linux-kernel - Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local

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Date:   Mon, 23 Jan 2023 18:14:15 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Ira Weiny <ira.weiny@...el.com>, linux-parisc@...r.kernel.org
Subject: Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local_page()

On venerdì 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:
> > > > 
> > > > static inline void __kunmap_local(const void *addr)
> > > > {
> > > > #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
> > > > 
> > > >         kunmap_flush_on_unmap(addr);
> > > > 
> > > > #endif
> > > > }
> > > > 
> > > > Are you sure that the guts of that thing will be happy with address 
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...
> > > 
> > > Ugh, PA-RISC (the only implementer) definitely will flush the wrong
> > > addresses.  I think we should do this, as having bugs that only manifest
> > > on one not-well-tested architecture seems Bad.
> > > 
> > >  static inline void __kunmap_local(const void *addr)
> > >  {
> > >  #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
> > > 
> > > -       kunmap_flush_on_unmap(addr);
> > > +       kunmap_flush_on_unmap(PAGE_ALIGN_DOWN(addr));
> > > 
> > >  #endif
> > >  }
> > 
> > PTR_ALIGN_DOWN(addr, PAGE_SIZE), perhaps?
> 
> 	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 
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,

Fabio