On Wed, 12 Oct 2022, Fabio M. De Francesco wrote:
> The use of kmap() is being deprecated in favor of kmap_local_page().
>
> There are two main problems with kmap(): (1) It comes with an overhead as
> the mapping space is restricted and protected by a global lock for
> synchronization and (2) it also requires global TLB invalidation when the
> kmap’s pool wraps and it might block when the mapping space is fully
> utilized until a slot becomes available.
>
> With kmap_local_page() the mappings are per thread, CPU local, can take
> page faults, and can be called from any context (including interrupts).
> It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
> the tasks can be preempted and, when they are scheduled to run again, the
> kernel virtual addresses are restored and still valid.
>
> Since its use in fs/cramfs is safe everywhere, it should be preferred.
>
> Therefore, replace kmap() with kmap_local_page() in fs/cramfs. Instead
> of open-coding kmap_local_page() + memcpy(), use memcpy_from_page().
>
> Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@xxxxxxxxx>
> Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
Reviewed-by: Nicolas Pitre <nico@xxxxxxxxxxx>
> ---
> fs/cramfs/inode.c | 9 ++++-----
> 1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
> index 61ccf7722fc3..c17cbba5d45e 100644
> --- a/fs/cramfs/inode.c
> +++ b/fs/cramfs/inode.c
> @@ -238,8 +238,7 @@ static void *cramfs_blkdev_read(struct super_block *sb, unsigned int offset,
> struct page *page = pages[i];
>
> if (page) {
> - memcpy(data, kmap(page), PAGE_SIZE);
> - kunmap(page);
> + memcpy_from_page(data, page, 0, PAGE_SIZE);
> put_page(page);
> } else
> memset(data, 0, PAGE_SIZE);
> @@ -815,7 +814,7 @@ static int cramfs_read_folio(struct file *file, struct folio *folio)
>
> maxblock = (inode->i_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
> bytes_filled = 0;
> - pgdata = kmap(page);
> + pgdata = kmap_local_page(page);
>
> if (page->index < maxblock) {
> struct super_block *sb = inode->i_sb;
> @@ -903,13 +902,13 @@ static int cramfs_read_folio(struct file *file, struct folio *folio)
>
> memset(pgdata + bytes_filled, 0, PAGE_SIZE - bytes_filled);
> flush_dcache_page(page);
> - kunmap(page);
> + kunmap_local(pgdata);
> SetPageUptodate(page);
> unlock_page(page);
> return 0;
>
> err:
> - kunmap(page);
> + kunmap_local(pgdata);
> ClearPageUptodate(page);
> SetPageError(page);
> unlock_page(page);
> --
> 2.37.3
>
>
