable VMAs, the patch
introduces a dedicated flag ZERO_PAGE_FLAG to mark the rmap_items of those
shared zero_pages. and guarantee that these rmap_items will be not freed
during the time of zero_pages not being writing, so we can only unshare
the *KSM-placed* zero_pages.
The patch will not degrade the performance of use_zero_pages as it doesn't
change the way of merging empty pages in use_zero_pages's feature.
Fixes: e86c59b1b12d ("mm/ksm: improve deduplication of zero pages with colouring")
Reported-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Cc: Xuexin Jiang <jiang.xuexin@xxxxxxxxxx>
Signed-off-by: xu xin <xu.xin16@xxxxxxxxxx>
Co-developed-by: Xiaokai Ran <ran.xiaokai@xxxxxxxxxx>
Signed-off-by: Xiaokai Ran <ran.xiaokai@xxxxxxxxxx>
Co-developed-by: Yang Yang <yang.yang29@xxxxxxxxxx>
Signed-off-by: Yang Yang <yang.yang29@xxxxxxxxxx>
Signed-off-by: xu xin <xu.xin16@xxxxxxxxxx>
---
mm/ksm.c | 136 ++++++++++++++++++++++++++++++++++++++++++-------------
1 file changed, 105 insertions(+), 31 deletions(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index 13c60f1071d8..e351d7b6d15e 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -213,6 +213,7 @@ struct ksm_rmap_item {
#define SEQNR_MASK 0x0ff /* low bits of unstable tree seqnr */
#define UNSTABLE_FLAG 0x100 /* is a node of the unstable tree */
#define STABLE_FLAG 0x200 /* is listed from the stable tree */
+#define ZERO_PAGE_FLAG 0x400 /* is zero page placed by KSM */
/* The stable and unstable tree heads */
static struct rb_root one_stable_tree[1] = { RB_ROOT };
@@ -381,14 +382,6 @@ static inline struct ksm_rmap_item *alloc_rmap_item(void)
return rmap_item;
}
-static inline void free_rmap_item(struct ksm_rmap_item *rmap_item)
-{
- ksm_rmap_items--;
- rmap_item->mm->ksm_rmap_items--;
- rmap_item->mm = NULL; /* debug safety */
- kmem_cache_free(rmap_item_cache, rmap_item);
-}
-
static inline struct ksm_stable_node *alloc_stable_node(void)
{
/*
@@ -420,7 +413,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
}
/*
- * We use break_ksm to break COW on a ksm page: it's a stripped down
+ * We use break_ksm to break COW on a ksm page or KSM-placed zero page (only
+ * happen when enabling use_zero_pages): it's a stripped down
*
* if (get_user_pages(addr, 1, FOLL_WRITE, &page, NULL) == 1)
* put_page(page);
@@ -434,7 +428,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
* of the process that owns 'vma'. We also do not want to enforce
* protection keys here anyway.
*/
-static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
+static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
+ bool ksm_check_bypass)
{
struct page *page;
vm_fault_t ret = 0;
@@ -449,6 +444,16 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
ret = handle_mm_fault(vma, addr,
FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
NULL);
+ else if (ksm_check_bypass && is_zero_pfn(page_to_pfn(page))) {
+ /*
+ * Although it's not ksm page, it's zero page as placed by
+ * KSM use_zero_page, so we should unshare it when
+ * ksm_check_bypass is true.
+ */
+ ret = handle_mm_fault(vma, addr,
+ FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
+ NULL);
+ }
