On Wed, Aug 08, 2012 at 09:10:17PM -0600, Miao Xie wrote: > If we create several snapshots at the same time, the following BUG_ON() will be > triggered. > > kernel BUG at fs/btrfs/extent-tree.c:6047! > > Steps to reproduce: > # mkfs.btrfs <partition> > # mount <partition> <mnt> > # cd <mnt> > # for ((i=0;i<2400;i++)); do touch long_name_to_make_tree_more_deep$i; done > # for ((i=0; i<4; i++)) > > do > > mkdir $i > > for ((j=0; j<200; j++)) > > do > > btrfs sub snap . $i/$j > > done & > > done > > The reason is: > Before transaction commit, some operations changed the fs tree and new tree > blocks were allocated because of COW. We used the implicit non-shared back > reference for those newly allocated tree blocks because they were not shared by > two or more trees. > > And then we created the first snapshot for the fs tree, according to the back > reference rules, we also used implicit back refs for the child tree blocks of > the root node of the fs tree, now those child nodes/leaves were shared by two > trees. > > Then We didn't deal with the delayed references, and continued to change the fs > tree(created the second snapshot and inserted the dir item of the new snapshot > into the fs tree). According to the rules of the back reference, we added full > back refs for those tree blocks whose parents have be shared by two trees. > Now some newly allocated tree blocks had two types of the references. > > As we know, the delayed reference system handles these delayed references from > back to front, and the full delayed reference is inserted after the implicit > ones. So when we dealt with the back references of those newly allocated tree > blocks, the full references was dealt with at first. And if the first reference > is a shared back reference and the tree block that the reference points to is > newly allocated, It would be considered as a tree block which is shared by two > or more trees when it is allocated and should be a full back reference not a > implicit one, the flag of its reference also should be set to FULL_BACKREF. > But in fact, it was a non-shared tree block with a implicit reference at > beginning, so it was not compulsory to set the flags to FULL_BACKREF. So BUG_ON > was triggered. > > We have several methods to fix this bug: > 1. deal with delayed references after the snapshot is created and before we > change the source tree of the snapshot. This is the easiest and safest way. > 2. modify the sort method of the delayed reference tree, make the full delayed > references be inserted before the implicit ones. It is also very easy, but > I don't know if it will introduce some problems or not. Thanks for tracking this down, FWIW I like option 2 the most, it would be intereseting to see if it does actually introduce new issues. Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
