On Fri, May 09, 2025 at 01:13:34PM +0100, Lorenzo Stoakes wrote:
> Provide a means by which drivers can specify which fields of those
> permitted to be changed should be altered to prior to mmap()'ing a
> range (which may either result from a merge or from mapping an entirely new
> VMA).
>
> Doing so is substantially safer than the existing .mmap() calback which
> provides unrestricted access to the part-constructed VMA and permits
> drivers and file systems to do 'creative' things which makes it hard to
> reason about the state of the VMA after the function returns.
>
> The existing .mmap() callback's freedom has caused a great deal of issues,
> especially in error handling, as unwinding the mmap() state has proven to
> be non-trivial and caused significant issues in the past, for instance
> those addressed in commit 5de195060b2e ("mm: resolve faulty mmap_region()
> error path behaviour").
>
> It also necessitates a second attempt at merge once the .mmap() callback
> has completed, which has caused issues in the past, is awkward, adds
> overhead and is difficult to reason about.
>
> The .mmap_prepare() callback eliminates this requirement, as we can update
> fields prior to even attempting the first merge. It is safer, as we heavily
> restrict what can actually be modified, and being invoked very early in the
> mmap() process, error handling can be performed safely with very little
> unwinding of state required.
>
> The .mmap_prepare() and deprecated .mmap() callbacks are mutually
> exclusive, so we permit only one to be invoked at a time.
>
> Update vma userland test stubs to account for changes.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Reviewed-by: Pedro Falcato <pfalcato@xxxxxxx>
Neat idea, thanks. This should also help out with the insane proliferation of
vm_flags_set in ->mmap() callbacks all over. Hopefully.
However, could we:
1) Add a small writeup to Documentation/filesystems/vfs.rst for this callback
2) Possibly add a ->mmap_finish()? With a fully constructed vma at that point.
So things like remap_pfn_range can still be used by drivers' mmap()
implementation.
1) is particularly important so our VFS and driver friends know this is supposed
to be The Way Forward.
--
Pedro
