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

Lorenzo Stoakes (Oracle)
2026-03-16T14:52:02Z

On Thu, Mar 12, 2026 at 05:12:04PM -0700, Randy Dunlap wrote:
> (Andrew: patch attached)
>
>
> On 3/12/26 1:27 PM, Lorenzo Stoakes (Oracle) wrote:
>
> Documentation/filesystems/mmap_prepare.rst: WARNING: document isn't included in any toctree [toc.not_included]
>
> Should be in some index.rst file. In filesystems I suppose.

Ack thanks.

>
> > ---
> > 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
> > +############
>
> Kernel style is "=============" above instead of "############".

Ack

>
> > +
> > +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
>
> .. code-block:: C
>
> Documentation/filesystems/mmap_prepare.rst:60: WARNING: Pygments lexer name 'Cw' is not known [misc.highlighting_failure]
>
> Maybe a typo?

Yeah is a typo thanks!

>
> > +
> > + 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
> > +=========
>
> and then use "---------------" here instead of "==============".

Ack

>
> (from Documentation/doc-guide/sphinx.rst)
>
> > +
> > +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 also see this warning, but I don't know what it is referring to:
>
> Documentation/filesystems/mmap_prepare.rst:132: ERROR: Anonymous hyperlink mismatch: 1 references but 0 targets.
> See "backrefs" attribute for IDs. [docutils]
>
> (OK, I found/fixed that also.)
>
> There are also lots of single ` marks which mean italics. I thought those were
> not what was intended, so I changed (most of) them to `` marks, which means
> "code block / monospace". I can fix those if needed.
>
> from the patch file:
> @Lorenzo: ISTR that you prefer explicit quoting on structs and
> functions. I didn't do that here since kernel automarkup does that,
> but if you prefer, I can redo the patch with those changes.

The issue was in another document it didn't seem to properly recognise the types
AFAICT (but I might have been mistaken anyway!) But I'm fine without.

>
> HTH.
> --
> ~Randy

Thanks for this, will fold the patch into the respin also!

Cheers, Lorenzo