mm: fold userfaultfd_rwp() to false without CONFIG_ARCH_HAS_PTE_PROTNONE

RWP tracks accesses by installing PAGE_NONE (protnone) PTEs, so its code
paths are gated on userfaultfd_rwp().  Without
CONFIG_ARCH_HAS_PTE_PROTNONE there is no PAGE_NONE -- <linux/pgtable.h>
defines it to a BUILD_BUG() stub, relying on callers folding such paths to
dead code via IS_ENABLED(CONFIG_ARCH_HAS_PTE_PROTNONE).

userfaultfd_rwp() was not a compile-time constant, so the compiler could
not fold those paths.  With an older compiler (gcc 8.5.0, sparc64) the
PAGE_NONE reference in move_pages_huge_pmd() survived to codegen:

  mm/huge_memory.c:2874: _dst_pmd = pmd_modify(_dst_pmd, PAGE_NONE);
  compiler_types.h:702: error: call to '__compiletime_assert_501'
    declared with attribute error: BUILD_BUG failed

RWP cannot exist without protnone, so return a compile-time false when
CONFIG_ARCH_HAS_PTE_PROTNONE is unset; every RWP path then folds away.

Link: https://lore.kernel.org/amcitKvUvFYr8W38@thinkstation
Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202607250853.VaJWGLeA-lkp@intel.com/
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: James Houghton <jthoughton@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions