ext4: Convert to new freezing mechanism

We remove most of frozen checks since upper layer takes care of blocking all
writes. We have to handle protection in ext4_page_mkwrite() in a special way
because we cannot use generic block_page_mkwrite(). Also we add a freeze
protection to ext4_evict_inode() so that iput() of unlinked inode cannot modify
a frozen filesystem (we cannot easily instrument ext4_journal_start() /
ext4_journal_stop() with freeze protection because we are missing the
superblock pointer in ext4_journal_stop() in nojournal mode).

CC: linux-ext4@vger.kernel.org
CC: "Theodore Ts'o" <tytso@mit.edu>
BugLink: https://bugs.launchpad.net/bugs/897421
Tested-by: Kamal Mostafa <kamal@canonical.com>
Tested-by: Peter M. Petrakis <peter.petrakis@canonical.com>
Tested-by: Dann Frazier <dann.frazier@canonical.com>
Tested-by: Massimo Morana <massimo.morana@canonical.com>
Acked-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

3 files changed, 23 insertions, 29 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 02bc8cbe7281..301e1c2db891 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -233,6 +233,11 @@ void ext4_evict_inode(struct inode *inode)
         if (is_bad_inode(inode))
                 goto no_delete;
 
+        /*
+         * Protect us against freezing - iput() caller didn't have to have any
+         * protection against it
+         */
+        sb_start_intwrite(inode->i_sb);
         handle = ext4_journal_start(inode, ext4_blocks_for_truncate(inode)+3);
         if (IS_ERR(handle)) {
                 ext4_std_error(inode->i_sb, PTR_ERR(handle));
@@ -242,6 +247,7 @@ void ext4_evict_inode(struct inode *inode)
                  * cleaned up.
                  */
                 ext4_orphan_del(NULL, inode);
+                sb_end_intwrite(inode->i_sb);
                 goto no_delete;
         }
 
@@ -273,6 +279,7 @@ void ext4_evict_inode(struct inode *inode)
                 stop_handle:
                         ext4_journal_stop(handle);
                         ext4_orphan_del(NULL, inode);
+                        sb_end_intwrite(inode->i_sb);
                         goto no_delete;
                 }
         }
@@ -301,6 +308,7 @@ void ext4_evict_inode(struct inode *inode)
         else
                 ext4_free_inode(handle, inode);
         ext4_journal_stop(handle);
+        sb_end_intwrite(inode->i_sb);
         return;
 no_delete:
         ext4_clear_inode(inode);        /* We must guarantee clearing of inode... */
@@ -4701,11 +4709,7 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
         get_block_t *get_block;
         int retries = 0;
 
-        /*
-         * This check is racy but catches the common case. We rely on
-         * __block_page_mkwrite() to do a reliable check.
-         */
-        vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
+        sb_start_pagefault(inode->i_sb);
         /* Delalloc case is easy... */
         if (test_opt(inode->i_sb, DELALLOC) &&
             !ext4_should_journal_data(inode) &&
@@ -4773,5 +4777,6 @@ retry_alloc:
 out_ret:
         ret = block_page_mkwrite_return(ret);
 out:
+        sb_end_pagefault(inode->i_sb);
         return ret;
 }
diff --git a/fs/ext4/mmp.c b/fs/ext4/mmp.c
index f99a1311e847..fe7c63f4717e 100644
--- a/fs/ext4/mmp.c
+++ b/fs/ext4/mmp.c
@@ -44,6 +44,11 @@ static int write_mmp_block(struct super_block *sb, struct buffer_head *bh)
 {
         struct mmp_struct *mmp = (struct mmp_struct *)(bh->b_data);
 
+        /*
+         * We protect against freezing so that we don't create dirty buffers
+         * on frozen filesystem.
+         */
+        sb_start_write(sb);
         ext4_mmp_csum_set(sb, mmp);
         mark_buffer_dirty(bh);
         lock_buffer(bh);
@@ -51,6 +56,7 @@ static int write_mmp_block(struct super_block *sb, struct buffer_head *bh)
         get_bh(bh);
         submit_bh(WRITE_SYNC, bh);
         wait_on_buffer(bh);
+        sb_end_write(sb);
         if (unlikely(!buffer_uptodate(bh)))
                 return 1;
 
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index d8759401ecae..9cc9bfd5176b 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -332,33 +332,17 @@ static void ext4_put_nojournal(handle_t *handle)
  * journal_end calls result in the superblock being marked dirty, so
  * that sync() will call the filesystem's write_super callback if
  * appropriate.
- *
- * To avoid j_barrier hold in userspace when a user calls freeze(),
- * ext4 prevents a new handle from being started by s_frozen, which
- * is in an upper layer.
  */
 handle_t *ext4_journal_start_sb(struct super_block *sb, int nblocks)
 {
         journal_t *journal;
-        handle_t  *handle;
 
         trace_ext4_journal_start(sb, nblocks, _RET_IP_);
         if (sb->s_flags & MS_RDONLY)
                 return ERR_PTR(-EROFS);
 
+        WARN_ON(sb->s_writers.frozen == SB_FREEZE_COMPLETE);
         journal = EXT4_SB(sb)->s_journal;
-        handle = ext4_journal_current_handle();
-
-        /*
-         * If a handle has been started, it should be allowed to
-         * finish, otherwise deadlock could happen between freeze
-         * and others(e.g. truncate) due to the restart of the
-         * journal handle if the filesystem is forzen and active
-         * handles are not stopped.
-         */
-        if (!handle)
-                vfs_check_frozen(sb, SB_FREEZE_TRANS);
-
         if (!journal)
                 return ext4_get_nojournal();
         /*
@@ -2723,6 +2707,7 @@ static int ext4_run_li_request(struct ext4_li_request *elr)
         sb = elr->lr_super;
         ngroups = EXT4_SB(sb)->s_groups_count;
 
+        sb_start_write(sb);
         for (group = elr->lr_next_group; group < ngroups; group++) {
                 gdp = ext4_get_group_desc(sb, group, NULL);
                 if (!gdp) {
@@ -2749,6 +2734,7 @@ static int ext4_run_li_request(struct ext4_li_request *elr)
                 elr->lr_next_sched = jiffies + elr->lr_timeout;
                 elr->lr_next_group = group + 1;
         }
+        sb_end_write(sb);
 
         return ret;
 }
@@ -4302,10 +4288,8 @@ int ext4_force_commit(struct super_block *sb)
                 return 0;
 
         journal = EXT4_SB(sb)->s_journal;
-        if (journal) {
-                vfs_check_frozen(sb, SB_FREEZE_TRANS);
+        if (journal)
                 ret = ext4_journal_force_commit(journal);
-        }
 
         return ret;
 }
@@ -4342,9 +4326,8 @@ static int ext4_sync_fs(struct super_block *sb, int wait)
  * gives us a chance to flush the journal completely and mark the fs clean.
  *
  * Note that only this function cannot bring a filesystem to be in a clean
- * state independently, because ext4 prevents a new handle from being started
- * by @sb->s_frozen, which stays in an upper layer.  It thus needs help from
- * the upper layer.
+ * state independently. It relies on upper layer to stop all data & metadata
+ * modifications.
  */
 static int ext4_freeze(struct super_block *sb)
 {
@@ -4371,7 +4354,7 @@ static int ext4_freeze(struct super_block *sb)
         EXT4_CLEAR_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_RECOVER);
         error = ext4_commit_super(sb, 1);
 out:
-        /* we rely on s_frozen to stop further updates */
+        /* we rely on upper layer to stop further updates */
         jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal);
         return error;
 }