linux-kernel - Re: [PATCH] btrfs: Convert zlib_compress_pages() to use kmap_local

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Date:   Thu, 30 Jun 2022 07:51:14 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
CC:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Nick Terrell <terrelln@...com>,
        Chris Down <chris@...isdown.name>, Qu Wenruo <wqu@...e.com>,
        Filipe Manana <fdmanana@...e.com>,
        Gabriel Niebler <gniebler@...e.com>,
        <linux-btrfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] btrfs: Convert zlib_compress_pages() to use
 kmap_local_page()

On Mon, Jun 27, 2022 at 06:33:05PM +0200, Fabio M. De Francesco wrote:
> The use of kmap() is being deprecated in favor of kmap_local_page(). With
> kmap_local_page(), the mapping is per thread, CPU local and not globally
> visible.
> 
> Therefore, use kmap_local_page() / kunmap_local() in zlib_compress_pages()
> because in this function the mappings are per thread and are not visible
> in other contexts. Furthermore, drop the mappings of "out_page" which is
> allocated within zlib_compress_pages() with alloc_page(GFP_NOFS) and use
> page_address() (thanks to David Sterba).
> 
> Tested with xfstests on a QEMU + KVM 32-bits VM with 4GB of RAM booting
> a kernel with HIGHMEM64G enabled. This patch passes 26/26 tests of group
> "compress".
> 
> Cc: Qu Wenruo <wqu@...e.com>
> Suggested-by: David Sterba <dsterba@...e.com>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>

Reviewed-by: Ira Weiny <ira.weiny@...el.com>

> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---
>  fs/btrfs/zlib.c | 32 +++++++++++++-------------------
>  1 file changed, 13 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/btrfs/zlib.c b/fs/btrfs/zlib.c
> index 770c4c6bbaef..b4f44662cda7 100644
> --- a/fs/btrfs/zlib.c
> +++ b/fs/btrfs/zlib.c
> @@ -97,7 +97,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
>  {
>  	struct workspace *workspace = list_entry(ws, struct workspace, list);
>  	int ret;
> -	char *data_in;
> +	char *data_in = NULL;
>  	char *cpage_out;
>  	int nr_pages = 0;
>  	struct page *in_page = NULL;
> @@ -126,7 +126,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
>  		ret = -ENOMEM;
>  		goto out;
>  	}
> -	cpage_out = kmap(out_page);
> +	cpage_out = page_address(out_page);
>  	pages[0] = out_page;
>  	nr_pages = 1;
>  
> @@ -148,26 +148,26 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
>  				int i;
>  
>  				for (i = 0; i < in_buf_pages; i++) {
> -					if (in_page) {
> -						kunmap(in_page);
> +					if (data_in) {
> +						kunmap_local(data_in);
>  						put_page(in_page);
>  					}
>  					in_page = find_get_page(mapping,
>  								start >> PAGE_SHIFT);
> -					data_in = kmap(in_page);
> +					data_in = kmap_local_page(in_page);
>  					memcpy(workspace->buf + i * PAGE_SIZE,
>  					       data_in, PAGE_SIZE);
>  					start += PAGE_SIZE;
>  				}
>  				workspace->strm.next_in = workspace->buf;
>  			} else {
> -				if (in_page) {
> -					kunmap(in_page);
> +				if (data_in) {
> +					kunmap_local(data_in);
>  					put_page(in_page);
>  				}
>  				in_page = find_get_page(mapping,
>  							start >> PAGE_SHIFT);
> -				data_in = kmap(in_page);
> +				data_in = kmap_local_page(in_page);
>  				start += PAGE_SIZE;
>  				workspace->strm.next_in = data_in;
>  			}
> @@ -196,9 +196,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
>  		 * the stream end if required
>  		 */
>  		if (workspace->strm.avail_out == 0) {
> -			kunmap(out_page);
>  			if (nr_pages == nr_dest_pages) {
> -				out_page = NULL;
>  				ret = -E2BIG;
>  				goto out;
>  			}
> @@ -207,7 +205,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
>  				ret = -ENOMEM;
>  				goto out;
>  			}
> -			cpage_out = kmap(out_page);
> +			cpage_out = page_address(out_page);
>  			pages[nr_pages] = out_page;
>  			nr_pages++;
>  			workspace->strm.avail_out = PAGE_SIZE;
> @@ -234,9 +232,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
>  			goto out;
>  		} else if (workspace->strm.avail_out == 0) {
>  			/* get another page for the stream end */
> -			kunmap(out_page);
>  			if (nr_pages == nr_dest_pages) {
> -				out_page = NULL;
>  				ret = -E2BIG;
>  				goto out;
>  			}
> @@ -245,7 +241,7 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
>  				ret = -ENOMEM;
>  				goto out;
>  			}
> -			cpage_out = kmap(out_page);
> +			cpage_out = page_address(out_page);
>  			pages[nr_pages] = out_page;
>  			nr_pages++;
>  			workspace->strm.avail_out = PAGE_SIZE;
> @@ -264,13 +260,11 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
>  	*total_in = workspace->strm.total_in;
>  out:
>  	*out_pages = nr_pages;
> -	if (out_page)
> -		kunmap(out_page);
> -
> -	if (in_page) {
> -		kunmap(in_page);
> +	if (data_in) {
> +		kunmap_local(data_in);
>  		put_page(in_page);
>  	}
> +
>  	return ret;
>  }
>  
> -- 
> 2.36.1
>