ext4: fix deadlock in ext4_symlink() in ENOSPC conditions

ext4_symlink() cannot call __page_symlink() with transaction open.
__page_symlink() calls ext4_write_begin() which can wait for
transaction commit if we are running out of space thus causing a
deadlock. Also error recovery in ext4_truncate_failed_write() does not
count with the transaction being already started (although I'm not
aware of any particular deadlock here).

Fix the problem by stopping a transaction before calling
__page_symlink() (we have to be careful and put inode to orphan list
so that it gets deleted in case of crash) and starting another one
after __page_symlink() returns for addition of symlink into a
directory.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 55 insertions, 11 deletions

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index cadf04b924aa..3c7a06e58469 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -2250,6 +2250,7 @@ static int ext4_symlink(struct inode *dir,
         handle_t *handle;
         struct inode *inode;
         int l, err, retries = 0;
+        int credits;
 
         l = strlen(symname)+1;
         if (l > dir->i_sb->s_blocksize)
@@ -2257,10 +2258,26 @@ static int ext4_symlink(struct inode *dir,
 
         dquot_initialize(dir);
 
+        if (l > EXT4_N_BLOCKS * 4) {
+                /*
+                 * For non-fast symlinks, we just allocate inode and put it on
+                 * orphan list in the first transaction => we need bitmap,
+                 * group descriptor, sb, inode block, quota blocks.
+                 */
+                credits = 4 + EXT4_MAXQUOTAS_INIT_BLOCKS(dir->i_sb);
+        } else {
+                /*
+                 * Fast symlink. We have to add entry to directory
+                 * (EXT4_DATA_TRANS_BLOCKS + EXT4_INDEX_EXTRA_TRANS_BLOCKS),
+                 * allocate new inode (bitmap, group descriptor, inode block,
+                 * quota blocks, sb is already counted in previous macros).
+                 */
+                credits = EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
+                          EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3 +
+                          EXT4_MAXQUOTAS_INIT_BLOCKS(dir->i_sb);
+        }
 retry:
-        handle = ext4_journal_start(dir, EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
-                                        EXT4_INDEX_EXTRA_TRANS_BLOCKS + 5 +
-                                        EXT4_MAXQUOTAS_INIT_BLOCKS(dir->i_sb));
+        handle = ext4_journal_start(dir, credits);
         if (IS_ERR(handle))
                 return PTR_ERR(handle);
 
@@ -2273,21 +2290,44 @@ retry:
         if (IS_ERR(inode))
                 goto out_stop;
 
-        if (l > sizeof(EXT4_I(inode)->i_data)) {
+        if (l > EXT4_N_BLOCKS * 4) {
                 inode->i_op = &ext4_symlink_inode_operations;
                 ext4_set_aops(inode);
                 /*
-                 * page_symlink() calls into ext4_prepare/commit_write.
-                 * We have a transaction open.  All is sweetness.  It also sets
-                 * i_size in generic_commit_write().
+                 * We cannot call page_symlink() with transaction started
+                 * because it calls into ext4_write_begin() which can wait
+                 * for transaction commit if we are running out of space
+                 * and thus we deadlock. So we have to stop transaction now
+                 * and restart it when symlink contents is written.
+                 * 
+                 * To keep fs consistent in case of crash, we have to put inode
+                 * to orphan list in the mean time.
                  */
+                drop_nlink(inode);
+                err = ext4_orphan_add(handle, inode);
+                ext4_journal_stop(handle);
+                if (err)
+                        goto err_drop_inode;
                 err = __page_symlink(inode, symname, l, 1);
+                if (err)
+                        goto err_drop_inode;
+                /*
+                 * Now inode is being linked into dir (EXT4_DATA_TRANS_BLOCKS
+                 * + EXT4_INDEX_EXTRA_TRANS_BLOCKS), inode is also modified
+                 */
+                handle = ext4_journal_start(dir,
+                                EXT4_DATA_TRANS_BLOCKS(dir->i_sb) +
+                                EXT4_INDEX_EXTRA_TRANS_BLOCKS + 1);
+                if (IS_ERR(handle)) {
+                        err = PTR_ERR(handle);
+                        goto err_drop_inode;
+                }
+                inc_nlink(inode);
+                err = ext4_orphan_del(handle, inode);
                 if (err) {
+                        ext4_journal_stop(handle);
                         clear_nlink(inode);
-                        unlock_new_inode(inode);
-                        ext4_mark_inode_dirty(handle, inode);
-                        iput(inode);
-                        goto out_stop;
+                        goto err_drop_inode;
                 }
         } else {
                 /* clear the extent format for fast symlink */
@@ -2303,6 +2343,10 @@ out_stop:
         if (err == -ENOSPC && ext4_should_retry_alloc(dir->i_sb, &retries))
                 goto retry;
         return err;
+err_drop_inode:
+        unlock_new_inode(inode);
+        iput(inode);
+        return err;
 }
 
 static int ext4_link(struct dentry *old_dentry,