[PATCH 3/3] mm: Pass FGP_NOWAIT in generic_file_buffered_read and enable ext4

[Kirill Tkhai]

All page-obtaining functions, which are used by ext4, look
to go thru pagecache_get_page() path, so all taken uncharged
pages will be properly charged. Thus, we enable AS_KEEP_MEMCG_RECLAIM
for ext4 regular files.

Since memcg accounting requires page lock, and function
generic_file_buffered_read() is the only of ext4-used
functions, which does not care about FGP_NOWAIT, we make
it use find_get_page_flags() and pass the flag. This allows
pagecache_get_page() to use lock_page(), when it's possible.

Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
---
 fs/ext4/inode.c |    1 +
 mm/filemap.c    |    8 +++++++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index b1d7ddd70eee..2fc9e4a7c0db 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5065,6 +5065,7 @@ struct inode *__ext4_iget(struct super_block *sb, unsigned long ino,
 		inode->i_op = &ext4_file_inode_operations;
 		inode->i_fop = &ext4_file_operations;
 		ext4_set_aops(inode);
+		set_bit(AS_KEEP_MEMCG_RECLAIM, &inode->i_mapping->flags);
 	} else if (S_ISDIR(inode->i_mode)) {
 		inode->i_op = &ext4_dir_inode_operations;
 		inode->i_fop = &ext4_dir_operations;
diff --git a/mm/filemap.c b/mm/filemap.c
index 2603c44fc74a..46922003811f 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2061,6 +2061,11 @@ static void shrink_readahead_size_eio(struct file *filp,
 	ra->ra_pages /= 4;
 }
 
+static int kiocb_fgp_flags(struct kiocb *iocb)
+{
+	return (iocb->ki_flags & IOCB_NOWAIT) ? FGP_NOWAIT : 0;
+}
+
 /**
  * generic_file_buffered_read - generic file read routine
  * @iocb:	the iocb to read
@@ -2111,7 +2116,8 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 			goto out;
 		}
 
-		page = find_get_page(mapping, index);
+		page = find_get_page_flags(mapping, index,
+					   kiocb_fgp_flags(iocb));
 		if (!page) {
 			if (iocb->ki_flags & IOCB_NOWAIT)
 				goto would_block;