linux-kernel - Re: [PATCH v3] kexec: Replace kmap() with kmap_local

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Date:   Fri, 5 Aug 2022 11:39:41 +0800
From:   Baoquan He <bhe@...hat.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        akpm@...ux-foundation.org
Cc:     Eric Biederman <ebiederm@...ssion.com>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH v3] kexec: Replace kmap() with kmap_local_page()

On 08/03/22 at 06:00pm, Fabio M. De Francesco wrote:

Thanks for the log update. And Cc Andrew.

> 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.
> 
> Since its use in kexec_core.c is safe everywhere, it should be preferred.
> 
> Therefore, replace kmap() with kmap_local_page() in kexec_core.c.
> 
> Tested on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
> HIGHMEM64GB enabled.
> 
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Reviewed-by: Ira Weiny <ira.weiny@...el.com>
> Acked-by: Baoquan He <bhe@...hat.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---
> 
> v2->v3: Add more information in the commit message as requested by Baoquan
> He and forward two tags, respectively from Ira Weiny and Baoquan He (many
> thanks to them two!).
> 
> v1->v2: A sentence of the commit message contained an error due to a
> mistake in copy-pasting from a previous patch. Replace "aio.c" with
> "kexec_core.c".
> 
>  kernel/kexec_core.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index 4d34c78334ce..6f98274765d4 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -814,7 +814,7 @@ static int kimage_load_normal_segment(struct kimage *image,
>  		if (result < 0)
>  			goto out;
>  
> -		ptr = kmap(page);
> +		ptr = kmap_local_page(page);
>  		/* Start with a clear page */
>  		clear_page(ptr);
>  		ptr += maddr & ~PAGE_MASK;
> @@ -827,7 +827,7 @@ static int kimage_load_normal_segment(struct kimage *image,
>  			memcpy(ptr, kbuf, uchunk);
>  		else
>  			result = copy_from_user(ptr, buf, uchunk);
> -		kunmap(page);
> +		kunmap_local(ptr);
>  		if (result) {
>  			result = -EFAULT;
>  			goto out;
> @@ -878,7 +878,7 @@ static int kimage_load_crash_segment(struct kimage *image,
>  			goto out;
>  		}
>  		arch_kexec_post_alloc_pages(page_address(page), 1, 0);
> -		ptr = kmap(page);
> +		ptr = kmap_local_page(page);
>  		ptr += maddr & ~PAGE_MASK;
>  		mchunk = min_t(size_t, mbytes,
>  				PAGE_SIZE - (maddr & ~PAGE_MASK));
> @@ -894,7 +894,7 @@ static int kimage_load_crash_segment(struct kimage *image,
>  		else
>  			result = copy_from_user(ptr, buf, uchunk);
>  		kexec_flush_icache_page(page);
> -		kunmap(page);
> +		kunmap_local(ptr);
>  		arch_kexec_pre_free_pages(page_address(page), 1);
>  		if (result) {
>  			result = -EFAULT;
> -- 
> 2.37.1
>