linux-kernel - [PATCH] nvmet-tcp: Don't kmap() pages which can't come from HIGHMEM

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Message-Id: <20220817170427.15682-1-fmdefrancesco@gmail.com>
Date:   Wed, 17 Aug 2022 19:04:27 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Christoph Hellwig <hch@....de>, Sagi Grimberg <sagi@...mberg.me>,
        Chaitanya Kulkarni <kch@...dia.com>,
        linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Chaitanya Kulkarni <chaitanyak@...dia.com>,
        Keith Busch <kbusch@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH] nvmet-tcp: Don't kmap() pages which can't come from HIGHMEM

kmap() is being deprecated in favor of kmap_local_page().[1]

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.

The pages which will be mapped are allocated in nvmet_tcp_map_data(),
using the GFP_KERNEL flag. This assures that they cannot come from
HIGHMEM. This imply that a straight page_address() can replace the kmap()
of sg_page(sg) in nvmet_tcp_map_pdu_iovec(). As a side effect, we might
also delete the field "nr_mapped" from struct "nvmet_tcp_cmd" because,
after removing the kmap() calls, there would be no longer any need of it.

Therefore, replace the kmap() of sg_page(sg) with a page_address() and
delete the "nr_mapped" field from "nvmet_tcp_cmd".

[1] "[PATCH] checkpatch: Add kmap and kmap_atomic to the deprecated
list" https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com/

Cc: Chaitanya Kulkarni <chaitanyak@...dia.com>
Cc: Keith Busch <kbusch@...nel.org>
Cc: Sagi Grimberg <sagi@...mberg.me>
Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---

Thanks so much to Chaitanya, Keith, Sagi, for the answers and the comments
on the RFC which gave birth to this patch. The RFC is at:
https://lore.kernel.org/all/20220816091808.23236-1-fmdefrancesco@gmail.com/

 drivers/nvme/target/tcp.c | 26 ++------------------------
 1 file changed, 2 insertions(+), 24 deletions(-)

diff --git a/drivers/nvme/target/tcp.c b/drivers/nvme/target/tcp.c
index dc3b4dc8fe08..6ed44531de55 100644
--- a/drivers/nvme/target/tcp.c
+++ b/drivers/nvme/target/tcp.c
@@ -77,7 +77,6 @@ struct nvmet_tcp_cmd {
 	u32				pdu_len;
 	u32				pdu_recv;
 	int				sg_idx;
-	int				nr_mapped;
 	struct msghdr			recv_msg;
 	struct kvec			*iov;
 	u32				flags;
@@ -167,7 +166,6 @@ 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)
@@ -301,27 +299,12 @@ static int nvmet_tcp_check_ddgst(struct nvmet_tcp_queue *queue, void *pdu)
 
 static void nvmet_tcp_free_cmd_buffers(struct nvmet_tcp_cmd *cmd)
 {
-	WARN_ON(unlikely(cmd->nr_mapped > 0));
-
 	kfree(cmd->iov);
 	sgl_free(cmd->req.sg);
 	cmd->iov = NULL;
 	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;
@@ -329,7 +312,6 @@ static void nvmet_tcp_map_pdu_iovec(struct nvmet_tcp_cmd *cmd)
 	u32 length, offset, sg_offset;
 
 	length = cmd->pdu_len;
-	cmd->nr_mapped = DIV_ROUND_UP(length, PAGE_SIZE);
 	offset = cmd->rbytes_done;
 	cmd->sg_idx = offset / PAGE_SIZE;
 	sg_offset = offset % PAGE_SIZE;
@@ -338,7 +320,7 @@ 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;
@@ -347,8 +329,7 @@ static void nvmet_tcp_map_pdu_iovec(struct nvmet_tcp_cmd *cmd)
 		sg_offset = 0;
 	}
 
-	iov_iter_kvec(&cmd->recv_msg.msg_iter, READ, cmd->iov,
-		cmd->nr_mapped, cmd->pdu_len);
+	iov_iter_kvec(&cmd->recv_msg.msg_iter, READ, cmd->iov, 0, cmd->pdu_len);
 }
 
 static void nvmet_tcp_fatal_error(struct nvmet_tcp_queue *queue)
@@ -1141,7 +1122,6 @@ 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 +1391,6 @@ 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 +1403,6 @@ 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