[PATCH v8 03/13] mm: introduce AS_NO_DIRECT_MAP

["Kalyazin, Nikita" <kalyazin@amazon.co.uk>]

From: Patrick Roy <patrick.roy@linux.dev>

Add AS_NO_DIRECT_MAP for mappings where direct map entries of folios are
set to not present. Currently, mappings that match this description are
secretmem mappings (memfd_secret()). Later, some guest_memfd
configurations will also fall into this category.

Reject this new type of mappings in all locations that currently reject
secretmem mappings, on the assumption that if secretmem mappings are
rejected somewhere, it is precisely because of an inability to deal with
folios without direct map entries, and then make memfd_secret() use
AS_NO_DIRECT_MAP on its address_space to drop its special
vma_is_secretmem()/secretmem_mapping() checks.

This drops a optimization in gup_fast_folio_allowed() where
secretmem_mapping() was only called if CONFIG_SECRETMEM=y. secretmem is
enabled by default since commit b758fe6df50d ("mm/secretmem: make it on
by default"), so the secretmem check did not actually end up elided in
most cases anymore anyway.

Use a new flag instead of overloading AS_INACCESSIBLE (which is already
set by guest_memfd) because not all guest_memfd mappings will end up
being direct map removed (e.g. in pKVM setups, parts of guest_memfd that
can be mapped to userspace should also be GUP-able, and generally not
have restrictions on who can access it).

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Acked-by: David Hildenbrand (Red Hat)" <david@kernel.org>
Signed-off-by: Patrick Roy <patrick.roy@linux.dev>
Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
---
 include/linux/pagemap.h   | 16 ++++++++++++++++
 include/linux/secretmem.h | 18 ------------------
 lib/buildid.c             |  4 ++--
 mm/gup.c                  | 19 +++++--------------
 mm/mlock.c                |  2 +-
 mm/secretmem.c            |  8 ++------
 6 files changed, 26 insertions(+), 41 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index a17fabbc0269..d51e0c0404e2 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -213,6 +213,7 @@ enum mapping_flags {
 	AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
 	AS_KERNEL_FILE = 10,	/* mapping for a fake kernel file that shouldn't
 				   account usage to user cgroups */
+	AS_NO_DIRECT_MAP = 11,	/* Folios in the mapping are not in the direct map */
 	/* Bits 16-25 are used for FOLIO_ORDER */
 	AS_FOLIO_ORDER_BITS = 5,
 	AS_FOLIO_ORDER_MIN = 16,
@@ -348,6 +349,21 @@ static inline bool mapping_writeback_may_deadlock_on_reclaim(const struct addres
 	return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
 }
 
+static inline void mapping_set_no_direct_map(struct address_space *mapping)
+{
+	set_bit(AS_NO_DIRECT_MAP, &mapping->flags);
+}
+
+static inline bool mapping_no_direct_map(const struct address_space *mapping)
+{
+	return test_bit(AS_NO_DIRECT_MAP, &mapping->flags);
+}
+
+static inline bool vma_has_no_direct_map(const struct vm_area_struct *vma)
+{
+	return vma->vm_file && mapping_no_direct_map(vma->vm_file->f_mapping);
+}
+
 static inline gfp_t mapping_gfp_mask(const struct address_space *mapping)
 {
 	return mapping->gfp_mask;
diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
index e918f96881f5..0ae1fb057b3d 100644
--- a/include/linux/secretmem.h
+++ b/include/linux/secretmem.h
@@ -4,28 +4,10 @@
 
 #ifdef CONFIG_SECRETMEM
 
-extern const struct address_space_operations secretmem_aops;
-
-static inline bool secretmem_mapping(struct address_space *mapping)
-{
-	return mapping->a_ops == &secretmem_aops;
-}
-
-bool vma_is_secretmem(struct vm_area_struct *vma);
 bool secretmem_active(void);
 
 #else
 
-static inline bool vma_is_secretmem(struct vm_area_struct *vma)
-{
-	return false;
-}
-
-static inline bool secretmem_mapping(struct address_space *mapping)
-{
-	return false;
-}
-
 static inline bool secretmem_active(void)
 {
 	return false;
diff --git a/lib/buildid.c b/lib/buildid.c
index c4b0f376fb34..89e567954284 100644
--- a/lib/buildid.c
+++ b/lib/buildid.c
@@ -65,8 +65,8 @@ static int freader_get_folio(struct freader *r, loff_t file_off)
 
 	freader_put_folio(r);
 
-	/* reject secretmem folios created with memfd_secret() */
-	if (secretmem_mapping(r->file->f_mapping))
+	/* reject folios without direct map entries (e.g. from memfd_secret() or guest_memfd()) */
+	if (mapping_no_direct_map(r->file->f_mapping))
 		return -EFAULT;
 
 	r->folio = filemap_get_folio(r->file->f_mapping, file_off >> PAGE_SHIFT);
diff --git a/mm/gup.c b/mm/gup.c
index a8ba5112e4d0..719e4dbecc09 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -11,7 +11,6 @@
 #include <linux/rmap.h>
 #include <linux/swap.h>
 #include <linux/swapops.h>
-#include <linux/secretmem.h>
 
 #include <linux/sched/signal.h>
 #include <linux/rwsem.h>
@@ -1216,7 +1215,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
 	if ((gup_flags & FOLL_SPLIT_PMD) && is_vm_hugetlb_page(vma))
 		return -EOPNOTSUPP;
 
-	if (vma_is_secretmem(vma))
+	if (vma_has_no_direct_map(vma))
 		return -EFAULT;
 
 	if (write) {
@@ -2724,7 +2723,7 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
  * This call assumes the caller has pinned the folio, that the lowest page table
  * level still points to this folio, and that interrupts have been disabled.
  *
- * GUP-fast must reject all secretmem folios.
+ * GUP-fast must reject all folios without direct map entries (such as secretmem).
  *
  * Writing to pinned file-backed dirty tracked folios is inherently problematic
  * (see comment describing the writable_file_mapping_allowed() function). We
@@ -2739,7 +2738,6 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
 {
 	bool reject_file_backed = false;
 	struct address_space *mapping;
-	bool check_secretmem = false;
 	unsigned long mapping_flags;
 
 	/*
@@ -2751,18 +2749,10 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
 		reject_file_backed = true;
 
 	/* We hold a folio reference, so we can safely access folio fields. */
-
-	/* secretmem folios are always order-0 folios. */
-	if (IS_ENABLED(CONFIG_SECRETMEM) && !folio_test_large(folio))
-		check_secretmem = true;
-
-	if (!reject_file_backed && !check_secretmem)
-		return true;
-
 	if (WARN_ON_ONCE(folio_test_slab(folio)))
 		return false;
 
-	/* hugetlb neither requires dirty-tracking nor can be secretmem. */
+	/* hugetlb neither requires dirty-tracking nor can be without direct map. */
 	if (folio_test_hugetlb(folio))
 		return true;
 
@@ -2800,8 +2790,9 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
 	 * At this point, we know the mapping is non-null and points to an
 	 * address_space object.
 	 */
-	if (check_secretmem && secretmem_mapping(mapping))
+	if (mapping_no_direct_map(mapping))
 		return false;
+
 	/* The only remaining allowed file system is shmem. */
 	return !reject_file_backed || shmem_mapping(mapping);
 }
diff --git a/mm/mlock.c b/mm/mlock.c
index bb0776f5ef7c..506452ee8cad 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -474,7 +474,7 @@ static int mlock_fixup(struct vma_iterator *vmi, struct vm_area_struct *vma,
 
 	if (newflags == oldflags || (oldflags & VM_SPECIAL) ||
 	    is_vm_hugetlb_page(vma) || vma == get_gate_vma(current->mm) ||
-	    vma_is_dax(vma) || vma_is_secretmem(vma) || (oldflags & VM_DROPPABLE))
+	    vma_is_dax(vma) || vma_has_no_direct_map(vma) || (oldflags & VM_DROPPABLE))
 		/* don't set VM_LOCKED or VM_LOCKONFAULT and don't count */
 		goto out;
 
diff --git a/mm/secretmem.c b/mm/secretmem.c
index b59350daffe3..c1eafe29c3e8 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c
@@ -134,11 +134,6 @@ static int secretmem_mmap_prepare(struct vm_area_desc *desc)
 	return 0;
 }
 
-bool vma_is_secretmem(struct vm_area_struct *vma)
-{
-	return vma->vm_ops == &secretmem_vm_ops;
-}
-
 static const struct file_operations secretmem_fops = {
 	.release	= secretmem_release,
 	.mmap_prepare	= secretmem_mmap_prepare,
@@ -156,7 +151,7 @@ static void secretmem_free_folio(struct folio *folio)
 	folio_zero_segment(folio, 0, folio_size(folio));
 }
 
-const struct address_space_operations secretmem_aops = {
+static const struct address_space_operations secretmem_aops = {
 	.dirty_folio	= noop_dirty_folio,
 	.free_folio	= secretmem_free_folio,
 	.migrate_folio	= secretmem_migrate_folio,
@@ -205,6 +200,7 @@ static struct file *secretmem_file_create(unsigned long flags)
 
 	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
 	mapping_set_unevictable(inode->i_mapping);
+	mapping_set_no_direct_map(inode->i_mapping);
 
 	inode->i_op = &secretmem_iops;
 	inode->i_mapping->a_ops = &secretmem_aops;
-- 
2.50.1