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

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Mar 12, 2026 at 1:27 PM Lorenzo Stoakes (Oracle) <ljs@kernel.org> wrote:
>
> 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
> new file mode 100644
> index 000000000000..76908200f3a1
> --- /dev/null
> +++ b/Documentation/filesystems/mmap_prepare.rst
> @@ -0,0 +1,131 @@
> +.. 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;

So, action still belongs to /* Write-only fields. */ section? This is
nitpicky, but it might be better to have this as:

        /* Write-only fields. */
        const struct vm_operations_struct *vm_ops;
        void *private_data;
        struct mmap_action action; /* Take further 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.

I'm guessing the start and size parameters passed to
mmap_action_remap() and such are restricted by vm_area_desc.start
vm_area_desc.end. If so, should we document those restrictions and
enforce them in the code?

> +    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;


> --
> 2.53.0
>