On Thu, Aug 09, 2012 at 06:23:19AM -0600, Josef Bacik wrote: > 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, For this release, I like the current patch ;) Great job tracking it down Miao. -chris -- 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
