[PATCH 7/9] mm: add a flag to indicate we used a cached page

[Josef Bacik <josef@toxicpanda.com>]

This is preparation for dropping the mmap_sem in page_mkwrite.  We need
to know if we used our cached page so we can be sure it is the page we
already did the page_mkwrite stuff on so we don't have to redo all of
that work.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 include/linux/mm.h | 6 +++++-
 mm/filemap.c       | 5 ++++-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 4a84ec976dfc..a7305d193c71 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -318,6 +318,9 @@ extern pgprot_t protection_map[16];
 #define FAULT_FLAG_USER		0x40	/* The fault originated in userspace */
 #define FAULT_FLAG_REMOTE	0x80	/* faulting for non current tsk/mm */
 #define FAULT_FLAG_INSTRUCTION  0x100	/* The fault was during an instruction fetch */
+#define FAULT_FLAG_USED_CACHED	0x200	/* Our vmf->page was from a previous
+					 * loop through the fault handler.
+					 */
 
 #define FAULT_FLAG_TRACE \
 	{ FAULT_FLAG_WRITE,		"WRITE" }, \
@@ -328,7 +331,8 @@ extern pgprot_t protection_map[16];
 	{ FAULT_FLAG_TRIED,		"TRIED" }, \
 	{ FAULT_FLAG_USER,		"USER" }, \
 	{ FAULT_FLAG_REMOTE,		"REMOTE" }, \
-	{ FAULT_FLAG_INSTRUCTION,	"INSTRUCTION" }
+	{ FAULT_FLAG_INSTRUCTION,	"INSTRUCTION" }, \
+	{ FAULT_FLAG_USED_CACHED,	"USED_CACHED" }
 
 /*
  * vm_fault is filled by the the pagefault handler and passed to the vma's
diff --git a/mm/filemap.c b/mm/filemap.c
index 5212ab637832..e9cb44bd35aa 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2556,6 +2556,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 		if (cached_page->mapping == mapping &&
 		    cached_page->index == offset) {
 			page = cached_page;
+			vmf->flags |= FAULT_FLAG_USED_CACHED;
 			goto have_cached_page;
 		}
 		unlock_page(cached_page);
@@ -2619,8 +2620,10 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
 	 * We have a locked page in the page cache, now we need to check
 	 * that it's up-to-date. If not, it is going to be due to an error.
 	 */
-	if (unlikely(!PageUptodate(page)))
+	if (unlikely(!PageUptodate(page))) {
+		vmf->flags &= ~(FAULT_FLAG_USED_CACHED);
 		goto page_not_uptodate;
+	}
 
 	/*
 	 * Found the page and have a reference on it.
-- 
2.14.3