linux-kernel - Re: [RFC PATCH] btrfs: Replace kmap() with kmap_local

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Date:   Tue, 14 Jun 2022 07:13:49 -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>,
        Filipe Manana <fdmanana@...e.com>, Qu Wenruo <wqu@...e.com>,
        Nikolay Borisov <nborisov@...e.com>,
        Gabriel Niebler <gniebler@...e.com>,
        <linux-btrfs@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] btrfs: Replace kmap() with kmap_local_page() in
 zlib.c

On Tue, Jun 14, 2022 at 12:47:18PM +0200, Fabio M. De Francesco wrote:

[snip]

> @@ -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;

This is moving start forward...

>  				}
>  				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 +196,14 @@ 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);
> +			kunmap_local(data_in);
> +			data_in = NULL;
> +			put_page(in_page);

I don't think you need to put/get the page to do the remapping.

> +
> +			kunmap_local(cpage_out);
>  			if (nr_pages == nr_dest_pages) {
> -				out_page = NULL;
> +				cpage_out = NULL;
> +				put_page(out_page);
>  				ret = -E2BIG;
>  				goto out;
>  			}
> @@ -207,7 +212,14 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
>  				ret = -ENOMEM;
>  				goto out;
>  			}
> -			cpage_out = kmap(out_page);
> +			cpage_out = kmap_local_page(out_page);
> +
> +			in_page = find_get_page(mapping, start >> PAGE_SHIFT);

I think start is the wrong value here.  It got incremented above.

> +			data_in = kmap_local_page(in_page);
> +			workspace->strm.next_in = data_in;
> +			workspace->strm.avail_in = min(bytes_left,
> +						       (unsigned long)workspace->buf_size);

I'm not sure about changing avail_in here either because nothing has happened
with the in buffer whilst doing this re-mapping AFAICS.

[snip]

I think the issue with the tests is contained to zlib_compress_pages().
But an idea to verify that is to split this patch between the changes in
zlib_compress_pages() [above] and zlib_decompress_bio() [below] because below
looks very straight forward.

Ira

> @@ -287,7 +311,7 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>  	unsigned long buf_start;
>  	struct page **pages_in = cb->compressed_pages;
>  
> -	data_in = kmap(pages_in[page_in_index]);
> +	data_in = kmap_local_page(pages_in[page_in_index]);
>  	workspace->strm.next_in = data_in;
>  	workspace->strm.avail_in = min_t(size_t, srclen, PAGE_SIZE);
>  	workspace->strm.total_in = 0;
> @@ -309,7 +333,7 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>  
>  	if (Z_OK != zlib_inflateInit2(&workspace->strm, wbits)) {
>  		pr_warn("BTRFS: inflateInit failed\n");
> -		kunmap(pages_in[page_in_index]);
> +		kunmap_local(data_in);
>  		return -EIO;
>  	}
>  	while (workspace->strm.total_in < srclen) {
> @@ -336,13 +360,14 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>  
>  		if (workspace->strm.avail_in == 0) {
>  			unsigned long tmp;
> -			kunmap(pages_in[page_in_index]);
> +
> +			kunmap_local(data_in);
>  			page_in_index++;
>  			if (page_in_index >= total_pages_in) {
>  				data_in = NULL;
>  				break;
>  			}
> -			data_in = kmap(pages_in[page_in_index]);
> +			data_in = kmap_local_page(pages_in[page_in_index]);
>  			workspace->strm.next_in = data_in;
>  			tmp = srclen - workspace->strm.total_in;
>  			workspace->strm.avail_in = min(tmp, PAGE_SIZE);
> @@ -355,7 +380,7 @@ int zlib_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>  done:
>  	zlib_inflateEnd(&workspace->strm);
>  	if (data_in)
> -		kunmap(pages_in[page_in_index]);
> +		kunmap_local(data_in);
>  	if (!ret)
>  		zero_fill_bio(cb->orig_bio);
>  	return ret;
> -- 
> 2.36.1
>