linux-kernel - Re: [PATCH] nvmet-tcp: Don't map pages which can't come from HIGHMEM

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Date:   Tue, 30 Aug 2022 23:40:09 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     linux-nvme@...ts.infradead.org, Sagi Grimberg <sagi@...mberg.me>
Cc:     linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Keith Busch <kbusch@...nel.org>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        James Smart <james.smart@...adcom.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Venkataramanan Anirudh <anirudh.venkataramanan@...el.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] nvmet-tcp: Don't map pages which can't come from HIGHMEM

On lunedì 29 agosto 2022 09:54:01 CEST Sagi Grimberg wrote:
> From: "Fabio M. De Francesco" <fmdefrancesco@...il.com>
> 
> 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.
> 
> In addition, there is no reason to use a kvec for the command receive
> data buffers iovec, use a bio_vec instead and let iov_iter handle the
> buffer mapping and data copy.
> 
> Test with blktests on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
> HIGHMEM64GB enabled.
> 
> [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: Christoph Hellwig <hch@....de>
> Cc: Keith Busch <kbusch@...nel.org>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> [sagi: added bio_vec plus minor naming changes]
> Signed-off-by: Sagi Grimberg <sagi@...mberg.me>
> ---
>  drivers/nvme/target/tcp.c | 44 ++++++++++++---------------------------
>  1 file changed, 13 insertions(+), 31 deletions(-)

Hi Sagi,

Thanks for changing the code according to the suggestions from Christoph and 
Al.

Differently from what I just wrote while thanking Al, and after Ira made me 
notice that you are one of the maintainers, I decided to leave everything like 
you did.

The only exceptions we'll be about sending a v2 with the "Suggested-by" tags 
from Christoph and Al. In the meantime checkpatch warned that the alignment of 
"nr_pages" doesn't match the open parenthesis, so I'm changing it too.

Again thanks,

Fabio