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Subject[PATCHv2 11/41] thp: try to free page's buffers before attempt split
Date
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 73e80d49c32e..a8fcfa3010c8 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>
@@ -2007,7 +2008,6 @@ 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)) {
@@ -2036,6 +2036,23 @@ 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;
--
2.8.1
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