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authorDavid Chinner <dgc@sgi.com>2008-03-27 18:00:45 +1100
committerLachlan McIlroy <lachlan@redback.melbourne.sgi.com>2008-04-18 11:42:21 +1000
commit59a33f9f776b051018ec98af95bd9fe8ba9d0f3e (patch)
treed8d93fcd6ef6a77a7efe722ed61febd3b4051bf7 /Documentation/usb
parent75de2a91c98a6f486f261c1367fe59f5583e15a3 (diff)
[XFS] Ensure a btree insert returns a valid cursor.
When writing into preallocated regions there is a case where XFS can oops or hang doing the unwritten extent conversion on I/O completion. It turns out that the problem is related to the btree cursor being invalid. When we do an insert into the tree, we may need to split blocks in the tree. When we only split at the leaf level (i.e. level 0), everything works just fine. However, if we have a multi-level split in the btreee, the cursor passed to the insert function is no longer valid once the insert is complete. The leaf level split is handled correctly because all the operations at level 0 are done using the original cursor, hence it is updated correctly. However, when we need to update the next level up the tree, we don't use that cursor - we use a cloned cursor that points to the index in the next level up where we need to do the insert. Hence if we need to split a second level, the changes to the tree are reflected in the cloned cursor and not the original cursor. This clone-and-move-up-a-level-on-split behaviour recurses all the way to the top of the tree. The complexity here is that these cloned cursors do not point to the original index that was inserted - they point to the newly allocated block (the right block) and the original cursor pointer to that level may still point to the left block. Hence, without deep examination of the cloned cursor and buffers, we cannot update the original cursor with the new path from the cloned cursor. In these cases the original cursor could be pointing to the wrong block(s) and hence a subsequent modification to the tree using that cursor will lead to corruption of the tree. The crash case occurs when the tree changes height - we insert a new level in the tree, and the cursor does not have a buffer in it's path for that level. Hence any attempt to walk back up the cursor to the root block will result in a null pointer dereference. To make matters even more complex, the BMAP BT is rooted in an inode, so we can have a change of height in the btree *without a root split*. That is, if the root block in the inode is full when we split a leaf node, we cannot fit the pointer to the new block in the root, so we allocate a new block, migrate all the ptrs out of the inode into the new block and point the inode root block at the newly allocated block. This changes the height of the tree without a root split having occurred and hence invalidates the path in the original cursor. The patch below prevents xfs_bmbt_insert() from returning with an invalid cursor by detecting the cases that invalidate the original cursor and refresh it by do a lookup into the btree for the original index we were inserting at. Note that the INOBT, AGFBNO and AGFCNT btree implementations also have this bug, but the cursor is currently always destroyed or revalidated after an insert for those trees. Hence this patch only address the problem in the BMBT code. SGI-PV: 979339 SGI-Modid: xfs-linux-melb:xfs-kern:30701a Signed-off-by: David Chinner <dgc@sgi.com> Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Diffstat (limited to 'Documentation/usb')
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