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Howlett wrote: > * David Hildenbrand [250430 17:58]: > > 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 > > > > > > 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 ... > > Although I don't really mind what we call this, I don't like the flags > name. Can we qualify it with vm_flags? It looks dumb most of the time > but we have had variables named "flags" set to the wrong flag type make > it through code review and into the kernel. Sure, will do! > > That is, we may see people set a struct vma_proto proto later do > proto.flags = map_flags. It sounds stupid here, but we have had cases > of exactly this making it through to a kernel release. > > I bring this up here because it may influence the prefix of the setup > call, or vice versa... and not _just_ to derail another renaming. ;) Yeah, 'flags' is one of the more ambigious names in the kernel generally... I did go back and forth on this one but this is a good point, and it's an easy mistake to make, sadly... > > > > > > > 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 > > >