Re: [PATCH] btrfs: Convert zlib_compress_pages() to use kmap_local_page()

Ira Weiny
2022-06-30T14:51:30Z

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@suse.com>
> Suggested-by: David Sterba <dsterba@suse.com>
> Suggested-by: Ira Weiny <ira.weiny@intel.com>

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

> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.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
>