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

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Message-ID: <Y9LqwvXy+gpFBerE@ZenIV>
Date:   Thu, 26 Jan 2023 21:04:02 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        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>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Helge Deller <deller@....de>
Subject: Re: [PATCH] mm/highmem: Align-down to page the address for
 kunmap_flush_on_unmap()

On Thu, Jan 26, 2023 at 08:48:03PM +0000, Matthew Wilcox wrote:
> On Thu, Jan 26, 2023 at 12:38:58PM -0800, Andrew Morton wrote:
> > 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?
> 
> The version of this patch I sent out includes this information,
> as well as the missed alignment for kunmap_atomic().

One point: AFAICS, the situation right now is
	* all callers of kunmap_local() pass page-aligned pointers
	* all callers of kunmap_atomic() seem to do the same
	* there's nothing in documentation that would say one can
pass anything other than the return value of original kmap_local_page()
or kmap_atomic() call to those.
	* there's nothing that would outright ban doing that.

So these patches really ought to touch Documentation/mm/highmem.rst
saying that from now on kunmap_local() and kunmap_atomic() callers
are allowed to pass any pointer within the mapped area.  And yes,
we want it in -stable before anything that relies upon that sucker
gets backported.