Messages in this thread Patch in this message | | | From | "Fabio M. De Francesco" <> | Subject | [RFC PATCH] nvmet-tcp: Don't kmap() pages which can't come from HIGHMEM | Date | Tue, 16 Aug 2022 11:18:08 +0200 |
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kmap() is being deprecated in favor of kmap_local_page().
There are two main problems with kmap(): (1) It comes with an overhead as mapping space is restricted and protected by a global lock for synchronization and (2) it also requires global TLB invalidation when the kmap’s pool wraps and it might block when the mapping space is fully utilized until a slot becomes available.
With kmap_local_page() the mappings are per thread, CPU local, can take page faults, and can be called from any context (including interrupts). It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore, the tasks can be preempted and, when they are scheduled to run again, the kernel virtual addresses are restored and are still valid.
However, there is a huge constraint which might block some conversions to kmap_local_page(): the kernel virtual address cannot be handed across different threads. Ira made me notice that the kmap() and kunmap() in this driver happen in two different workqueues. Therefore, kunmap_local() will try to unmap an invalid address.
Let me explain why I'm sending an RFC. When I hit the above mentioned issues I tried to refactor the code in ways where mapping and unmapping happen in a single thread (to not break the rules of threads locality).
However, while reading this code again I think I noticed an important prerequisite which may lead to a simpler solution... If I'm not wrong, it looks like the pages are allocated in nvmet_tcp_map_data(), using the GFP_KERNEL flag.
This would assure that those pages _cannot_ come from HIGHMEM. If I'm not missing something (again!), a plain page_address() could replace the kmap() of sg_page(sg); furthermore, we shouldn't need the unmappings any longer.
Unfortunately, I don't know this protocol and I'm not so experienced with kernel development to be able to understand this code properly.
Therefore, I have two questions: am I right about thinking that the pages mapped in nvmet_tcp_map_pdu_iovec() are allocated with GFP_KERNEL? If so, can anyone with more knowledge than mine please say if my changes make any sense?
Suggested-by: Ira Weiny <ira.weiny@intel.com> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com> --- drivers/nvme/target/tcp.c | 19 +------------------ 1 file changed, 1 insertion(+), 18 deletions(-)
diff --git a/drivers/nvme/target/tcp.c b/drivers/nvme/target/tcp.c index dc3b4dc8fe08..affba6d862fc 100644 --- a/drivers/nvme/target/tcp.c +++ b/drivers/nvme/target/tcp.c @@ -167,7 +167,6 @@ static const struct nvmet_fabrics_ops nvmet_tcp_ops; static void nvmet_tcp_free_cmd(struct nvmet_tcp_cmd *c); static void nvmet_tcp_finish_cmd(struct nvmet_tcp_cmd *cmd); static void nvmet_tcp_free_cmd_buffers(struct nvmet_tcp_cmd *cmd); -static void nvmet_tcp_unmap_pdu_iovec(struct nvmet_tcp_cmd *cmd); static inline u16 nvmet_tcp_cmd_tag(struct nvmet_tcp_queue *queue, struct nvmet_tcp_cmd *cmd) @@ -309,19 +308,6 @@ static void nvmet_tcp_free_cmd_buffers(struct nvmet_tcp_cmd *cmd) cmd->req.sg = NULL; } -static void nvmet_tcp_unmap_pdu_iovec(struct nvmet_tcp_cmd *cmd) -{ - struct scatterlist *sg; - int i; - - sg = &cmd->req.sg[cmd->sg_idx]; - - for (i = 0; i < cmd->nr_mapped; i++) - kunmap(sg_page(&sg[i])); - - cmd->nr_mapped = 0; -} - static void nvmet_tcp_map_pdu_iovec(struct nvmet_tcp_cmd *cmd) { struct kvec *iov = cmd->iov; @@ -338,7 +324,7 @@ static void nvmet_tcp_map_pdu_iovec(struct nvmet_tcp_cmd *cmd) while (length) { u32 iov_len = min_t(u32, length, sg->length - sg_offset); - iov->iov_base = kmap(sg_page(sg)) + sg->offset + sg_offset; + iov->iov_base = page_address(sg_page(sg)) + sg->offset + sg_offset; iov->iov_len = iov_len; length -= iov_len; @@ -1141,7 +1127,6 @@ static int nvmet_tcp_try_recv_data(struct nvmet_tcp_queue *queue) cmd->rbytes_done += ret; } - nvmet_tcp_unmap_pdu_iovec(cmd); if (queue->data_digest) { nvmet_tcp_prep_recv_ddgst(cmd); return 0; @@ -1411,7 +1396,6 @@ static void nvmet_tcp_restore_socket_callbacks(struct nvmet_tcp_queue *queue) static void nvmet_tcp_finish_cmd(struct nvmet_tcp_cmd *cmd) { nvmet_req_uninit(&cmd->req); - nvmet_tcp_unmap_pdu_iovec(cmd); nvmet_tcp_free_cmd_buffers(cmd); } @@ -1424,7 +1408,6 @@ static void nvmet_tcp_uninit_data_in_cmds(struct nvmet_tcp_queue *queue) if (nvmet_tcp_need_data_in(cmd)) nvmet_req_uninit(&cmd->req); - nvmet_tcp_unmap_pdu_iovec(cmd); nvmet_tcp_free_cmd_buffers(cmd); } -- 2.37.1
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