Newly allocated XFS metadata buffers are added to the LRU once the hold count is released, which typically occurs after I/O completion. There is no other mechanism at current that tracks the existence or I/O state of a new buffer. Further, readahead I/O tends to be submitted asynchronously by nature, which means the I/O can remain in flight and actually complete long after the calling context is gone. This means that file descriptors or any other holds on the filesystem can be released, allowing the filesystem to be unmounted while I/O is still in flight. When I/O completion occurs, core data structures may have been freed, causing completion to run into invalid memory accesses and likely to panic. This problem is reproduced on XFS via directory readahead. A filesystem is mounted, a directory is opened/closed and the filesystem immediately unmounted. The open/close cycle triggers a directory readahead that if delayed long enough, runs buffer I/O completion after the unmount has completed. To work around this problem, add readahead buffers to the LRU earlier than other buffers (when the buffer is allocated, specifically). The buffer hold count will ultimately remain until I/O completion, which means any shrinker activity will skip the buffer until then. This makes the buffer visible to xfs_wait_buftarg(), however, which ensures that an unmount or quiesce waits for I/O completion appropriately. Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx> --- This addresses the problem reproduced by the recently posted xfstests test: http://thread.gmane.org/gmane.comp.file-systems.fstests/2740 This could probably be made more involved, i.e., to create another list of buffers in flight or some such. This seems more simple/sane to me, however, and survives my testing so far... Brian fs/xfs/xfs_buf.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c index 4665ff6..3f03df9 100644 --- a/fs/xfs/xfs_buf.c +++ b/fs/xfs/xfs_buf.c @@ -590,8 +590,20 @@ xfs_buf_get_map( return NULL; } + /* + * If the buffer found doesn't match the one allocated above, somebody + * else beat us to insertion and we can toss the new one. + * + * If we did add the buffer and it happens to be readahead, add to the + * LRU now rather than waiting until the hold is released. Otherwise, + * the buffer is not visible to xfs_wait_buftarg() while in flight and + * nothing else prevents an unmount before I/O completion. + */ if (bp != new_bp) xfs_buf_free(new_bp); + else if (flags & XBF_READ_AHEAD && + list_lru_add(&bp->b_target->bt_lru, &bp->b_lru)) + atomic_inc(&bp->b_hold); found: if (!bp->b_addr) { -- 2.5.5 _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
