[PATCH 02/15] mm: add documentation for the mmap_prepare file operation callback

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

This documentation makes it easier for a driver/file system implementer to
correctly use this callback.

It covers the fundamentals, whilst intentionally leaving the less lovely
possible actions one might take undocumented (for instance - the
success_hook, error_hook fields in mmap_action).

The document also covers the new VMA flags implementation which is the only
one which will work correctly with mmap_prepare.

Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
---
 Documentation/filesystems/mmap_prepare.rst | 131 +++++++++++++++++++++
 1 file changed, 131 insertions(+)
 create mode 100644 Documentation/filesystems/mmap_prepare.rst

diff --git a/Documentation/filesystems/mmap_prepare.rst b/Documentation/filesystems/mmap_prepare.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+===========================
+mmap_prepare callback HOWTO
+===========================
+
+Introduction
+############
+
+The `struct file->f_op->mmap()` callback has been deprecated as it is both a
+stability and security risk, and doesn't always permit the merging of adjacent
+mappings resulting in unnecessary memory fragmentation.
+
+It has been replaced with the `file->f_op->mmap_prepare()` callback which solves
+these problems.
+
+## How To Use
+
+In your driver's `struct file_operations` struct, specify an `mmap_prepare`
+callback rather than an `mmap` one, e.g. for ext4:
+
+
+.. code-block:: C
+
+    const struct file_operations ext4_file_operations = {
+        ...
+        .mmap_prepare    = ext4_file_mmap_prepare,
+    };
+
+This has a signature of `int (*mmap_prepare)(struct vm_area_desc *)`.
+
+Examining the `struct vm_area_desc` type:
+
+.. code-block:: C
+
+    struct vm_area_desc {
+        /* Immutable state. */
+        const struct mm_struct *const mm;
+        struct file *const file; /* May vary from vm_file in stacked callers. */
+        unsigned long start;
+        unsigned long end;
+
+        /* Mutable fields. Populated with initial state. */
+        pgoff_t pgoff;
+        struct file *vm_file;
+        vma_flags_t vma_flags;
+        pgprot_t page_prot;
+
+        /* Write-only fields. */
+        const struct vm_operations_struct *vm_ops;
+        void *private_data;
+
+        /* Take further action? */
+        struct mmap_action action;
+    };
+
+This is straightforward - you have all the fields you need to set up the
+mapping, and you can update the mutable and writable fields, for instance:
+
+.. code-block:: Cw
+
+    static int ext4_file_mmap_prepare(struct vm_area_desc *desc)
+    {
+        int ret;
+        struct file *file = desc->file;
+        struct inode *inode = file->f_mapping->host;
+
+        ...
+
+        file_accessed(file);
+        if (IS_DAX(file_inode(file))) {
+            desc->vm_ops = &ext4_dax_vm_ops;
+            vma_desc_set_flags(desc, VMA_HUGEPAGE_BIT);
+        } else {
+            desc->vm_ops = &ext4_file_vm_ops;
+        }
+        return 0;
+    }
+
+Importantly, you no longer have to dance around with reference counts or locks
+when updating these fields - __you can simply go ahead and change them__.
+
+Everything is taken care of by the mapping code.
+
+VMA Flags
+=========
+
+Along with `mmap_prepare`, VMA flags have undergone an overhaul. Where before
+you would invoke one of `vm_flags_init()`, `vm_flags_reset()`, `vm_flags_set()`,
+`vm_flags_clear()`, and `vm_flags_mod()` to modify flags (and to have the
+locking done correctly for you, this is no longer necessary.
+
+Also, the legacy approach of specifying VMA flags via `VM_READ`, `VM_WRITE`,
+etc. - i.e. using a `VM_xxx` macro has changed too.
+
+When implementing `mmap_prepare()`, reference flags by their bit number, defined
+as a `VMA_xxx_BIT` macro, e.g. `VMA_READ_BIT`, `VMA_WRITE_BIT` etc., and use one
+of (where `desc` is a pointer to `struct vma_area_desc`):
+
+* `vma_desc_test_flags(desc, ...)` - Specify a comma-separated list of flags you
+  wish to test for (whether _any_ are set), e.g. - `vma_desc_test_flags(desc,
+  VMA_WRITE_BIT, VMA_MAYWRITE_BIT)` - returns `true` if either are set,
+  otherwise `false`.
+* `vma_desc_set_flags(desc, ...)` - Update the VMA descriptor flags to set
+  additional flags specified by a comma-separated list,
+  e.g. - `vma_desc_set_flags(desc, VMA_PFNMAP_BIT, VMA_IO_BIT)`.
+* `vma_desc_clear_flags(desc, ...)` - Update the VMA descriptor flags to clear
+  flags specified by a comma-separated list, e.g. - `vma_desc_clear_flags(desc,
+  VMA_WRITE_BIT, VMA_MAYWRITE_BIT)`.
+
+Actions
+=======
+
+You can now very easily have actions be performed upon a mapping once set up by
+utilising simple helper functions invoked upon the `struct vm_area_desc`
+pointer. These are:
+
+* `mmap_action_remap()` - Remaps a range consisting only of PFNs for a specific
+  range starting a virtual address and PFN number of a set size.
+
+* `mmap_action_remap_full()` - Same as `mmap_action_remap()`, only remaps the
+  entire mapping from `start_pfn` onward.
+
+* `mmap_action_ioremap()` - Same as `mmap_action_remap()`, only performs an I/O
+  remap.
+
+* `mmap_action_ioremap_full()` - Same as `mmap_action_ioremap()`, only remaps
+  the entire mapping from `start_pfn` onward.
+
+**NOTE:** The 'action' field should never normally be manipulated directly,
+rather you ought to use one of these helpers.
-- 
2.53.0