linux-kernel - Re: [PATCH] mm/highmem: Align-down to page the address for kunmap_flush_on

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Message-Id: <20230126123858.42d9377ad6e352a58c81668b@linux-foundation.org>
Date:   Thu, 26 Jan 2023 12:38:58 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Ira Weiny <ira.weiny@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Tony Luck <tony.luck@...el.com>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        David Sterba <dsterba@...e.com>,
        Kees Cook <keescook@...omium.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>, Helge Deller <deller@....de>
Subject: Re: [PATCH] mm/highmem: Align-down to page the address for
 kunmap_flush_on_unmap()

On Thu, 26 Jan 2023 15:33:46 +0100 "Fabio M. De Francesco" <fmdefrancesco@...il.com> wrote:

> If ARCH_HAS_FLUSH_ON_KUNMAP is defined (PA-RISC case), __kunmap_local()
> calls kunmap_flush_on_unmap(). The latter currently flushes the wrong
> address (as confirmed by Matthew Wilcox and Helge Deller). Al Viro
> proposed to call kunmap_flush_on_unmap() on an aligned-down to page
> address in order to fix this issue. Consensus has been reached on this
> solution.

What are the user-visible runtime effects of this flaw?

> Therefore, if ARCH_HAS_FLUSH_ON_KUNMAP is defined, call
> kunmap_flush_on_unmap() on an aligned-down to page address computed with
> the PTR_ALIGN_DOWN() macro.
> 
> Cc: Ira Weiny <ira.weiny@...el.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Suggested-by: Al Viro <viro@...iv.linux.org.uk>
> Confirmed-by: Helge Deller <deller@....de>
> Confirmed-by: Matthew Wilcox <willy@...radead.org>
> Fixes: f3ba3c710ac5 ("mm/highmem: Provide kmap_local*")
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---
> 
> I have (at least) two problems with this patch...
> 
> 1) checkpatch.pl complains about the use of the non-standard
> "Confirmed-by" tags. I don't know how else I can give credit to Helge
> and Matthew. However, this is not the first time that I see non-standard
> tags in patches applied upstream (I too had a non-standard
> "Analysed-by" tag in patch which fixes a SAC bug). Any objections?

Add a paragraph "this was confirmed by X and Y", then add Cc:X, Cc:y?

This gives you an opportunity to tell us what "confirmed" actually
means!  Did they confirm that it's a bug?  Or that the fix is correct? 
I dunno.

> 2) I'm not sure whether or not the "Fixes" tag is appropriate in this
> patch. Can someone either confirm or deny it?
> 
>  include/linux/highmem-internal.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
> index 034b1106d022..e247c9ac4583 100644
> --- a/include/linux/highmem-internal.h
> +++ b/include/linux/highmem-internal.h
> @@ -200,7 +200,7 @@ static inline void *kmap_local_pfn(unsigned long pfn)
>  static inline void __kunmap_local(const void *addr)
>  {
>  #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
> -	kunmap_flush_on_unmap(addr);
> +	kunmap_flush_on_unmap(PTR_ALIGN_DOWN(addr, PAGE_SIZE));
>  #endif
>  }
>  
> -- 
> 2.39.0