mm: implement sticky VMA flags

It is useful to be able to designate that certain flags are 'sticky', that
is, if two VMAs are merged one with a flag of this nature and one without,
the merged VMA sets this flag.

As a result we ignore these flags for the purposes of determining VMA flag
differences between VMAs being considered for merge.

This patch therefore updates the VMA merge logic to perform this action,
with flags possessing this property being described in the VM_STICKY
bitmap.

Those flags which ought to be ignored for the purposes of VMA merge are
described in the VM_IGNORE_MERGE bitmap, which the VMA merge logic is also
updated to use.

As part of this change we place VM_SOFTDIRTY in VM_IGNORE_MERGE as it
already had this behaviour, alongside VM_STICKY as sticky flags by
implication must not disallow merge.

Ultimately it seems that we should make VM_SOFTDIRTY a sticky flag in its
own right, but this change is out of scope for this series.

The only sticky flag designated as such is VM_MAYBE_GUARD, so as a result
of this change, once the VMA flag is set upon guard region installation,
VMAs with guard ranges will now not have their merge behaviour impacted as
a result and can be freely merged with other VMAs without VM_MAYBE_GUARD
set.

Also update the comments for vma_modify_flags() to directly reference
sticky flags now we have established the concept.

We also update the VMA userland tests to account for the changes.

Link: https://lkml.kernel.org/r/22ad5269f7669d62afb42ce0c79bad70b994c58d.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>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand (Red Hat) <david@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, 28 insertions, 0 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 03776aab3837..fea113d1d723 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -528,6 +528,34 @@ extern unsigned int kobjsize(const void *objp);
 #define VM_FLAGS_CLEAR	(ARCH_VM_PKEY_FLAGS | VM_ARCH_CLEAR)
 
 /*
+ * Flags which should be 'sticky' on merge - that is, flags which, when one VMA
+ * possesses it but the other does not, the merged VMA should nonetheless have
+ * applied to it:
+ *
+ * VM_MAYBE_GUARD - If a VMA may have guard regions in place it implies that
+ *                  mapped page tables may contain metadata not described by the
+ *                  VMA and thus any merged VMA may also contain this metadata,
+ *                  and thus we must make this flag sticky.
+ */
+#define VM_STICKY VM_MAYBE_GUARD
+
+/*
+ * VMA flags we ignore for the purposes of merge, i.e. one VMA possessing one
+ * of these flags and the other not does not preclude a merge.
+ *
+ * VM_SOFTDIRTY - Should not prevent from VMA merging, if we match the flags but
+ *                dirty bit -- the caller should mark merged VMA as dirty. If
+ *                dirty bit won't be excluded from comparison, we increase
+ *                pressure on the memory system forcing the kernel to generate
+ *                new VMAs when old one could be extended instead.
+ *
+ *    VM_STICKY - When merging VMAs, VMA flags must match, unless they are
+ *                'sticky'. If any sticky flags exist in either VMA, we simply
+ *                set all of them on the merged VMA.
+ */
+#define VM_IGNORE_MERGE (VM_SOFTDIRTY | VM_STICKY)
+
+/*
  * mapping from the currently active vm_flags protection bits (the
  * low four bits) to a page protection mask..
  */