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

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@node.shutemov.name
Fixes: 779750d20b93 ("shmem: split huge pages beyond i_size under memory pressure")
Signed-off-by: Gang Li <redacted>
---
 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