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

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Date:   Mon, 13 Jun 2022 20:39:13 +0200
From:   David Sterba <dsterba@...e.cz>
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>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, Filipe Manana <fdmanana@...nel.org>,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] btrfs: Replace kmap() with kmap_local_page() in zstd.c

On Sat, Jun 11, 2022 at 03:52:03PM +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 nested
> local mapping / unmapping ordering rules.
> 
> Tested with xfstests on a QEMU + KVM 32 bits VM with 4GB of RAM and
> HIGHMEM64G enabled.
> 
> Cc: Filipe Manana <fdmanana@...nel.org>
> Suggested-by: Ira Weiny <ira.weiny@...el.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---
> 
> @@ -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((void *)workspace->in_buf.src);

Why is the cast needed? I see that it leads to a warning but we pass a
const buffer and that breaks the API contract as in kunmap it would be
accessed as non-const and potentially changed without warning or
compiler error. If kunmap_local does not touch the buffer and 'const
void*' would work too, then it should be fixed.