[PATCH AUTOSEL for 4.9 50/54] mm: avoid returning VM_FAULT_RETRY from ->page_mkwrite handlers

[alexander.levin@verizon.com]

From: Jan Kara <jack@suse.cz>

[ Upstream commit 0911d0041c22922228ca52a977d7b0b0159fee4b ]

Some ->page_mkwrite handlers may return VM_FAULT_RETRY as its return
code (GFS2 or Lustre can definitely do this).  However VM_FAULT_RETRY
from ->page_mkwrite is completely unhandled by the mm code and results
in locking and writeably mapping the page which definitely is not what
the caller wanted.

Fix Lustre and block_page_mkwrite_ret() used by other filesystems
(notably GFS2) to return VM_FAULT_NOPAGE instead which results in
bailing out from the fault code, the CPU then retries the access, and we
fault again effectively doing what the handler wanted.

Link: http://lkml.kernel.org/r/20170203150729.15863-1-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
---
 drivers/staging/lustre/lustre/llite/llite_mmap.c | 4 +---
 include/linux/buffer_head.h                      | 4 +---
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/drivers/staging/lustre/lustre/llite/llite_mmap.c b/drivers/staging/lustre/lustre/llite/llite_mmap.c
index 436691814a5e..27333d973bcd 100644
--- a/drivers/staging/lustre/lustre/llite/llite_mmap.c
+++ b/drivers/staging/lustre/lustre/llite/llite_mmap.c
@@ -401,15 +401,13 @@ static int ll_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 		result = VM_FAULT_LOCKED;
 		break;
 	case -ENODATA:
+	case -EAGAIN:
 	case -EFAULT:
 		result = VM_FAULT_NOPAGE;
 		break;
 	case -ENOMEM:
 		result = VM_FAULT_OOM;
 		break;
-	case -EAGAIN:
-		result = VM_FAULT_RETRY;
-		break;
 	default:
 		result = VM_FAULT_SIGBUS;
 		break;
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 447a915db25d..4431ea2c8802 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -239,12 +239,10 @@ static inline int block_page_mkwrite_return(int err)
 {
 	if (err == 0)
 		return VM_FAULT_LOCKED;
-	if (err == -EFAULT)
+	if (err == -EFAULT || err == -EAGAIN)
 		return VM_FAULT_NOPAGE;
 	if (err == -ENOMEM)
 		return VM_FAULT_OOM;
-	if (err == -EAGAIN)
-		return VM_FAULT_RETRY;
 	/* -ENOSPC, -EDQUOT, -EIO ... */
 	return VM_FAULT_SIGBUS;
 }
-- 
2.11.0