[PATCHv3 11/41] thp: try to free page's buffers before attempt split

Kirill A. Shutemov
2016-09-15T11:54:53Z

We want page to be isolated from the rest of the system before spliting
it. We rely on page count to be 2 for file pages to make sure nobody
uses the page: one pin to caller, one to radix-tree.

Filesystems with backing storage can have page count increased if it has
buffers.

Let's try to free them, before attempt split. And remove one guarding
VM_BUG_ON_PAGE().

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
include/linux/buffer_head.h | 1 +
mm/huge_memory.c | 19 ++++++++++++++++++-
2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index ebbacd14d450..006a8a42acfb 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -395,6 +395,7 @@ extern int __set_page_dirty_buffers(struct page *page);
#else /* CONFIG_BLOCK */

static inline void buffer_init(void) {}
+static inline int page_has_buffers(struct page *page) { return 0; }
static inline int try_to_free_buffers(struct page *page) { return 1; }
static inline int inode_has_buffers(struct inode *inode) { return 0; }
static inline void invalidate_inode_buffers(struct inode *inode) {}
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 020a23d6e7f8..44bf0ba3d10f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -30,6 +30,7 @@
#include <linux/userfaultfd_k.h>
#include <linux/page_idle.h>
#include <linux/shmem_fs.h>
+#include <linux/buffer_head.h>

#include <asm/tlb.h>
#include <asm/pgalloc.h>
@@ -2012,7 +2013,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)

VM_BUG_ON_PAGE(is_huge_zero_page(page), page);
VM_BUG_ON_PAGE(!PageLocked(page), page);
- VM_BUG_ON_PAGE(!PageSwapBacked(page), page);
VM_BUG_ON_PAGE(!PageCompound(page), page);

if (PageAnon(head)) {
@@ -2041,6 +2041,23 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
goto out;
}

+ /* Try to free buffers before attempt split */
+ if (!PageSwapBacked(head) && PagePrivate(page)) {
+ /*
+ * We cannot trigger writeback from here due possible
+ * recursion if triggered from vmscan, only wait.
+ *
+ * Caller can trigger writeback it on its own, if safe.
+ */
+ wait_on_page_writeback(head);
+
+ if (page_has_buffers(head) &&
+ !try_to_free_buffers(head)) {
+ ret = -EBUSY;
+ goto out;
+ }
+ }
+
/* Addidional pin from radix tree */
extra_pins = 1;
anon_vma = NULL;