linux-kernel - Re: [PATCH] drivers/nvdimm: Use local kmaps

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Message-ID: <6225d231-1d9f-474b-9003-a3a56d528545@intel.com>
Date: Thu, 4 Dec 2025 08:32:33 -0700
From: Dave Jiang <dave.jiang@...el.com>
To: Davidlohr Bueso <dave@...olabs.net>, vishal.l.verma@...el.com,
 dan.j.williams@...el.com, ira.weiny@...el.com
Cc: nvdimm@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers/nvdimm: Use local kmaps



On 11/28/25 2:23 PM, Davidlohr Bueso wrote:
> Replace the now deprecated kmap_atomic() with kmap_local_page().
> 
> Optimizing nvdimm/pmem for highmem makes no sense as this is always
> 64bit, and the mapped regions for both btt and pmem do not require
> disabling preemption and pagefaults. Specifically, kmap does not care
> about the caller's atomic context (such as reads holding the btt arena
> spinlock) or NVDIMM_IO_ATOMIC semantics to avoid error handling when
> accessing the btt arena in general. Same for the memcpy cases. kmap
> local temporary mappings will hold valid across any context switches.
> 
> Signed-off-by: Davidlohr Bueso <dave@...olabs.net>

Reviewed-by: Dave Jiang <dave.jiang@...el.com>> ---
>  drivers/nvdimm/btt.c  | 12 ++++++------
>  drivers/nvdimm/pmem.c |  8 ++++----
>  2 files changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/nvdimm/btt.c b/drivers/nvdimm/btt.c
> index a933db961ed7..237edfa1c624 100644
> --- a/drivers/nvdimm/btt.c
> +++ b/drivers/nvdimm/btt.c
> @@ -1104,10 +1104,10 @@ static int btt_data_read(struct arena_info *arena, struct page *page,
>  {
>  	int ret;
>  	u64 nsoff = to_namespace_offset(arena, lba);
> -	void *mem = kmap_atomic(page);
> +	void *mem = kmap_local_page(page);
>  
>  	ret = arena_read_bytes(arena, nsoff, mem + off, len, NVDIMM_IO_ATOMIC);
> -	kunmap_atomic(mem);
> +	kunmap_local(mem);
>  
>  	return ret;
>  }
> @@ -1117,20 +1117,20 @@ static int btt_data_write(struct arena_info *arena, u32 lba,
>  {
>  	int ret;
>  	u64 nsoff = to_namespace_offset(arena, lba);
> -	void *mem = kmap_atomic(page);
> +	void *mem = kmap_local_page(page);
>  
>  	ret = arena_write_bytes(arena, nsoff, mem + off, len, NVDIMM_IO_ATOMIC);
> -	kunmap_atomic(mem);
> +	kunmap_local(mem);
>  
>  	return ret;
>  }
>  
>  static void zero_fill_data(struct page *page, unsigned int off, u32 len)
>  {
> -	void *mem = kmap_atomic(page);
> +	void *mem = kmap_local_page(page);
>  
>  	memset(mem + off, 0, len);
> -	kunmap_atomic(mem);
> +	kunmap_local(mem);
>  }
>  
>  #ifdef CONFIG_BLK_DEV_INTEGRITY
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index 05785ff21a8b..92c67fbbc1c8 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -128,10 +128,10 @@ static void write_pmem(void *pmem_addr, struct page *page,
>  	void *mem;
>  
>  	while (len) {
> -		mem = kmap_atomic(page);
> +		mem = kmap_local_page(page);
>  		chunk = min_t(unsigned int, len, PAGE_SIZE - off);
>  		memcpy_flushcache(pmem_addr, mem + off, chunk);
> -		kunmap_atomic(mem);
> +		kunmap_local(mem);
>  		len -= chunk;
>  		off = 0;
>  		page++;
> @@ -147,10 +147,10 @@ static blk_status_t read_pmem(struct page *page, unsigned int off,
>  	void *mem;
>  
>  	while (len) {
> -		mem = kmap_atomic(page);
> +		mem = kmap_local_page(page);
>  		chunk = min_t(unsigned int, len, PAGE_SIZE - off);
>  		rem = copy_mc_to_kernel(mem + off, pmem_addr, chunk);
> -		kunmap_atomic(mem);
> +		kunmap_local(mem);
>  		if (rem)
>  			return BLK_STS_IOERR;
>  		len -= chunk;