Re: [RFC PATCH 1/3] mm: introduce new .mmap_proto() f_op callback

[Christian Brauner <brauner@kernel.org>]

On Wed, Apr 30, 2025 at 11:58:14PM +0200, David Hildenbrand wrote:
> On 30.04.25 21:54, 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_proto() 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.
> > 
> > Update vma userland test stubs to account for changes.
> > 
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> 
> 
> I really don't like the "proto" terminology. :)
> 
> [yes, David and his naming :P ]
> 
> No, the problem is that it is fairly unintuitive what is happening here.
> 
> Coming from a different direction, the callback is trigger after
> __mmap_prepare() ... could we call it "->mmap_prepare" or something like
> that? (mmap_setup, whatever)
> 
> Maybe mmap_setup and vma_setup_param? Just a thought ...
> 
> 
> In general (although it's late in Germany), it does sound like an
> interesting approach.
> 
> How feasiable is it to remove ->mmap in the long run, and would we maybe
> need other callbacks to make that possible?

If mm needs new file operations that aim to replace the old ->mmap() I
want the old method to be ripped out within a reasonable time frame. I
don't want to have ->mmap() and ->mmap_$new() hanging around for the
next 5 years. We have enough of that already. And it would be great to
be clear whether that replacement can actually happen.