[PATCH 07/16] doc: update porting, vfs documentation for mmap_[complete, abort]

Lorenzo Stoakes
2025-09-08T11:12:17Z

We have introduced the mmap_complete() and mmap_abort() callbacks, which
work in conjunction with mmap_prepare(), so describe what they used for.

We update both the VFS documentation and the porting guide.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
Documentation/filesystems/porting.rst | 9 +++++++
Documentation/filesystems/vfs.rst | 35 +++++++++++++++++++++++++++
2 files changed, 44 insertions(+)

diff --git a/Documentation/filesystems/porting.rst b/Documentation/filesystems/porting.rst
index 85f590254f07..abc1b8c95d24 100644
--- a/Documentation/filesystems/porting.rst
+++ b/Documentation/filesystems/porting.rst
@@ -1285,3 +1285,12 @@ rather than a VMA, as the VMA at this stage is not yet valid.
The vm_area_desc provides the minimum required information for a filesystem
to initialise state upon memory mapping of a file-backed region, and output
parameters for the file system to set this state.
+
+In nearly all cases, this is all that is required for a filesystem. However,
+should there be a need to operate on the newly inserted VMA, the mmap_complete()
+can be specified to do so.
+
+Additionally, if mmap_prepare() and mmap_complete() are specified, mmap_abort()
+may also be provided which is invoked if the mapping fails between mmap_prepare
+and mmap_complete(). It is only valid to specify mmap_abort() if both other
+hooks are provided.
diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst
index 486a91633474..172d36a13e13 100644
--- a/Documentation/filesystems/vfs.rst
+++ b/Documentation/filesystems/vfs.rst
@@ -1114,6 +1114,10 @@ This describes how the VFS can manipulate an open file. As of kernel
int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *,
unsigned int poll_flags);
int (*mmap_prepare)(struct vm_area_desc *);
+ int (*mmap_complete)(struct file *, struct vm_area_struct *,
+ const void *context);
+ void (*mmap_abort)(const struct file *, const void *vm_private_data,
+ const void *context);
};

Again, all methods are called without any locks being held, unless
@@ -1236,6 +1240,37 @@ otherwise noted.
file-backed memory mapping, most notably establishing relevant
private state and VMA callbacks.

+``mmap_complete``
+ If mmap_prepare is provided, will be invoked after the mapping is fully
+ established, with the mmap and VMA write locks held.
+
+ It is useful for prepopulating VMAs before they may be accessed by
+ users.
+
+ The hook MUST NOT release either the VMA or mmap write locks. This is
+ asserted by the mmap logic.
+
+ If an error is returned by the hook, the VMA is unmapped and the
+ mmap() operation fails with that error.
+
+ It is not valid to specify this hook if mmap_prepare is not also
+ specified, doing so will result in an error upon mapping.
+
+``mmap_abort``
+ If mmap_prepare() and mmap_complete() are provided, then mmap_abort
+ may also be provided, which will be invoked if the mapping operation
+ fails between the two calls.
+
+ This is important, because mmap_prepare may succeed, but some other part
+ of the mapping operation may fail before mmap_complete can be called.
+
+ This allows a caller to acquire locks in mmap_prepare with certainty
+ that the locks will be released by either mmap_abort or mmap_complete no
+ matter what happens.
+
+ It is not valid to specify this unless mmap_prepare and mmap_complete
+ are both specified, doing so will result in an error upon mapping.
+
Note that the file operations are implemented by the specific
filesystem in which the inode resides. When opening a device node
(character or block special) most filesystems will call special
--
2.51.0