mm: add ability to take further action in vm_area_desc

Some drivers/filesystems need to perform additional tasks after the VMA is
set up. This is typically in the form of pre-population.

The forms of pre-population most likely to be performed are a PFN remap
or the insertion of normal folios and PFNs into a mixed map.

We start by implementing the PFN remap functionality, ensuring that we
perform the appropriate actions at the appropriate time - that is setting
flags at the point of .mmap_prepare, and performing the actual remap at the
point at which the VMA is fully established.

This prevents the driver from doing anything too crazy with a VMA at any
stage, and we retain complete control over how the mm functionality is
applied.

Unfortunately callers still do often require some kind of custom action,
so we add an optional success/error _hook to allow the caller to do
something after the action has succeeded or failed.

This is done at the point when the VMA has already been established, so
the harm that can be done is limited.

The error hook can be used to filter errors if necessary.

There may be cases in which the caller absolutely must hold the file rmap
lock until the operation is entirely complete. It is an edge case, but
certainly the hugetlbfs mmap hook requires it.

To accommodate this, we add the hide_from_rmap_until_complete flag to the
mmap_action type. In this case, if a new VMA is allocated, we will hold the
file rmap lock until the operation is entirely completed (including any
success/error hooks).

Note that we do not need to update __compat_vma_mmap() to accommodate this
flag, as this function will be invoked from an .mmap handler whose VMA is
not yet visible, so we implicitly hide it from the rmap.

If any error arises on these final actions, we simply unmap the VMA
altogether.

Also update the stacked filesystem compatibility layer to utilise the
action behaviour, and update the VMA tests accordingly.

While we're here, rename __compat_vma_mmap_prepare() to __compat_vma_mmap()
as we are now performing actions invoked by the mmap_prepare in addition to
just the mmap_prepare hook.

Link: https://lkml.kernel.org/r/2601199[e-mail]
Signed-off-by: Lorenzo Stoakes <[e-mail]>
Cc: Alexander Gordeev <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Andreas Larsson <[e-mail]>
Cc: Andrey Konovalov <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Baolin Wang <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Chatre, Reinette <[e-mail]>
Cc: Christian Borntraeger <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: Dave Jiang <[e-mail]>
Cc: Dave Martin <[e-mail]>
Cc: Dave Young <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: David S. Miller <[e-mail]>
Cc: Dmitriy Vyukov <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Guo Ren <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: James Morse <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Jason Gunthorpe <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Kevin Tian <[e-mail]>
Cc: Konstantin Komarov <[e-mail]>
Cc: Liam Howlett <[e-mail]>
Cc: "Luck, Tony" <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Muchun Song <[e-mail]>
Cc: Nicolas Pitre <[e-mail]>
Cc: Oscar Salvador <[e-mail]>
Cc: Pedro Falcato <[e-mail]>
Cc: Robin Murohy <[e-mail]>
Cc: Sumanth Korikkar <[e-mail]>
Cc: Suren Baghdasaryan <[e-mail]>
Cc: Sven Schnelle <[e-mail]>
Cc: Thomas Bogendoerfer <[e-mail]>
Cc: "Uladzislau Rezki (Sony)" <[e-mail]>
Cc: Vasily Gorbik <[e-mail]>
Cc: Vishal Verma <[e-mail]>
Cc: Vivek Goyal <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/fs.h

···
 	return true;
 }
 
-int __compat_vma_mmap_prepare(const struct file_operations *f_op,
+int __compat_vma_mmap(const struct file_operations *f_op,
 		struct file *file, struct vm_area_struct *vma);
-int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma);
+int compat_vma_mmap(struct file *file, struct vm_area_struct *vma);
 
 static inline int vfs_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	if (file->f_op->mmap_prepare)
-		return compat_vma_mmap_prepare(file, vma);
+		return compat_vma_mmap(file, vma);
 
 	return file->f_op->mmap(file, vma);
 }

include/linux/mm.h

···
 	return vma_desc_size(desc) >> PAGE_SHIFT;
 }
 
+/**
+ * mmap_action_remap - helper for mmap_prepare hook to specify that a pure PFN
+ * remap is required.
+ * @desc: The VMA descriptor for the VMA requiring remap.
+ * @start: The virtual address to start the remap from, must be within the VMA.
+ * @start_pfn: The first PFN in the range to remap.
+ * @size: The size of the range to remap, in bytes, at most spanning to the end
+ * of the VMA.
+ */
+static inline void mmap_action_remap(struct vm_area_desc *desc,
+				     unsigned long start,
+				     unsigned long start_pfn,
+				     unsigned long size)
+{
+	struct mmap_action *action = &desc->action;
+
+	/* [start, start + size) must be within the VMA. */
+	WARN_ON_ONCE(start < desc->start || start >= desc->end);
+	WARN_ON_ONCE(start + size > desc->end);
+
+	action->type = MMAP_REMAP_PFN;
+	action->remap.start = start;
+	action->remap.start_pfn = start_pfn;
+	action->remap.size = size;
+	action->remap.pgprot = desc->page_prot;
+}
+
+/**
+ * mmap_action_remap_full - helper for mmap_prepare hook to specify that the
+ * entirety of a VMA should be PFN remapped.
+ * @desc: The VMA descriptor for the VMA requiring remap.
+ * @start_pfn: The first PFN in the range to remap.
+ */
+static inline void mmap_action_remap_full(struct vm_area_desc *desc,
+					  unsigned long start_pfn)
+{
+	mmap_action_remap(desc, desc->start, start_pfn, vma_desc_size(desc));
+}
+
+/**
+ * mmap_action_ioremap - helper for mmap_prepare hook to specify that a pure PFN
+ * I/O remap is required.
+ * @desc: The VMA descriptor for the VMA requiring remap.
+ * @start: The virtual address to start the remap from, must be within the VMA.
+ * @start_pfn: The first PFN in the range to remap.
+ * @size: The size of the range to remap, in bytes, at most spanning to the end
+ * of the VMA.
+ */
+static inline void mmap_action_ioremap(struct vm_area_desc *desc,
+				       unsigned long start,
+				       unsigned long start_pfn,
+				       unsigned long size)
+{
+	mmap_action_remap(desc, start, start_pfn, size);
+	desc->action.type = MMAP_IO_REMAP_PFN;
+}
+
+/**
+ * mmap_action_ioremap_full - helper for mmap_prepare hook to specify that the
+ * entirety of a VMA should be PFN I/O remapped.
+ * @desc: The VMA descriptor for the VMA requiring remap.
+ * @start_pfn: The first PFN in the range to remap.
+ */
+static inline void mmap_action_ioremap_full(struct vm_area_desc *desc,
+					  unsigned long start_pfn)
+{
+	mmap_action_ioremap(desc, desc->start, start_pfn, vma_desc_size(desc));
+}
+
+void mmap_action_prepare(struct mmap_action *action,
+			 struct vm_area_desc *desc);
+int mmap_action_complete(struct mmap_action *action,
+			 struct vm_area_struct *vma);
+
 /* Look up the first VMA which exactly match the interval vm_start ... vm_end */
 static inline struct vm_area_struct *find_exact_vma(struct mm_struct *mm,
 				unsigned long vm_start, unsigned long vm_end)

include/linux/mm_types.h

···
 };
 #endif
 
+/* What action should be taken after an .mmap_prepare call is complete? */
+enum mmap_action_type {
+	MMAP_NOTHING,		/* Mapping is complete, no further action. */
+	MMAP_REMAP_PFN,		/* Remap PFN range. */
+	MMAP_IO_REMAP_PFN,	/* I/O remap PFN range. */
+};
+
+/*
+ * Describes an action an mmap_prepare hook can instruct to be taken to complete
+ * the mapping of a VMA. Specified in vm_area_desc.
+ */
+struct mmap_action {
+	union {
+		/* Remap range. */
+		struct {
+			unsigned long start;
+			unsigned long start_pfn;
+			unsigned long size;
+			pgprot_t pgprot;
+		} remap;
+	};
+	enum mmap_action_type type;
+
+	/*
+	 * If specified, this hook is invoked after the selected action has been
+	 * successfully completed. Note that the VMA write lock still held.
+	 *
+	 * The absolute minimum ought to be done here.
+	 *
+	 * Returns 0 on success, or an error code.
+	 */
+	int (*success_hook)(const struct vm_area_struct *vma);
+
+	/*
+	 * If specified, this hook is invoked when an error occurred when
+	 * attempting the selection action.
+	 *
+	 * The hook can return an error code in order to filter the error, but
+	 * it is not valid to clear the error here.
+	 */
+	int (*error_hook)(int err);
+
+	/*
+	 * This should be set in rare instances where the operation required
+	 * that the rmap should not be able to access the VMA until
+	 * completely set up.
+	 */
+	bool hide_from_rmap_until_complete :1;
+};
+
 /*
  * Describes a VMA that is about to be mmap()'ed. Drivers may choose to
  * manipulate mutable fields which will cause those fields to be updated in the
···
 	/* Write-only fields. */
 	const struct vm_operations_struct *vm_ops;
 	void *private_data;
+
+	/* Take further action? */
+	struct mmap_action action;
 };
 
 /*

mm/util.c

···
 #endif
 
 /**
- * __compat_vma_mmap_prepare() - See description for compat_vma_mmap_prepare()
+ * __compat_vma_mmap() - See description for compat_vma_mmap()
  * for details. This is the same operation, only with a specific file operations
  * struct which may or may not be the same as vma->vm_file->f_op.
  * @f_op: The file operations whose .mmap_prepare() hook is specified.
···
  * @vma: The VMA to apply the .mmap_prepare() hook to.
  * Returns: 0 on success or error.
  */
-int __compat_vma_mmap_prepare(const struct file_operations *f_op,
+int __compat_vma_mmap(const struct file_operations *f_op,
 		struct file *file, struct vm_area_struct *vma)
 {
 	struct vm_area_desc desc = {
···
 		.vm_file = vma->vm_file,
 		.vm_flags = vma->vm_flags,
 		.page_prot = vma->vm_page_prot,
+
+		.action.type = MMAP_NOTHING, /* Default */
 	};
 	int err;
 
 	err = f_op->mmap_prepare(&desc);
 	if (err)
 		return err;
-	set_vma_from_desc(vma, &desc);
 
-	return 0;
+	mmap_action_prepare(&desc.action, &desc);
+	set_vma_from_desc(vma, &desc);
+	return mmap_action_complete(&desc.action, vma);
 }
-EXPORT_SYMBOL(__compat_vma_mmap_prepare);
+EXPORT_SYMBOL(__compat_vma_mmap);
 
 /**
- * compat_vma_mmap_prepare() - Apply the file's .mmap_prepare() hook to an
- * existing VMA.
+ * compat_vma_mmap() - Apply the file's .mmap_prepare() hook to an
+ * existing VMA and execute any requested actions.
  * @file: The file which possesss an f_op->mmap_prepare() hook.
  * @vma: The VMA to apply the .mmap_prepare() hook to.
  *
···
  * .mmap_prepare() hook, as we are in a different context when we invoke the
  * .mmap() hook, already having a VMA to deal with.
  *
- * compat_vma_mmap_prepare() is a compatibility function that takes VMA state,
+ * compat_vma_mmap() is a compatibility function that takes VMA state,
  * establishes a struct vm_area_desc descriptor, passes to the underlying
  * .mmap_prepare() hook and applies any changes performed by it.
  *
···
  *
  * Returns: 0 on success or error.
  */
-int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
+int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	return __compat_vma_mmap_prepare(file->f_op, file, vma);
+	return __compat_vma_mmap(file->f_op, file, vma);
 }
-EXPORT_SYMBOL(compat_vma_mmap_prepare);
+EXPORT_SYMBOL(compat_vma_mmap);
 
 static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
 			 const struct page *page)
···
 		ps->idx = 0;
 	}
 }
+
+static int mmap_action_finish(struct mmap_action *action,
+		const struct vm_area_struct *vma, int err)
+{
+	/*
+	 * If an error occurs, unmap the VMA altogether and return an error. We
+	 * only clear the newly allocated VMA, since this function is only
+	 * invoked if we do NOT merge, so we only clean up the VMA we created.
+	 */
+	if (err) {
+		const size_t len = vma_pages(vma) << PAGE_SHIFT;
+
+		do_munmap(current->mm, vma->vm_start, len, NULL);
+
+		if (action->error_hook) {
+			/* We may want to filter the error. */
+			err = action->error_hook(err);
+
+			/* The caller should not clear the error. */
+			VM_WARN_ON_ONCE(!err);
+		}
+		return err;
+	}
+
+	if (action->success_hook)
+		return action->success_hook(vma);
+
+	return 0;
+}
+
+#ifdef CONFIG_MMU
+/**
+ * mmap_action_prepare - Perform preparatory setup for an VMA descriptor
+ * action which need to be performed.
+ * @desc: The VMA descriptor to prepare for @action.
+ * @action: The action to perform.
+ */
+void mmap_action_prepare(struct mmap_action *action,
+			 struct vm_area_desc *desc)
+{
+	switch (action->type) {
+	case MMAP_NOTHING:
+		break;
+	case MMAP_REMAP_PFN:
+		remap_pfn_range_prepare(desc, action->remap.start_pfn);
+		break;
+	case MMAP_IO_REMAP_PFN:
+		io_remap_pfn_range_prepare(desc, action->remap.start_pfn,
+					   action->remap.size);
+		break;
+	}
+}
+EXPORT_SYMBOL(mmap_action_prepare);
+
+/**
+ * mmap_action_complete - Execute VMA descriptor action.
+ * @action: The action to perform.
+ * @vma: The VMA to perform the action upon.
+ *
+ * Similar to mmap_action_prepare().
+ *
+ * Return: 0 on success, or error, at which point the VMA will be unmapped.
+ */
+int mmap_action_complete(struct mmap_action *action,
+			 struct vm_area_struct *vma)
+{
+	int err = 0;
+
+	switch (action->type) {
+	case MMAP_NOTHING:
+		break;
+	case MMAP_REMAP_PFN:
+		err = remap_pfn_range_complete(vma, action->remap.start,
+				action->remap.start_pfn, action->remap.size,
+				action->remap.pgprot);
+		break;
+	case MMAP_IO_REMAP_PFN:
+		err = io_remap_pfn_range_complete(vma, action->remap.start,
+				action->remap.start_pfn, action->remap.size,
+				action->remap.pgprot);
+		break;
+	}
+
+	return mmap_action_finish(action, vma, err);
+}
+EXPORT_SYMBOL(mmap_action_complete);
+#else
+void mmap_action_prepare(struct mmap_action *action,
+			struct vm_area_desc *desc)
+{
+	switch (action->type) {
+	case MMAP_NOTHING:
+		break;
+	case MMAP_REMAP_PFN:
+	case MMAP_IO_REMAP_PFN:
+		WARN_ON_ONCE(1); /* nommu cannot handle these. */
+		break;
+	}
+}
+EXPORT_SYMBOL(mmap_action_prepare);
+
+int mmap_action_complete(struct mmap_action *action,
+			struct vm_area_struct *vma)
+{
+	int err = 0;
+
+	switch (action->type) {
+	case MMAP_NOTHING:
+		break;
+	case MMAP_REMAP_PFN:
+	case MMAP_IO_REMAP_PFN:
+		WARN_ON_ONCE(1); /* nommu cannot handle this. */
+
+		err = -EINVAL;
+		break;
+	}
+
+	return mmap_action_finish(action, vma, err);
+}
+EXPORT_SYMBOL(mmap_action_complete);
+#endif
 
 #ifdef CONFIG_MMU
 /**

mm/vma.c

···
 	struct maple_tree mt_detach;
 
 	/* Determine if we can check KSM flags early in mmap() logic. */
-	bool check_ksm_early;
+	bool check_ksm_early :1;
+	/* If we map new, hold the file rmap lock on mapping. */
+	bool hold_file_rmap_lock :1;
 };
 
 #define MMAP_STATE(name, mm_, vmi_, addr_, len_, pgoff_, vm_flags_, file_) \
···
 		unlink_file_vma_batch_process(vb);
 }
 
-static void vma_link_file(struct vm_area_struct *vma)
+static void vma_link_file(struct vm_area_struct *vma, bool hold_rmap_lock)
 {
 	struct file *file = vma->vm_file;
 	struct address_space *mapping;
···
 		mapping = file->f_mapping;
 		i_mmap_lock_write(mapping);
 		__vma_link_file(vma, mapping);
-		i_mmap_unlock_write(mapping);
+		if (!hold_rmap_lock)
+			i_mmap_unlock_write(mapping);
 	}
 }
 
···
 
 	vma_start_write(vma);
 	vma_iter_store_new(&vmi, vma);
-	vma_link_file(vma);
+	vma_link_file(vma, /* hold_rmap_lock= */false);
 	mm->map_count++;
 	validate_mm(mm);
 	return 0;
···
 	map->vm_flags = ksm_vma_flags(map->mm, map->file, map->vm_flags);
 }
 
+static void set_desc_from_map(struct vm_area_desc *desc,
+		const struct mmap_state *map)
+{
+	desc->start = map->addr;
+	desc->end = map->end;
+
+	desc->pgoff = map->pgoff;
+	desc->vm_file = map->file;
+	desc->vm_flags = map->vm_flags;
+	desc->page_prot = map->page_prot;
+}
+
 /*
  * __mmap_setup() - Prepare to gather any overlapping VMAs that need to be
  * unmapped once the map operation is completed, check limits, account mapping
  * and clean up any pre-existing VMAs.
  *
+ * As a result it sets up the @map and @desc objects.
+ *
  * @map: Mapping state.
+ * @desc: VMA descriptor
  * @uf:  Userfaultfd context list.
  *
  * Returns: 0 on success, error code otherwise.
  */
-static int __mmap_setup(struct mmap_state *map, struct list_head *uf)
+static int __mmap_setup(struct mmap_state *map, struct vm_area_desc *desc,
+			struct list_head *uf)
 {
 	int error;
 	struct vma_iterator *vmi = map->vmi;
···
 	 */
 	vms_clean_up_area(vms, &map->mas_detach);
 
+	set_desc_from_map(desc, map);
 	return 0;
 }
 
···
 	vma_start_write(vma);
 	vma_iter_store_new(vmi, vma);
 	map->mm->map_count++;
-	vma_link_file(vma);
+	vma_link_file(vma, map->hold_file_rmap_lock);
 
 	/*
 	 * vma_merge_new_range() calls khugepaged_enter_vma() too, the below
···
 	vma_set_page_prot(vma);
 }
 
+static void call_action_prepare(struct mmap_state *map,
+				struct vm_area_desc *desc)
+{
+	struct mmap_action *action = &desc->action;
+
+	mmap_action_prepare(action, desc);
+
+	if (action->hide_from_rmap_until_complete)
+		map->hold_file_rmap_lock = true;
+}
+
 /*
  * Invoke the f_op->mmap_prepare() callback for a file-backed mapping that
  * specifies it.
···
  *
  * Returns 0 on success, or an error code otherwise.
  */
-static int call_mmap_prepare(struct mmap_state *map)
+static int call_mmap_prepare(struct mmap_state *map,
+		struct vm_area_desc *desc)
 {
 	int err;
-	struct vm_area_desc desc = {
-		.mm = map->mm,
-		.file = map->file,
-		.start = map->addr,
-		.end = map->end,
-
-		.pgoff = map->pgoff,
-		.vm_file = map->file,
-		.vm_flags = map->vm_flags,
-		.page_prot = map->page_prot,
-	};
 
 	/* Invoke the hook. */
-	err = vfs_mmap_prepare(map->file, &desc);
+	err = vfs_mmap_prepare(map->file, desc);
 	if (err)
 		return err;
 
+	call_action_prepare(map, desc);
+
 	/* Update fields permitted to be changed. */
-	map->pgoff = desc.pgoff;
-	map->file = desc.vm_file;
-	map->vm_flags = desc.vm_flags;
-	map->page_prot = desc.page_prot;
+	map->pgoff = desc->pgoff;
+	map->file = desc->vm_file;
+	map->vm_flags = desc->vm_flags;
+	map->page_prot = desc->page_prot;
 	/* User-defined fields. */
-	map->vm_ops = desc.vm_ops;
-	map->vm_private_data = desc.private_data;
+	map->vm_ops = desc->vm_ops;
+	map->vm_private_data = desc->private_data;
 
 	return 0;
 }
···
 	return false;
 }
 
+static int call_action_complete(struct mmap_state *map,
+				struct vm_area_desc *desc,
+				struct vm_area_struct *vma)
+{
+	struct mmap_action *action = &desc->action;
+	int ret;
+
+	ret = mmap_action_complete(action, vma);
+
+	/* If we held the file rmap we need to release it. */
+	if (map->hold_file_rmap_lock) {
+		struct file *file = vma->vm_file;
+
+		i_mmap_unlock_write(file->f_mapping);
+	}
+	return ret;
+}
+
 static unsigned long __mmap_region(struct file *file, unsigned long addr,
 		unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
 		struct list_head *uf)
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma = NULL;
-	int error;
 	bool have_mmap_prepare = file && file->f_op->mmap_prepare;
 	VMA_ITERATOR(vmi, mm, addr);
 	MMAP_STATE(map, mm, &vmi, addr, len, pgoff, vm_flags, file);
+	struct vm_area_desc desc = {
+		.mm = mm,
+		.file = file,
+		.action = {
+			.type = MMAP_NOTHING, /* Default to no further action. */
+		},
+	};
+	bool allocated_new = false;
+	int error;
 
 	map.check_ksm_early = can_set_ksm_flags_early(&map);
 
-	error = __mmap_setup(&map, uf);
+	error = __mmap_setup(&map, &desc, uf);
 	if (!error && have_mmap_prepare)
-		error = call_mmap_prepare(&map);
+		error = call_mmap_prepare(&map, &desc);
 	if (error)
 		goto abort_munmap;
 
···
 		error = __mmap_new_vma(&map, &vma);
 		if (error)
 			goto unacct_error;
+		allocated_new = true;
 	}
 
 	if (have_mmap_prepare)
 		set_vma_user_defined_fields(vma, &map);
 
 	__mmap_complete(&map, vma);
+
+	if (have_mmap_prepare && allocated_new) {
+		error = call_action_complete(&map, &desc, vma);
+
+		if (error)
+			return error;
+	}
 
 	return addr;
 

tools/testing/vma/vma_internal.h

···
 
 struct vm_area_struct;
 
+
+/* What action should be taken after an .mmap_prepare call is complete? */
+enum mmap_action_type {
+	MMAP_NOTHING,		/* Mapping is complete, no further action. */
+	MMAP_REMAP_PFN,		/* Remap PFN range. */
+	MMAP_IO_REMAP_PFN,	/* I/O remap PFN range. */
+};
+
+/*
+ * Describes an action an mmap_prepare hook can instruct to be taken to complete
+ * the mapping of a VMA. Specified in vm_area_desc.
+ */
+struct mmap_action {
+	union {
+		/* Remap range. */
+		struct {
+			unsigned long start;
+			unsigned long start_pfn;
+			unsigned long size;
+			pgprot_t pgprot;
+		} remap;
+	};
+	enum mmap_action_type type;
+
+	/*
+	 * If specified, this hook is invoked after the selected action has been
+	 * successfully completed. Note that the VMA write lock still held.
+	 *
+	 * The absolute minimum ought to be done here.
+	 *
+	 * Returns 0 on success, or an error code.
+	 */
+	int (*success_hook)(const struct vm_area_struct *vma);
+
+	/*
+	 * If specified, this hook is invoked when an error occurred when
+	 * attempting the selection action.
+	 *
+	 * The hook can return an error code in order to filter the error, but
+	 * it is not valid to clear the error here.
+	 */
+	int (*error_hook)(int err);
+
+	/*
+	 * This should be set in rare instances where the operation required
+	 * that the rmap should not be able to access the VMA until
+	 * completely set up.
+	 */
+	bool hide_from_rmap_until_complete :1;
+};
+
 /*
  * Describes a VMA that is about to be mmap()'ed. Drivers may choose to
  * manipulate mutable fields which will cause those fields to be updated in the
···
 	/* Write-only fields. */
 	const struct vm_operations_struct *vm_ops;
 	void *private_data;
+
+	/* Take further action? */
+	struct mmap_action action;
 };
 
 struct file_operations {
···
 static inline void set_vma_from_desc(struct vm_area_struct *vma,
 		struct vm_area_desc *desc);
 
-static inline int __compat_vma_mmap_prepare(const struct file_operations *f_op,
+static inline void mmap_action_prepare(struct mmap_action *action,
+					   struct vm_area_desc *desc)
+{
+}
+
+static inline int mmap_action_complete(struct mmap_action *action,
+					   struct vm_area_struct *vma)
+{
+	return 0;
+}
+
+static inline int __compat_vma_mmap(const struct file_operations *f_op,
 		struct file *file, struct vm_area_struct *vma)
 {
 	struct vm_area_desc desc = {
 		.mm = vma->vm_mm,
-		.file = vma->vm_file,
+		.file = file,
 		.start = vma->vm_start,
 		.end = vma->vm_end,
 
···
 		.vm_file = vma->vm_file,
 		.vm_flags = vma->vm_flags,
 		.page_prot = vma->vm_page_prot,
+
+		.action.type = MMAP_NOTHING, /* Default */
 	};
 	int err;
 
 	err = f_op->mmap_prepare(&desc);
 	if (err)
 		return err;
-	set_vma_from_desc(vma, &desc);
 
-	return 0;
+	mmap_action_prepare(&desc.action, &desc);
+	set_vma_from_desc(vma, &desc);
+	return mmap_action_complete(&desc.action, vma);
 }
 
-static inline int compat_vma_mmap_prepare(struct file *file,
+static inline int compat_vma_mmap(struct file *file,
 		struct vm_area_struct *vma)
 {
-	return __compat_vma_mmap_prepare(file->f_op, file, vma);
+	return __compat_vma_mmap(file->f_op, file, vma);
 }
 
 /* Did the driver provide valid mmap hook configuration? */
···
 static inline int vfs_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	if (file->f_op->mmap_prepare)
-		return compat_vma_mmap_prepare(file, vma);
+		return compat_vma_mmap(file, vma);
 
 	return file->f_op->mmap(file, vma);
 }
···
 		const struct file *file, vm_flags_t vm_flags)
 {
 	return vm_flags;
+}
+
+static inline void remap_pfn_range_prepare(struct vm_area_desc *desc, unsigned long pfn)
+{
+}
+
+static inline int remap_pfn_range_complete(struct vm_area_struct *vma, unsigned long addr,
+		unsigned long pfn, unsigned long size, pgprot_t pgprot)
+{
+	return 0;
+}
+
+static inline int do_munmap(struct mm_struct *, unsigned long, size_t,
+		struct list_head *uf)
+{
+	return 0;
 }
 
 #endif	/* __MM_VMA_INTERNAL_H */