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

[David Hildenbrand <david@redhat.com>]

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?


-- 
Cheers,

David / dhildenb