Re: please review snapshot corruption path with delayed metadata insertion

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Tsutomu Itoh's message of 2011-07-07 19:51:09 -0400:
> Hi, Chris,
> 
> (2011/07/08 5:26), Chris Mason wrote:
> > Excerpts from Tsutomu Itoh's message of 2011-07-01 04:11:28 -0400:
> >> Hi, Miao,
> >>
> >> (2011/06/30 15:32), Miao Xie wrote:
> >>> Hi, Itoh-san
> >>>
> >>> Could you test the following patch to check whether it can fix the bug or not?
> >>> I have tested it on my x86_64 machine by your test script for two days, it worked well.
> >>
> >> I ran my test script about a day, I was not able to reproduce this BUG.
> > 
> > Can you please try this patch with the inode_cache option (in addition
> > to Miao's code).
> 
> In my clarification.
> 
> I do only have to apply this patch to 'btrfs-unstable + (current)for-linus'?
> or, other patches also necessary?
> 

Hi, sorry that I wasn't clear.  You can apply it to the current
for-linus branch, which has Miao's fix to keep from doing delayed
metadata updates on the relocation inode.

-chris

> Thanks,
> Tsutomu
> 
> > 
> > commit d0243d46f7a1e4cd57c74fa14556be65b454687d
> > Author: Chris Mason <chris.mason@oracle.com>
> > Date:   Thu Jul 7 15:53:12 2011 -0400
> > 
> >     Btrfs: write out free inode cache before taking snapshots
> >     
> >     The btrfs snapshotting code requires that once a root has been
> >     snapshotted, we don't change it during a commit
> >     
> >     But the free inode cache was changing the roots when it root the cache,
> >     which lead to corruptions.
> >     
> >     This fixes things by making sure we write the cache while we are taking
> >     the snapshot, and that we don't write it again later.
> >     
> >     Signed-off-by: Chris Mason <chris.mason@oracle.com>
> > 
> > diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
> > index bf0d615..d594cf7 100644
> > --- a/fs/btrfs/free-space-cache.c
> > +++ b/fs/btrfs/free-space-cache.c
> > @@ -1651,6 +1651,7 @@ int __btrfs_add_free_space(struct btrfs_free_space_ctl *ctl,
> >      info->bytes = bytes;
> >  
> >      spin_lock(&ctl->tree_lock);
> > +    ctl->dirty = 1;
> >  
> >      if (try_merge_free_space(ctl, info, true))
> >          goto link;
> > @@ -1691,6 +1692,7 @@ int btrfs_remove_free_space(struct btrfs_block_group_cache *block_group,
> >      int ret = 0;
> >  
> >      spin_lock(&ctl->tree_lock);
> > +    ctl->dirty = 1;
> >  
> >  again:
> >      info = tree_search_offset(ctl, offset, 0, 0);
> > @@ -2589,6 +2591,7 @@ u64 btrfs_find_ino_for_alloc(struct btrfs_root *fs_root)
> >          if (entry->bytes == 0)
> >              free_bitmap(ctl, entry);
> >      }
> > +    ctl->dirty = 1;
> >  out:
> >      spin_unlock(&ctl->tree_lock);
> >  
> > @@ -2688,6 +2691,10 @@ int btrfs_write_out_ino_cache(struct btrfs_root *root,
> >          printk(KERN_ERR "btrfs: failed to write free ino cache "
> >                 "for root %llu\n", root->root_key.objectid);
> >  
> > +    /* we write out at transaction commit time, there's no racing. */
> > +    if (ret == 0)
> > +        ctl->dirty = 0;
> > +
> >      iput(inode);
> >      return ret;
> >  }
> > diff --git a/fs/btrfs/free-space-cache.h b/fs/btrfs/free-space-cache.h
> > index 8f2613f..1e92c93 100644
> > --- a/fs/btrfs/free-space-cache.h
> > +++ b/fs/btrfs/free-space-cache.h
> > @@ -35,6 +35,11 @@ struct btrfs_free_space_ctl {
> >      int free_extents;
> >      int total_bitmaps;
> >      int unit;
> > +    /*
> > +     * record if we've changed since written.  This can turn
> > +     * into a bit field if we need more flags
> > +     */
> > +    unsigned long dirty;
> >      u64 start;
> >      struct btrfs_free_space_op *op;
> >      void *private;
> > diff --git a/fs/btrfs/inode-map.c b/fs/btrfs/inode-map.c
> > index b4087e0..e7c1493 100644
> > --- a/fs/btrfs/inode-map.c
> > +++ b/fs/btrfs/inode-map.c
> > @@ -376,6 +376,7 @@ void btrfs_init_free_ino_ctl(struct btrfs_root *root)
> >      ctl->start = 0;
> >      ctl->private = NULL;
> >      ctl->op = &free_ino_op;
> > +    ctl->dirty = 1;
> >  
> >      /*
> >       * Initially we allow to use 16K of ram to cache chunks of
> > @@ -417,6 +418,9 @@ int btrfs_save_ino_cache(struct btrfs_root *root,
> >      if (!btrfs_test_opt(root, INODE_MAP_CACHE))
> >          return 0;
> >  
> > +    if (!ctl->dirty)
> > +        return 0;
> > +
> >      path = btrfs_alloc_path();
> >      if (!path)
> >          return -ENOMEM;
> > @@ -485,6 +489,24 @@ out:
> >      return ret;
> >  }
> >  
> > +/*
> > + * this tries to save the cache, but if it fails for any reason we clear
> > + * the dirty flag so that it won't be saved again during this commit.
> > + *
> > + * This is used by the snapshotting code to make sure we don't corrupt the
> > + * FS by saving the inode cache after the snapshot is taken.
> > + */
> > +int btrfs_force_save_ino_cache(struct btrfs_root *root,
> > +                   struct btrfs_trans_handle *trans)
> > +{
> > +    struct btrfs_free_space_ctl *ctl = root->free_ino_ctl;
> > +    int ret;
> > +    ret = btrfs_save_ino_cache(root, trans);
> > +
> > +    ctl->dirty = 0;
> > +    return ret;
> > +}
> > +
> >  static int btrfs_find_highest_objectid(struct btrfs_root *root, u64 *objectid)
> >  {
> >      struct btrfs_path *path;
> > diff --git a/fs/btrfs/inode-map.h b/fs/btrfs/inode-map.h
> > index ddb347b..2be060e 100644
> > --- a/fs/btrfs/inode-map.h
> > +++ b/fs/btrfs/inode-map.h
> > @@ -7,7 +7,8 @@ void btrfs_return_ino(struct btrfs_root *root, u64 objectid);
> >  int btrfs_find_free_ino(struct btrfs_root *root, u64 *objectid);
> >  int btrfs_save_ino_cache(struct btrfs_root *root,
> >               struct btrfs_trans_handle *trans);
> > -
> > +int btrfs_force_save_ino_cache(struct btrfs_root *root,
> > +                   struct btrfs_trans_handle *trans);
> >  int btrfs_find_free_objectid(struct btrfs_root *root, u64 *objectid);
> >  
> >  #endif
> > diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> > index 51dcec8..e34827c 100644
> > --- a/fs/btrfs/transaction.c
> > +++ b/fs/btrfs/transaction.c
> > @@ -966,6 +966,15 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
> >      ret = btrfs_run_delayed_items(trans, root);
> >      BUG_ON(ret);
> >  
> > +    /*
> > +     * there are a few transient reasons the free inode cache writeback can fail.
> > +     * and if it does, we'll try again later in the commit.  This forces the
> > +     * clean bit, tossing the cache.  Tossing the cache isn't really good, but
> > +     * if we try to write it again later in the commit we'll corrupt things.
> > +     */
> > +    btrfs_force_save_ino_cache(parent_root, trans);
> > +
> > +
> >      record_root_in_trans(trans, root);
> >      btrfs_set_root_last_snapshot(&root->root_item, trans->transid);
> >      memcpy(new_root_item, &root->root_item, sizeof(*new_root_item));
> > 
> > 
>