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) <[email protected]> --- Documentation/filesystems/index.rst | 1 + Documentation/filesystems/mmap_prepare.rst | 142 +++++++++++++++++++++ 2 files changed, 143 insertions(+) create mode 100644 Documentation/filesystems/mmap_prepare.rst diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst index f4873197587d..6cbc3e0292ae 100644 --- a/Documentation/filesystems/index.rst +++ b/Documentation/filesystems/index.rst @@ -29,6 +29,7 @@ algorithms work. fiemap files locks + mmap_prepare multigrain-ts mount_api quota diff --git a/Documentation/filesystems/mmap_prepare.rst b/Documentation/filesystems/mmap_prepare.rst new file mode 100644 index 000000000000..65a1f094e469 --- /dev/null +++ b/Documentation/filesystems/mmap_prepare.rst @@ -0,0 +1,142 @@ +.. 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. + +This hook is called right at the beginning of setting up the mapping, and +importantly it is invoked *before* any merging of adjacent mappings has taken +place. + +If an error arises upon mapping, it might arise after this callback has been +invoked, therefore it should be treated as effectively stateless. + +That is - no resources should be allocated nor state updated to reflect that a +mapping has been established, as the mapping may either be merged, or fail to be +mapped after the callback is complete. + +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:: C + + 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 vm_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
