Re: [PATCH 06/16] mm: introduce the f_op->mmap_complete, mmap_abort hooks

[David Hildenbrand <david@redhat.com>]

On 08.09.25 13:10, Lorenzo Stoakes wrote:
> We have introduced the f_op->mmap_prepare hook to allow for setting up a
> VMA far earlier in the process of mapping memory, reducing problematic
> error handling paths, but this does not provide what all
> drivers/filesystems need.
> 
> In order to supply this, and to be able to move forward with removing
> f_op->mmap altogether, introduce f_op->mmap_complete.
> 
> This hook is called once the VMA is fully mapped and everything is done,
> however with the mmap write lock and VMA write locks held.
> 
> The hook is then provided with a fully initialised VMA which it can do what
> it needs with, though the mmap and VMA write locks must remain held
> throughout.
> 
> It is not intended that the VMA be modified at this point, attempts to do
> so will end in tears.
> 
> This allows for operations such as pre-population typically via a remap, or
> really anything that requires access to the VMA once initialised.
> 
> In addition, a caller may need to take a lock in mmap_prepare, when it is
> possible to modify the VMA, and release it on mmap_complete. In order to
> handle errors which may arise between the two operations, f_op->mmap_abort
> is provided.
> 
> This hook should be used to drop any lock and clean up anything before the
> VMA mapping operation is aborted. After this point the VMA will not be
> added to any mapping and will not exist.
> 
> We also add a new mmap_context field to the vm_area_desc type which can be
> used to pass information pertinent to any locks which are held or any state
> which is required for mmap_complete, abort to operate correctly.
> 
> We also update the compatibility layer for nested filesystems which
> currently still only specify an f_op->mmap() handler so that it correctly
> invokes f_op->mmap_complete as necessary (note that no error can occur
> between mmap_prepare and mmap_complete so mmap_abort will never be called
> in this case).
> 
> Also update the VMA tests to account for the changes.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>   include/linux/fs.h               |  4 ++
>   include/linux/mm_types.h         |  5 ++
>   mm/util.c                        | 18 +++++--
>   mm/vma.c                         | 82 ++++++++++++++++++++++++++++++--
>   tools/testing/vma/vma_internal.h | 31 ++++++++++--
>   5 files changed, 129 insertions(+), 11 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 594bd4d0521e..bb432924993a 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2195,6 +2195,10 @@ struct file_operations {
>   	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);

Do we have a description somewhere what these things do, when they are 
called, and what a driver may be allowed to do with a VMA?

In particular, the mmap_complete() looks like another candidate for 
letting a driver just go crazy on the vma? :)

-- 
Cheers

David / dhildenb