Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local_page()

[Helge Deller]

On 1/20/23 06:56, 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 implementor) 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.

No, on PA2.0 (64bit) CPUs the lower bits of the address of pdtlb
encodes the amount of memory (page size) to be flushed, see:
http://ftp.parisc-linux.org/docs/arch/parisc2.0.pdf (page 7-106)
So, the proposed page alignment with e.g. PTR_ALIGN_DON() is needed.

Helge