mm: introduce copy-on-fork VMAs and make VM_MAYBE_GUARD one

Gather all the VMA flags whose presence implies that page tables must be
copied on fork into a single bitmap - VM_COPY_ON_FORK - and use this
rather than specifying individual flags in vma_needs_copy().

We also add VM_MAYBE_GUARD to this list, as it being set on a VMA implies
that there may be metadata contained in the page tables (that is - guard
markers) which would will not and cannot be propagated upon fork.

This was already being done manually previously in vma_needs_copy(), but
this makes it very explicit, alongside VM_PFNMAP, VM_MIXEDMAP and
VM_UFFD_WP all of which imply the same.

Note that VM_STICKY flags ought generally to be marked VM_COPY_ON_FORK too
- because equally a flag being VM_STICKY indicates that the VMA contains
metadat that is not propagated by being faulted in - i.e.  that the VMA
metadata does not fully describe the VMA alone, and thus we must propagate
whatever metadata there is on a fork.

However, for maximum flexibility, we do not make this necessarily the case
here.

Link: https://lkml.kernel.org/r/5d41b24e7bc622cda0af92b6d558d7f4c0d1bc8c.1763460113.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 26 insertions, 0 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index fea113d1d723..af2904aeb163 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -556,6 +556,32 @@ extern unsigned int kobjsize(const void *objp);
 #define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
 
 /*
+ * Flags which should result in page tables being copied on fork. These are
+ * flags which indicate that the VMA maps page tables which cannot be
+ * reconsistuted upon page fault, so necessitate page table copying upon
+ *
+ * VM_PFNMAP / VM_MIXEDMAP - These contain kernel-mapped data which cannot be
+ *                           reasonably reconstructed on page fault.
+ *
+ *              VM_UFFD_WP - Encodes metadata about an installed uffd
+ *                           write protect handler, which cannot be
+ *                           reconstructed on page fault.
+ *
+ *                           We always copy pgtables when dst_vma has uffd-wp
+ *                           enabled even if it's file-backed
+ *                           (e.g. shmem). Because when uffd-wp is enabled,
+ *                           pgtable contains uffd-wp protection information,
+ *                           that's something we can't retrieve from page cache,
+ *                           and skip copying will lose those info.
+ *
+ *          VM_MAYBE_GUARD - Could contain page guard region markers which
+ *                           by design are a property of the page tables
+ *                           only and thus cannot be reconstructed on page
+ *                           fault.
+ */
+#define VM_COPY_ON_FORK (VM_PFNMAP | VM_MIXEDMAP | VM_UFFD_WP | VM_MAYBE_GUARD)
+
+/*
  * mapping from the currently active vm_flags protection bits (the
  * low four bits) to a page protection mask..
  */