Re: [RESEND PATCH v4 2/2] btrfs: Replace kmap() with kmap_local_page() in zstd.c

[Ira Weiny <ira.weiny@intel.com>]

On Thu, Jun 16, 2022 at 11:00:36PM +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 zstd.c because in
> this file the mappings are per thread and are not visible in other
> contexts; meanwhile refactor zstd_compress_pages() to comply with the
> ordering rules about nested local mapping / unmapping.
> 
> Tested with xfstests on QEMU + KVM 32 bits VM with 4GB of RAM and
> HIGHMEM64G enabled. These changes passed all the tests of the group
> "compress".
> 
> Cc: Filipe Manana <fdmanana@kernel.org>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> ---
> 
> v3->v4: Cc Maintainers and lists that had been overlooked when v3 was
>         sent (mostly regarding patch 1/2).
> 
> v2->v3: Remove unnecessary casts to arguments of kunmap_local() now that
>         this API can take pointers to const void.
> 
> v1->v2: No changes.
> 
> Thanks to Ira Weiny for his invaluable help and persevering support.
> Thanks also to Filipe Manana for identifying a fundamental detail I had
> overlooked in RFC:
> https://lore.kernel.org/lkml/20220611093411.GA3779054@falcondesktop/
> 
>  fs/btrfs/zstd.c | 42 +++++++++++++++++++++++-------------------
>  1 file changed, 23 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
> index 0fe31a6f6e68..5d2ab0bac9d2 100644
> --- a/fs/btrfs/zstd.c
> +++ b/fs/btrfs/zstd.c
> @@ -391,6 +391,8 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  	*out_pages = 0;
>  	*total_out = 0;
>  	*total_in = 0;
> +	workspace->in_buf.src = NULL;
> +	workspace->out_buf.dst = NULL;

I don't think either of these are needed as they are both set straight away
below.

>  
>  	/* Initialize the stream */
>  	stream = zstd_init_cstream(&params, len, workspace->mem,
> @@ -403,7 +405,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  
>  	/* map in the first page of input data */
>  	in_page = find_get_page(mapping, start >> PAGE_SHIFT);
> -	workspace->in_buf.src = kmap(in_page);
> +	workspace->in_buf.src = kmap_local_page(in_page);
>  	workspace->in_buf.pos = 0;
>  	workspace->in_buf.size = min_t(size_t, len, PAGE_SIZE);
>  
> @@ -415,7 +417,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  		goto out;
>  	}
>  	pages[nr_pages++] = out_page;
> -	workspace->out_buf.dst = kmap(out_page);
> +	workspace->out_buf.dst = kmap_local_page(out_page);

Given the conversation in the other thread for zlib_compress_pages(); I think
we should also just use page_address() here.  That simplifies the algorithm
immensely.

I know there was a lot of thought put into how to make this conversion when
this was posted but that is now the cleaner solution.

Ira

>  	workspace->out_buf.pos = 0;
>  	workspace->out_buf.size = min_t(size_t, max_out, PAGE_SIZE);
>  
> @@ -450,9 +452,9 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  		if (workspace->out_buf.pos == workspace->out_buf.size) {
>  			tot_out += PAGE_SIZE;
>  			max_out -= PAGE_SIZE;
> -			kunmap(out_page);
> +			kunmap_local(workspace->out_buf.dst);
>  			if (nr_pages == nr_dest_pages) {
> -				out_page = NULL;
> +				workspace->out_buf.dst = NULL;
>  				ret = -E2BIG;
>  				goto out;
>  			}
> @@ -462,7 +464,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  				goto out;
>  			}
>  			pages[nr_pages++] = out_page;
> -			workspace->out_buf.dst = kmap(out_page);
> +			workspace->out_buf.dst = kmap_local_page(out_page);
>  			workspace->out_buf.pos = 0;
>  			workspace->out_buf.size = min_t(size_t, max_out,
>  							PAGE_SIZE);
> @@ -477,15 +479,16 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  		/* Check if we need more input */
>  		if (workspace->in_buf.pos == workspace->in_buf.size) {
>  			tot_in += PAGE_SIZE;
> -			kunmap(in_page);
> +			kunmap_local(workspace->out_buf.dst);
> +			kunmap_local(workspace->in_buf.src);
>  			put_page(in_page);
> -
>  			start += PAGE_SIZE;
>  			len -= PAGE_SIZE;
>  			in_page = find_get_page(mapping, start >> PAGE_SHIFT);
> -			workspace->in_buf.src = kmap(in_page);
> +			workspace->in_buf.src = kmap_local_page(in_page);
>  			workspace->in_buf.pos = 0;
>  			workspace->in_buf.size = min_t(size_t, len, PAGE_SIZE);
> +			workspace->out_buf.dst = kmap_local_page(out_page);
>  		}
>  	}
>  	while (1) {
> @@ -510,9 +513,9 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  
>  		tot_out += PAGE_SIZE;
>  		max_out -= PAGE_SIZE;
> -		kunmap(out_page);
> +		kunmap_local(workspace->out_buf.dst);
>  		if (nr_pages == nr_dest_pages) {
> -			out_page = NULL;
> +			workspace->out_buf.dst = NULL;
>  			ret = -E2BIG;
>  			goto out;
>  		}
> @@ -522,7 +525,7 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  			goto out;
>  		}
>  		pages[nr_pages++] = out_page;
> -		workspace->out_buf.dst = kmap(out_page);
> +		workspace->out_buf.dst = kmap_local_page(out_page);
>  		workspace->out_buf.pos = 0;
>  		workspace->out_buf.size = min_t(size_t, max_out, PAGE_SIZE);
>  	}
> @@ -538,12 +541,12 @@ int zstd_compress_pages(struct list_head *ws, struct address_space *mapping,
>  out:
>  	*out_pages = nr_pages;
>  	/* Cleanup */
> -	if (in_page) {
> -		kunmap(in_page);
> +	if (workspace->out_buf.dst)
> +		kunmap_local(workspace->out_buf.dst);
> +	if (workspace->in_buf.src) {
> +		kunmap_local(workspace->in_buf.src);
>  		put_page(in_page);
>  	}
> -	if (out_page)
> -		kunmap(out_page);
>  	return ret;
>  }
>  
> @@ -567,7 +570,7 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>  		goto done;
>  	}
>  
> -	workspace->in_buf.src = kmap(pages_in[page_in_index]);
> +	workspace->in_buf.src = kmap_local_page(pages_in[page_in_index]);
>  	workspace->in_buf.pos = 0;
>  	workspace->in_buf.size = min_t(size_t, srclen, PAGE_SIZE);
>  
> @@ -603,14 +606,15 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>  			break;
>  
>  		if (workspace->in_buf.pos == workspace->in_buf.size) {
> -			kunmap(pages_in[page_in_index++]);
> +			kunmap_local(workspace->in_buf.src);
> +			page_in_index++;
>  			if (page_in_index >= total_pages_in) {
>  				workspace->in_buf.src = NULL;
>  				ret = -EIO;
>  				goto done;
>  			}
>  			srclen -= PAGE_SIZE;
> -			workspace->in_buf.src = kmap(pages_in[page_in_index]);
> +			workspace->in_buf.src = kmap_local_page(pages_in[page_in_index]);
>  			workspace->in_buf.pos = 0;
>  			workspace->in_buf.size = min_t(size_t, srclen, PAGE_SIZE);
>  		}
> @@ -619,7 +623,7 @@ int zstd_decompress_bio(struct list_head *ws, struct compressed_bio *cb)
>  	zero_fill_bio(cb->orig_bio);
>  done:
>  	if (workspace->in_buf.src)
> -		kunmap(pages_in[page_in_index]);
> +		kunmap_local(workspace->in_buf.src);
>  	return ret;
>  }
>  
> -- 
> 2.36.1
>