[4.19.y PATCH] tmpfs: fix unable to remount nr_inodes from limited to unlimited

[yu kuai <yukuai3@huawei.com>]

tmpfs support 'size', 'nr_blocks' and 'nr_inodes' mount options. mount or
remount them to zero means unlimited. 'size' and 'br_blocks' can remount
from limited to unlimited, while 'nr_inodes' can't.

The problem is fixed since upstream commit 0b5071dd323d ("
shmem_parse_options(): use a separate structure to keep the results"). But
in order to backport it, the amount of related patches need to backport is
huge. 

So, I made some local changes to fix the problem.

Signed-off-by: yu kuai <yukuai3@huawei.com>
---
 mm/shmem.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 3c8742655756..966fc69ee8fb 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3444,7 +3444,7 @@ static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
 	inodes = sbinfo->max_inodes - sbinfo->free_inodes;
 	if (percpu_counter_compare(&sbinfo->used_blocks, config.max_blocks) > 0)
 		goto out;
-	if (config.max_inodes < inodes)
+	if (config.max_inodes && config.max_inodes < inodes)
 		goto out;
 	/*
 	 * Those tests disallow limited->unlimited while any are in use;
@@ -3460,7 +3460,10 @@ static int shmem_remount_fs(struct super_block *sb, int *flags, char *data)
 	sbinfo->huge = config.huge;
 	sbinfo->max_blocks  = config.max_blocks;
 	sbinfo->max_inodes  = config.max_inodes;
-	sbinfo->free_inodes = config.max_inodes - inodes;
+	if (!config.max_inodes)
+		sbinfo->free_inodes = 0;
+	else
+		sbinfo->free_inodes = config.max_inodes - inodes;
 
 	/*
 	 * Preserve previous mempolicy unless mpol remount option was specified.
-- 
2.17.2