Re: [PATCH] zero_user_page uses in fs/buffer.c and fs/libfs.c

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 30 Apr 2007 00:04:31 -0700 (PDT)
Christoph Lameter <clameter@sgi.com> wrote:

> 
> Replace open-coded kmap_atomic() and kunmap_atomic()
> surrounding two memory clear operations with zero_user_page(), as both
> memory operations act on the same page.
> 
> Cc: Nate Diller <nate.diller@gmail.com>
> Signed-off-by: Christoph Lameter <clameter@sgi.com>
> 
> ---
>  fs/buffer.c |   29 ++++++++++-------------------
>  fs/libfs.c  |   10 +++++-----
>  2 files changed, 15 insertions(+), 24 deletions(-)
> 
> Index: linux-2.6.21-rc7-mm2/fs/buffer.c
> ===================================================================
> --- linux-2.6.21-rc7-mm2.orig/fs/buffer.c	2007-04-27 22:51:27.000000000 -0700
> +++ linux-2.6.21-rc7-mm2/fs/buffer.c	2007-04-29 23:58:48.000000000 -0700
> @@ -1796,19 +1796,12 @@ static int __block_prepare_write(struct 
>  					set_buffer_uptodate(bh);
>  					continue;
>  				}
> -				if (block_end > to || block_start < from) {
> -					void *kaddr;
> -
> -					kaddr = kmap_atomic(page, KM_USER0);
> -					if (block_end > to)
> -						memset(kaddr+to, 0,
> -							block_end-to);
> -					if (block_start < from)
> -						memset(kaddr+block_start,
> -							0, from-block_start);
> -					flush_dcache_page(page);
> -					kunmap_atomic(kaddr, KM_USER0);
> -				}
> +				if (block_end > to)
> +					zero_user_page(page, to,
> +						block_end - to, KM_USER0);
> +				if (block_start < from)
> +					zero_user_page(page, block_start,
> +						from - block_start, KM_USER0);
>  				continue;
>  			}
>  		}
> @@ -2224,7 +2217,6 @@ int nobh_prepare_write(struct page *page
>  	unsigned block_in_page;
>  	unsigned block_start;
>  	sector_t block_in_file;
> -	char *kaddr;
>  	int nr_reads = 0;
>  	int i;
>  	int ret = 0;
> @@ -2264,13 +2256,12 @@ int nobh_prepare_write(struct page *page
>  		if (PageUptodate(page))
>  			continue;
>  		if (buffer_new(&map_bh) || !buffer_mapped(&map_bh)) {
> -			kaddr = kmap_atomic(page, KM_USER0);
>  			if (block_start < from)
> -				memset(kaddr+block_start, 0, from-block_start);
> +				zero_user_page(page, block_start,
> +					from - block_start, KM_USER0);
>  			if (block_end > to)
> -				memset(kaddr + to, 0, block_end - to);
> -			flush_dcache_page(page);
> -			kunmap_atomic(kaddr, KM_USER0);
> +				zero_user_page(page, to,
> +					block_end - to, KM_USER0);
>  			continue;
>  		}
>  		if (buffer_uptodate(&map_bh))
> Index: linux-2.6.21-rc7-mm2/fs/libfs.c
> ===================================================================
> --- linux-2.6.21-rc7-mm2.orig/fs/libfs.c	2007-04-27 22:51:27.000000000 -0700
> +++ linux-2.6.21-rc7-mm2/fs/libfs.c	2007-04-27 22:55:12.000000000 -0700
> @@ -338,11 +338,11 @@ int simple_prepare_write(struct file *fi
>  {
>  	if (!PageUptodate(page)) {
>  		if (to - from != PAGE_CACHE_SIZE) {
> -			void *kaddr = kmap_atomic(page, KM_USER0);
> -			memset(kaddr, 0, from);
> -			memset(kaddr + to, 0, PAGE_CACHE_SIZE - to);
> -			flush_dcache_page(page);
> -			kunmap_atomic(kaddr, KM_USER0);
> +			if (from)
> +				zero_user_page(page, 0, from, KM_USER0);
> +			if (to < PAGE_CACHE_SIZE)
> +				zero_user_page(page, to,
> +					PAGE_CACHE_SIZE - to, KM_USER0);
>  		}
>  	}
>  	return 0;

As Satyam said, this will sometimes cause us to map and unmap the page
twice, and to run flush_dcache_page() twice.  In not-terribly-uncommon
circumstances in very frequently called functions.

Doesn't seem worth it to me.