On Tue, Jul 23 2019 at 11:12pm -0400, Ming Lei ming.lei@redhat.com wrote:
dm-rq needs to free request which has been dispatched and not completed by underlying queue. However, the underlying queue may have allocated private data for this request in .queue_rq(), so the request private data will be leaked in dm multipath IO code path.
Add one new callback of .cleanup_rq() to fix the memory leak.
Think the above kind of glosses over the nature of the issue. While this issue _is_ unique to request-based DM multipath's use of blk-mq it ultimately is a failing of the existing blk-mq interface that SCSI's per-request private data is leaking. SO all said, I'd rather this patch header reflect the nuance of why you skinned the cat like you have.
Something like this would be a better header IMHO:
SCSI maintains its own driver private data hooked off of each SCSI request. An upper layer driver (e.g. dm-rq) may need to retry these SCSI requests, before SCSI has fully dispatched them, due to a lower level SCSI driver's resource limitation identified in scsi_queue_rq(). Currently SCSI's per-request private data is leaked when the upper layer driver (dm-rq) frees and then retries these requests in response to BLK_STS_RESOURCE or BLK_STS_DEV_RESOURCE returns from scsi_queue_rq().
This usecase is so specialized that it doesn't warrant training an existing blk-mq interface (e.g. blk_mq_free_request) to allow SCSI to account for freeing its driver private data -- doing so would add an extra branch for handling a special case that all other consumers of SCSI (and blk-mq) won't ever need to worry about.
So the most pragmatic way forward is to delegate freeing SCSI driver private data to the upper layer driver (dm-rq). Do so by calling a new blk_mq_cleanup_rq() method from dm-rq. A following commit will implement the .cleanup_rq() hook in scsi_mq_ops.
Another use case is to free request when the hctx is dead during cpu hotplug context.
Not seeing any point forecasting this .cleanup_rq() hook's potential ability to address that cpu hotplug case; the future patch that provides that fix can deal with it. Reality is the existing SCSI per-request private data leak justifies this new hook on its own.
Mike