From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Matthew Wilcox <willy@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>
Cc:     Christoph Hellwig <hch@infradead.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Ira Weiny <ira.weiny@intel.com>, linux-parisc@vger.kernel.org
Subject: Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local_page()
Date:   Mon, 23 Jan 2023 18:14:15 +0100
Message-ID: <3146373.5fSG56mABF@suse>
In-Reply-To: <Y8os8QR1pRXyu4N8@ZenIV>
References: <20230119153232.29750-1-fmdefrancesco@gmail.com> <Y8ohpDtqI8bPAgRn@ZenIV>
 <Y8os8QR1pRXyu4N8@ZenIV>
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On venerd=EC 20 gennaio 2023 06:56:01 CET Al Viro wrote:
> On Fri, Jan 20, 2023 at 05:07:48AM +0000, Al Viro wrote:
> > On Fri, Jan 20, 2023 at 04:54:51AM +0000, Matthew Wilcox wrote:
> > > > Sure, but... there's also this:
> > > >=20
> > > > static inline void __kunmap_local(const void *addr)
> > > > {
> > > > #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
> > > >=20
> > > >         kunmap_flush_on_unmap(addr);
> > > >=20
> > > > #endif
> > > > }
> > > >=20
> > > > Are you sure that the guts of that thing will be happy with address=
=20
that
> > > > is not page-aligned?  I've looked there at some point, got scared of
> > > > parisc (IIRC) MMU details and decided not to rely upon that...
> > >=20
> > > Ugh, PA-RISC (the only implementer) definitely will flush the wrong
> > > addresses.  I think we should do this, as having bugs that only manif=
est
> > > on one not-well-tested architecture seems Bad.
> > >=20
> > >  static inline void __kunmap_local(const void *addr)
> > >  {
> > >  #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
> > >=20
> > > -       kunmap_flush_on_unmap(addr);
> > > +       kunmap_flush_on_unmap(PAGE_ALIGN_DOWN(addr));
> > >=20
> > >  #endif
> > >  }
> >=20
> > PTR_ALIGN_DOWN(addr, PAGE_SIZE), perhaps?
>=20
> 	Anyway, that's a question to parisc folks; I _think_ pdtlb
> quietly ignores the lower bits of address, so that part seems
> to be safe, but I wouldn't bet upon that.  And when I got to
> flush_kernel_dcache_page_asm I gave up - it's been a long time
> since I've dealt with parisc assembler.

There seems to be consensus that __kunmap_local() needs to be fixed for the=
=20
parisc case (ARCH_HAS_FLUSH_ON_KUNMAP).

Is anyone doing this task?

If you agree, I could make this change and give proper credits for the tip.

Thank you,

=46abio