fd8ad2128d
unionfs uses a per-directory hashtable to cache subdirectory nodes. Currently this hashtable is looked up using the directory name, but since unionfs nodes aren't removed from the cache until they're reclaimed, this poses some problems. For example, if a directory is created on a unionfs mount shortly after deleting a previous directory with the same path, the cache may end up reusing the node for the previous directory, including its upper/lower FS vnodes. Operations against those vnodes with then likely fail because the vnodes represent deleted files; for example UFS will reject VOP_MKDIR() against such a vnode because its effective link count is 0. This may then manifest as e.g. mkdir(2) or open(2) returning ENOENT for an attempt to create a file under the re-created directory. While it would be possible to fix this by explicitly managing the name-based cache during delete or rename operations, or by rejecting cache hits if the underlying FS vnodes don't match those passed to unionfs_nodeget(), it seems cleaner to instead hash the unionfs nodes based on their underlying FS vnodes. Since unionfs prefers to operate against the upper vnode if one is present, the lower vnode will only be used for hashing as long as the upper vnode is NULL. This should also make hashing faster by eliminating string traversal and using the already-computed hash index stored in each vnode. While here, fix a couple of other cache-related issues: --Remove 8 bytes of unnecessary baggage from each unionfs node by getting rid of the stored hash mask field. The mask is knowable at compile time. --When a matching node is found in the cache, reference its vnode using vrefl() while still holding the vnode interlock. Previously unionfs_nodeget() would vref() the vnode after the interlock was dropped, but the vnode may be reclaimed during that window. This caused intermittent panics from vn_lock(9) during unionfs stress testing. Reviewed by: kib, markj Tested by: pho Differential Revision: https://reviews.freebsd.org/D32533
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|---|---|
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