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

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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




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