[PATCH 2/5] mm/mempolicy: add mpol_set_shared_policy_range()

[Gregory Price <gourry@gourry.net>]

mpol_set_shared_policy() installs a policy over a VMA's page-offset
range, which is the only programmatic way to populate an inode's
shared policy after init.

Two limitations make it unusable for binding an entire backing inode
from in-kernel code:

- It requires a VMA, so it cannot cover unmapped offsets.
  (e.g. unmapped file folios faulted by pagecache)

- mpol_shared_policy_init(), the only no-VMA installer, reconstructs
  the policy from mpol->w.user_nodemask.  That field is only populated
  for static/relative or mount-string policies.

  a policy built programmatically (e.g. by mempolicy_create()) leaves it
  empty and stores w.cpuset_mems_allowed instead and _init mangles it.

Both matter to a caller that wants a whole inode bound at creation.
guest_memfd is one: it has no VMA at that point and may never gain one
for a given offset, and it builds its policy in-kernel rather than from
a mount string, so neither existing installer can express it.

Add mpol_set_shared_policy_range(), which installs an already-built,
fully contextualised policy verbatim over an arbitrary [start, end)
page range with no VMA.

Reimplement mpol_set_shared_policy() as a thin wrapper that derives
the range from the VMA, so both share a single underlying path.

Suggested-by: Dave Jiang <dave.jiang@intel.com>
Co-developed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Gregory Price <gourry@gourry.net>
Assisted-by: Claude:claude-opus-4-8
---
 include/linux/mempolicy.h |  2 ++
 mm/mempolicy.c            | 35 +++++++++++++++++++++++++++++------
 2 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
index aef018ad92317..398318175cec7 100644
--- a/include/linux/mempolicy.h
+++ b/include/linux/mempolicy.h
@@ -124,6 +124,8 @@ int vma_dup_policy(struct vm_area_struct *src, struct vm_area_struct *dst);
 void mpol_shared_policy_init(struct shared_policy *sp, struct mempolicy *mpol);
 int mpol_set_shared_policy(struct shared_policy *sp,
 			   struct vm_area_struct *vma, struct mempolicy *mpol);
+int mpol_set_shared_policy_range(struct shared_policy *sp, pgoff_t start,
+				 pgoff_t end, struct mempolicy *mpol);
 void mpol_free_shared_policy(struct shared_policy *sp);
 struct mempolicy *mpol_shared_policy_lookup(struct shared_policy *sp,
 					    pgoff_t idx);
diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index da133ffe0b1c8..ce10ce4374643 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -3312,24 +3312,47 @@ void mpol_shared_policy_init(struct shared_policy *sp, struct mempolicy *mpol)
 }
 EXPORT_SYMBOL_FOR_MODULES(mpol_shared_policy_init, "kvm");
 
-int mpol_set_shared_policy(struct shared_policy *sp,
-			struct vm_area_struct *vma, struct mempolicy *pol)
+/**
+ * mpol_set_shared_policy_range - install @pol over [@start, @end) of @sp
+ * @sp:    the shared policy tree
+ * @start: first page offset (inclusive)
+ * @end:   last page offset (exclusive)
+ * @pol:   a fully-built, validated policy, or NULL to clear the range
+ *
+ * Installs @pol over the given range, replacing any overlapping policy.
+ * @sp takes its own reference, the caller retains its reference on @pol.
+ *
+ * The policy is not reconstructed, so the policy is preserved exactly.
+ *
+ * Unlike mpol_set_shared_policy(), no VMA is required, so a range that
+ * is never mapped into a VMA can be covered, including the whole file.
+ *
+ * Return: 0 on success, -ENOMEM on allocation failure.
+ */
+int mpol_set_shared_policy_range(struct shared_policy *sp, pgoff_t start,
+				 pgoff_t end, struct mempolicy *pol)
 {
-	const pgoff_t pgoff = vma_start_pgoff(vma);
-	const pgoff_t pgoff_end = vma_end_pgoff(vma);
 	struct sp_node *new = NULL;
 	int err;
 
 	if (pol) {
-		new = sp_alloc(pgoff, pgoff_end, pol);
+		new = sp_alloc(start, end, pol);
 		if (!new)
 			return -ENOMEM;
 	}
-	err = shared_policy_replace(sp, pgoff, pgoff_end, new);
+	err = shared_policy_replace(sp, start, end, new);
 	if (err && new)
 		sp_free(new);
 	return err;
 }
+EXPORT_SYMBOL_FOR_MODULES(mpol_set_shared_policy_range, "kvm");
+
+int mpol_set_shared_policy(struct shared_policy *sp,
+			struct vm_area_struct *vma, struct mempolicy *pol)
+{
+	return mpol_set_shared_policy_range(sp, vma->vm_pgoff,
+					   vma->vm_pgoff + vma_pages(vma), pol);
+}
 EXPORT_SYMBOL_FOR_MODULES(mpol_set_shared_policy, "kvm");
 
 /* Free a backing policy store on inode delete. */
-- 
2.53.0-Meta