[PATCH] fix an ext3 deadlock

mpage_writepages() does a lock_page() on pages to be written back, even
when it is being used for page reclaim writeback.

This is normally OK, because the page is unlocked quickly - pages are
unlocked during writeback and nobody should be performing __GFP_FS
allocations inside lock_page().

But it has introduced a ranking problem in ext3:

generic_file_write
->lock_page
  ->ext3_prepare_write
    ->journal_start	(waits for a commit)

versus

ext3_create()
->journal_start()
  ->ext3_new_inode(GFP_KERNEL)
    ->page reclaim
      ->mpage_writepages
        ->lock_page	(locks up, transaction is held open)

Maybe sometime, I'll have to turn mpage_writepages' lock_page into a
trylock if the caller is PF_MEMALLOC.  But for now, let's make ext3's
inside-transaction allocations use GFP_NOFS.  There is only one of them.

fs/ext3/super.c

@@ -443,12 +443,16 @@ void ext3_put_super (struct super_block * sb)
 	return;
 }
 
-static kmem_cache_t * ext3_inode_cachep;
+static kmem_cache_t *ext3_inode_cachep;
 
+/*
+ * Called inside transaction, so use GFP_NOFS
+ */
 static struct inode *ext3_alloc_inode(struct super_block *sb)
 {
 	struct ext3_inode_info *ei;
-	ei = (struct ext3_inode_info *)kmem_cache_alloc(ext3_inode_cachep, SLAB_KERNEL);
+
+	ei = kmem_cache_alloc(ext3_inode_cachep, SLAB_NOFS);
 	if (!ei)
 		return NULL;
 	return &ei->vfs_inode;