linux-kernel - Re: [PATCH] fs/cramfs: Convert kmap() to kmap_local

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Message-ID: <87672pr-o821-qns6-11rp-902sp1s34r3@syhkavp.arg>
Date:   Wed, 12 Oct 2022 14:54:03 -0400 (EDT)
From:   Nicolas Pitre <nico@...xnic.net>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
cc:     Christian Brauner <brauner@...nel.org>,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] fs/cramfs: Convert kmap() to kmap_local_data()

On Wed, 12 Oct 2022, Fabio M. De Francesco wrote:

> The use of kmap() is being deprecated in favor of kmap_local_page().
> 
> There are two main problems with kmap(): (1) It comes with an overhead as
> the 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 still valid.
> 
> Since its use in fs/cramfs is safe everywhere, it should be preferred.
> 
> Therefore, replace kmap() with kmap_local_page() in fs/cramfs. Instead
> of open-coding kmap_local_page() + memcpy(), use memcpy_from_page().
> 
> Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@...el.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>

Reviewed-by: Nicolas Pitre <nico@...xnic.net>

> ---
>  fs/cramfs/inode.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
> index 61ccf7722fc3..c17cbba5d45e 100644
> --- a/fs/cramfs/inode.c
> +++ b/fs/cramfs/inode.c
> @@ -238,8 +238,7 @@ static void *cramfs_blkdev_read(struct super_block *sb, unsigned int offset,
>  		struct page *page = pages[i];
>  
>  		if (page) {
> -			memcpy(data, kmap(page), PAGE_SIZE);
> -			kunmap(page);
> +			memcpy_from_page(data, page, 0, PAGE_SIZE);
>  			put_page(page);
>  		} else
>  			memset(data, 0, PAGE_SIZE);
> @@ -815,7 +814,7 @@ static int cramfs_read_folio(struct file *file, struct folio *folio)
>  
>  	maxblock = (inode->i_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
>  	bytes_filled = 0;
> -	pgdata = kmap(page);
> +	pgdata = kmap_local_page(page);
>  
>  	if (page->index < maxblock) {
>  		struct super_block *sb = inode->i_sb;
> @@ -903,13 +902,13 @@ static int cramfs_read_folio(struct file *file, struct folio *folio)
>  
>  	memset(pgdata + bytes_filled, 0, PAGE_SIZE - bytes_filled);
>  	flush_dcache_page(page);
> -	kunmap(page);
> +	kunmap_local(pgdata);
>  	SetPageUptodate(page);
>  	unlock_page(page);
>  	return 0;
>  
>  err:
> -	kunmap(page);
> +	kunmap_local(pgdata);
>  	ClearPageUptodate(page);
>  	SetPageError(page);
>  	unlock_page(page);
> -- 
> 2.37.3
> 
>