ext4: fix races between changing inode journal mode and ext4_writepages

In ext4, there is a race condition between changing inode journal mode
and ext4_writepages(). While ext4_writepages() is executed on a
non-journalled mode inode, the inode's journal mode could be enabled
by ioctl() and then, some pages dirtied after switching the journal
mode will be still exposed to ext4_writepages() in non-journaled mode.
To resolve this problem, we use fs-wide per-cpu rw semaphore by Jan
Kara's suggestion because we don't want to waste ext4_inode_info's
space for this extra rare case.

Signed-off-by: Daeho Jeong <daeho.jeong@samsung.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

4 files changed, 21 insertions, 3 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index cb00b1119ec9..ba5aecc07fbc 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -33,6 +33,7 @@
 #include <linux/ratelimit.h>
 #include <crypto/hash.h>
 #include <linux/falloc.h>
+#include <linux/percpu-rwsem.h>
 #ifdef __KERNEL__
 #include <linux/compat.h>
 #endif
@@ -1508,6 +1509,9 @@ struct ext4_sb_info {
         struct ratelimit_state s_err_ratelimit_state;
         struct ratelimit_state s_warning_ratelimit_state;
         struct ratelimit_state s_msg_ratelimit_state;
+
+        /* Barrier between changing inodes' journal flags and writepages ops. */
+        struct percpu_rw_semaphore s_journal_flag_rwsem;
 };
 
 static inline struct ext4_sb_info *EXT4_SB(struct super_block *sb)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 779ef4c11bc1..4d8ebbe00456 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2612,11 +2612,14 @@ static int ext4_writepages(struct address_space *mapping,
         struct blk_plug plug;
         bool give_up_on_write = false;
 
+        percpu_down_read(&sbi->s_journal_flag_rwsem);
         trace_ext4_writepages(inode, wbc);
 
-        if (dax_mapping(mapping))
-                return dax_writeback_mapping_range(mapping, inode->i_sb->s_bdev,
-                                                   wbc);
+        if (dax_mapping(mapping)) {
+                ret = dax_writeback_mapping_range(mapping, inode->i_sb->s_bdev,
+                                                  wbc);
+                goto out_writepages;
+        }
 
         /*
          * No pages to write? This is mainly a kludge to avoid starting
@@ -2786,6 +2789,7 @@ retry:
 out_writepages:
         trace_ext4_writepages_result(inode, wbc, ret,
                                      nr_to_write - wbc->nr_to_write);
+        percpu_up_read(&sbi->s_journal_flag_rwsem);
         return ret;
 }
 
@@ -5436,6 +5440,7 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
         journal_t *journal;
         handle_t *handle;
         int err;
+        struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 
         /*
          * We have to be very careful here: changing a data block's
@@ -5475,6 +5480,7 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
                 }
         }
 
+        percpu_down_write(&sbi->s_journal_flag_rwsem);
         jbd2_journal_lock_updates(journal);
 
         /*
@@ -5491,6 +5497,7 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
                 err = jbd2_journal_flush(journal);
                 if (err < 0) {
                         jbd2_journal_unlock_updates(journal);
+                        percpu_up_write(&sbi->s_journal_flag_rwsem);
                         ext4_inode_resume_unlocked_dio(inode);
                         return err;
                 }
@@ -5499,6 +5506,8 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
         ext4_set_aops(inode);
 
         jbd2_journal_unlock_updates(journal);
+        percpu_up_write(&sbi->s_journal_flag_rwsem);
+
         if (val)
                 up_write(&EXT4_I(inode)->i_mmap_sem);
         ext4_inode_resume_unlocked_dio(inode);
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 304c712dbe12..20c5d52253b4 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -859,6 +859,7 @@ static void ext4_put_super(struct super_block *sb)
         percpu_counter_destroy(&sbi->s_freeinodes_counter);
         percpu_counter_destroy(&sbi->s_dirs_counter);
         percpu_counter_destroy(&sbi->s_dirtyclusters_counter);
+        percpu_free_rwsem(&sbi->s_journal_flag_rwsem);
         brelse(sbi->s_sbh);
 #ifdef CONFIG_QUOTA
         for (i = 0; i < EXT4_MAXQUOTAS; i++)
@@ -3930,6 +3931,9 @@ no_journal:
         if (!err)
                 err = percpu_counter_init(&sbi->s_dirtyclusters_counter, 0,
                                           GFP_KERNEL);
+        if (!err)
+                err = percpu_init_rwsem(&sbi->s_journal_flag_rwsem);
+
         if (err) {
                 ext4_msg(sb, KERN_ERR, "insufficient memory");
                 goto failed_mount6;
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index f231e0bb311c..bec0b647f9cc 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -37,6 +37,7 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
         free_percpu(brw->fast_read_ctr);
         brw->fast_read_ctr = NULL; /* catch use after free bugs */
 }
+EXPORT_SYMBOL_GPL(percpu_free_rwsem);
 
 /*
  * This is the fast-path for down_read/up_read. If it succeeds we rely