linux-kernel - Re: [PATCH] x86/hyperv: Replace kmap() with kmap_local

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Date:   Mon, 03 Oct 2022 19:43:49 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     "K . Y . Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, linux-hyperv@...r.kernel.org,
        linux-kernel@...r.kernel.org, Zhao Liu <zhao1.liu@...ux.intel.com>,
        Ira Weiny <ira.weiny@...el.com>
Cc:     Zhenyu Wang <zhenyu.z.wang@...el.com>,
        Zhao Liu <zhao1.liu@...el.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH] x86/hyperv: Replace kmap() with kmap_local_page()

On Wednesday, September 28, 2022 11:56:40 AM CEST Zhao Liu wrote:
> From: Zhao Liu <zhao1.liu@...el.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.
> 
> 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.
> 
> In the fuction hyperv_init() of hyperv/hv_init.c, the mapping is used in a
> single thread and is short live. So, in this case, it's safe to simply use
> kmap_local_page() to create mapping, and this avoids the wasted cost of
> kmap() for global synchronization.
> 
> In addtion, the fuction hyperv_init() checks if kmap() fails by BUG_ON().
> From the original discussion[2], the BUG_ON() here is just used to
> explicitly panic NULL pointer. So still keep the BUG_ON() in place to check
> if kmap_local_page() fails. 

How might kmap_local_page() return invalid kernel addresses? 

I think that, if this function returns, the pointer is always a valid kernel 
address. Am I missing something?

> Based on this consideration, memcpy_to_page()
> is not selected here but only kmap_local_page() is used.

I can't agree with you, if the premises are that kmap_local_page() might 
provide invalid addresses.

Thanks,

Fabio

> Therefore, replace kmap() with kmap_local_page() in hyperv/hv_init.c.
> 
> [1]: https://lore.kernel.org/all/20220813220034.806698-1-ira.weiny@intel.com
> [2]: https://lore.kernel.org/lkml/20200915103710.cqmdvzh5lys4wsqo@liuwe-devbox-debian-v2/
> 
> Suggested-by: Dave Hansen <dave.hansen@...el.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> Signed-off-by: Zhao Liu <zhao1.liu@...el.com>
> 
> ---
> Suggested by credits.
> 	Dave: Referred to his comments about whether kmap() can fail and 
the
> 	      suggestion to keep BUG_ON() in place.
> 	Ira: Referred to his task documentation and review comments about
> 	     keeping BUG_ON() for kmap_local_page().
> 	Fabio: Stole some of his boiler plate commit message.
> ---
>  arch/x86/hyperv/hv_init.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>