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Message-Id: <20221011022215.322269-1-xu.xin16@zte.com.cn>
Date: Tue, 11 Oct 2022 02:22:15 +0000
From: xu.xin.sc@...il.com
To: akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
xu xin <xu.xin16@....com.cn>,
David Hildenbrand <david@...hat.com>,
Claudio Imbrenda <imbrenda@...ux.ibm.com>,
Xuexin Jiang <jiang.xuexin@....com.cn>,
Xiaokai Ran <ran.xiaokai@....com.cn>,
Yang Yang <yang.yang29@....com.cn>
Subject: [PATCH v3 2/5] ksm: support unsharing zero pages placed by KSM
From: xu xin <xu.xin16@....com.cn>
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@...hat.com>
Cc: Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc: Xuexin Jiang <jiang.xuexin@....com.cn>
Signed-off-by: xu xin <xu.xin16@....com.cn>
Co-developed-by: Xiaokai Ran <ran.xiaokai@....com.cn>
Signed-off-by: Xiaokai Ran <ran.xiaokai@....com.cn>
Co-developed-by: Yang Yang <yang.yang29@....com.cn>
Signed-off-by: Yang Yang <yang.yang29@....com.cn>
Signed-off-by: xu xin <xu.xin16@....com.cn>
---
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);
+ }
else
ret = VM_FAULT_WRITE;
put_page(page);
@@ -496,6 +501,11 @@ static struct vm_area_struct *find_mergeable_vma(struct mm_struct *mm,
return vma;
}
+/*
+ * Note: Don't call break_cow() in the context protected by
+ * mmap_read_lock(), which may cause dead lock because inside
+ * break_cow mmap_read_lock exists.
+ */
static void break_cow(struct ksm_rmap_item *rmap_item)
{
struct mm_struct *mm = rmap_item->mm;
@@ -511,10 +521,35 @@ static void break_cow(struct ksm_rmap_item *rmap_item)
mmap_read_lock(mm);
vma = find_mergeable_vma(mm, addr);
if (vma)
- break_ksm(vma, addr);
+ break_ksm(vma, addr, false);
mmap_read_unlock(mm);
}
+/* Only called when rmap_item->address is with ZERO_PAGE_FLAG */
+static inline int unshare_zero_pages(struct ksm_rmap_item *rmap_item)
+{
+ struct mm_struct *mm = rmap_item->mm;
+ struct vm_area_struct *vma;
+ unsigned long addr = rmap_item->address;
+ int err = -EFAULT;
+
+ vma = vma_lookup(mm, addr);
+ if (vma)
+ err = break_ksm(vma, addr, true);
+
+ return err;
+}
+
+static inline void free_rmap_item(struct ksm_rmap_item *rmap_item)
+{
+ if (rmap_item->address & ZERO_PAGE_FLAG)
+ unshare_zero_pages(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 struct page *get_mergeable_page(struct ksm_rmap_item *rmap_item)
{
struct mm_struct *mm = rmap_item->mm;
@@ -825,7 +860,7 @@ static int unmerge_ksm_pages(struct vm_area_struct *vma,
if (signal_pending(current))
err = -ERESTARTSYS;
else
- err = break_ksm(vma, addr);
+ err = break_ksm(vma, addr, false);
}
return err;
}
@@ -2017,6 +2052,36 @@ static void stable_tree_append(struct ksm_rmap_item *rmap_item,
rmap_item->mm->ksm_merging_pages++;
}
+static int try_to_merge_with_kernel_zero_page(struct mm_struct *mm,
+ struct ksm_rmap_item *rmap_item,
+ struct page *page)
+{
+ int err = 0;
+
+ if (!(rmap_item->address & ZERO_PAGE_FLAG)) {
+ struct vm_area_struct *vma;
+
+ mmap_read_lock(mm);
+ vma = find_mergeable_vma(mm, rmap_item->address);
+ if (vma) {
+ err = try_to_merge_one_page(vma, page,
+ ZERO_PAGE(rmap_item->address));
+ } else {
+ /* If the vma is out of date, we do not need to continue. */
+ err = 0;
+ }
+ mmap_read_unlock(mm);
+ /*
+ * In case of failure, the page was not really empty, so we
+ * need to continue. Otherwise we're done.
+ */
+ if (!err)
+ rmap_item->address |= ZERO_PAGE_FLAG;
+ }
+
+ return err;
+}
+
/*
* cmp_and_merge_page - first see if page can be merged into the stable tree;
* if not, compare checksum to previous and if it's the same, see if page can
@@ -2101,29 +2166,21 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
* Same checksum as an empty page. We attempt to merge it with the
* appropriate zero page if the user enabled this via sysfs.
*/
- if (ksm_use_zero_pages && (checksum == zero_checksum)) {
- struct vm_area_struct *vma;
-
- mmap_read_lock(mm);
- vma = find_mergeable_vma(mm, rmap_item->address);
- if (vma) {
- err = try_to_merge_one_page(vma, page,
- ZERO_PAGE(rmap_item->address));
- } else {
+ if (ksm_use_zero_pages) {
+ if (checksum == zero_checksum) {
+ /* If success, just return. Otherwise, continue */
+ if (!try_to_merge_with_kernel_zero_page(mm, rmap_item, page))
+ return;
+ } else if (rmap_item->address & ZERO_PAGE_FLAG) {
/*
- * If the vma is out of date, we do not need to
- * continue.
+ * The page now is not kernel zero page (COW happens to it)
+ * but the flag of its rmap_item is still zero-page, so need
+ * to reset the flag and update the corresponding count.
*/
- err = 0;
+ rmap_item->address &= PAGE_MASK;
}
- mmap_read_unlock(mm);
- /*
- * In case of failure, the page was not really empty, so we
- * need to continue. Otherwise we're done.
- */
- if (!err)
- return;
}
+
tree_rmap_item =
unstable_tree_search_insert(rmap_item, page, &tree_page);
if (tree_rmap_item) {
@@ -2337,6 +2394,23 @@ static struct ksm_rmap_item *scan_get_next_rmap_item(struct page **page)
mmap_read_unlock(mm);
return rmap_item;
}
+ /*
+ * Because we want to monitor ksm zero pages which is
+ * non-anonymous, we must try to return the rmap_items
+ * of those kernel zero pages which replaces its
+ * original anonymous empty page due to use_zero_pages's
+ * feature.
+ */
+ if (is_zero_pfn(page_to_pfn(*page))) {
+ rmap_item = try_to_get_old_rmap_item(ksm_scan.address,
+ ksm_scan.rmap_list);
+ if (rmap_item && (rmap_item->address & ZERO_PAGE_FLAG)) {
+ ksm_scan.rmap_list = &rmap_item->rmap_list;
+ ksm_scan.address += PAGE_SIZE;
+ mmap_read_unlock(mm);
+ return rmap_item;
+ }
+ }
next_page:
put_page(*page);
ksm_scan.address += PAGE_SIZE;
--
2.25.1
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