[PATCH v1] shmem: change shrinklist_lock form spinlock to mutex and move iput into it

[Gang Li <ligang.bdlg@xxxxxxxxxxxxx>]

This patch fixes commit 779750d20b93 ("shmem: split huge pages
beyond i_size under memory pressure").

iput out of sbinfo->shrinklist_lock will let shmem_evict_inode grab
and delete the inode, which will berak the consistency between
shrinklist_len and shrinklist. The simultaneous deletion of adjacent
elements in the local list "list" by shmem_unused_huge_shrink and
shmem_evict_inode will also break the list.

iput must in lock or after lock, but shrinklist_lock is a spinlock
which can not sleep and iput may sleep.[1]

Fix it by changing shrinklist_lock from spinlock to mutex and moving iput
into this lock.

[1]. Link: http://lkml.kernel.org/r/20170131093141.GA15899@xxxxxxxxxxxxxxxxxx
Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure")
Signed-off-by: Gang Li <ligang.bdlg@xxxxxxxxxxxxx>
---
 include/linux/shmem_fs.h |  2 +-
 mm/shmem.c               | 16 +++++++---------
 2 files changed, 8 insertions(+), 10 deletions(-)

diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 166158b6e917..65804fd264d0 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -41,7 +41,7 @@ struct shmem_sb_info {
 	ino_t next_ino;		    /* The next per-sb inode number to use */
 	ino_t __percpu *ino_batch;  /* The next per-cpu inode number to use */
 	struct mempolicy *mpol;     /* default memory policy for mappings */
-	spinlock_t shrinklist_lock;   /* Protects shrinklist */
+	struct mutex shrinklist_mutex;/* Protects shrinklist */
 	struct list_head shrinklist;  /* List of shinkable inodes */
 	unsigned long shrinklist_len; /* Length of shrinklist */
 };
diff --git a/mm/shmem.c b/mm/shmem.c
index 18f93c2d68f1..2165a28631c5 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -559,7 +559,7 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
 	if (list_empty(&sbinfo->shrinklist))
 		return SHRINK_STOP;
 
-	spin_lock(&sbinfo->shrinklist_lock);
+	mutex_lock(&sbinfo->shrinklist_mutex);
 	list_for_each_safe(pos, next, &sbinfo->shrinklist) {
 		info = list_entry(pos, struct shmem_inode_info, shrinklist);
 
@@ -586,7 +586,6 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
 		if (!--batch)
 			break;
 	}
-	spin_unlock(&sbinfo->shrinklist_lock);
 
 	list_for_each_safe(pos, next, &to_remove) {
 		info = list_entry(pos, struct shmem_inode_info, shrinklist);
@@ -643,10 +642,9 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
 		iput(inode);
 	}
 
-	spin_lock(&sbinfo->shrinklist_lock);
 	list_splice_tail(&list, &sbinfo->shrinklist);
 	sbinfo->shrinklist_len -= removed;
-	spin_unlock(&sbinfo->shrinklist_lock);
+	mutex_unlock(&sbinfo->shrinklist_mutex);
 
 	return split;
 }
@@ -1137,12 +1135,12 @@ static void shmem_evict_inode(struct inode *inode)
 		inode->i_size = 0;
 		shmem_truncate_range(inode, 0, (loff_t)-1);
 		if (!list_empty(&info->shrinklist)) {
-			spin_lock(&sbinfo->shrinklist_lock);
+		    mutex_lock(&sbinfo->shrinklist_mutex);
 			if (!list_empty(&info->shrinklist)) {
 				list_del_init(&info->shrinklist);
 				sbinfo->shrinklist_len--;
 			}
-			spin_unlock(&sbinfo->shrinklist_lock);
+		    mutex_unlock(&sbinfo->shrinklist_mutex);
 		}
 		while (!list_empty(&info->swaplist)) {
 			/* Wait while shmem_unuse() is scanning this inode... */
@@ -1954,7 +1952,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 		 * Part of the huge page is beyond i_size: subject
 		 * to shrink under memory pressure.
 		 */
-		spin_lock(&sbinfo->shrinklist_lock);
+		mutex_lock(&sbinfo->shrinklist_mutex);
 		/*
 		 * _careful to defend against unlocked access to
 		 * ->shrink_list in shmem_unused_huge_shrink()
@@ -1964,7 +1962,7 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
 				      &sbinfo->shrinklist);
 			sbinfo->shrinklist_len++;
 		}
-		spin_unlock(&sbinfo->shrinklist_lock);
+		mutex_unlock(&sbinfo->shrinklist_mutex);
 	}
 
 	/*
@@ -3652,7 +3650,7 @@ static int shmem_fill_super(struct super_block *sb, struct fs_context *fc)
 	raw_spin_lock_init(&sbinfo->stat_lock);
 	if (percpu_counter_init(&sbinfo->used_blocks, 0, GFP_KERNEL))
 		goto failed;
-	spin_lock_init(&sbinfo->shrinklist_lock);
+	mutex_init(&sbinfo->shrinklist_mutex);
 	INIT_LIST_HEAD(&sbinfo->shrinklist);
 
 	sb->s_maxbytes = MAX_LFS_FILESIZE;
-- 
2.20.1