mprotect() does mmu notifiers in PMD levels.  It's there since 2014 of
commit a5338093 ("mm: move mmu notifier call from change_protection to
change_pmd_range").

At that time, the issue was that NUMA balancing can be applied on a huge
range of VM memory, even if nothing was populated.  The notification can
be avoided in this case if no valid pmd detected, which includes either
THP or a PTE pgtable page.

Now to pave way for PUD handling, this isn't enough.  We need to generate
mmu notifications even on PUD entries properly.  mprotect() is currently
broken on PUD (e.g., one can easily trigger kernel error with dax 1G
mappings already), this is the start to fix it.

To fix that, this patch proposes to push such notifications to the PUD
layers.

There is risk on regressing the problem Rik wanted to resolve before, but I
think it shouldn't really happen, and I still chose this solution because
of a few reasons:

  1) Consider a large VM that should definitely contain more than GBs of
  memory, it's highly likely that PUDs are also none.  In this case there
  will have no regression.

  2) KVM has evolved a lot over the years to get rid of rmap walks, which
  might be the major cause of the previous soft-lockup.  At least TDP MMU
  already got rid of rmap as long as not nested (which should be the major
  use case, IIUC), then the TDP MMU pgtable walker will simply see empty VM
  pgtable (e.g. EPT on x86), the invalidation of a full empty region in
  most cases could be pretty fast now, comparing to 2014.

  3) KVM has explicit code paths now to even give way for mmu notifiers
  just like this one, e.g. in commit d02c357e ("KVM: x86/mmu: Retry
  fault before acquiring mmu_lock if mapping is changing").  It'll also
  avoid contentions that may also contribute to a soft-lockup.

  4) Stick with PMD layer simply don't work when PUD is there...  We need
  one way or another to fix PUD mappings on mprotect().

Pushing it to PUD should be the safest approach as of now, e.g. there's yet
no sign of huge P4D coming on any known archs.

Link: https://lkml.kernel.org/r/20240812181225.1360970-3-peterx@redhat.comSigned-off-by: Peter Xu <peterx@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>