On 09.08.22 22:30, Linus Torvalds wrote: > On Tue, Aug 9, 2022 at 1:20 PM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >> IIUC VM_MAYSHARE is always set in a MAP_SHARED mapping, but for file >> mappings we only set VM_SHARED if the file allows for writes > > Heh. > > This is a horrific hack, and probably should go away. > > Yeah, we have that > > if (!(file->f_mode & FMODE_WRITE)) > vm_flags &= ~(VM_MAYWRITE | VM_SHARED); > > > but I think that's _entirely_ historical. > > Long long ago, in a galaxy far away, we didn't handle shared mmap() > very well. In fact, we used to not handle it at all. > > But nntpd would use write() to update the spool file, adn them read it > through a shared mmap. > > And since our mmap() *was* coherent with people doing write() system > calls, but didn't handle actual dirty shared mmap, what Linux used to > do was to just say "Oh, you want a read-only shared file mmap? I can > do that - I'll just downgrade it to a read-only _private_ mapping, and > it actually ends up with the same semantics". > > And here we are, 30 years later, and it still does that, but it leaves > the VM_MAYSHARE flag so that /proc/<pid>/maps can show that it's a > shared mapping. I was suspecting that this code is full of legacy :) What would make sense to me is to just have VM_SHARED and make it correspond to MAP_SHARED, that would at least confuse me less. Once I have some spare cycles I'll see how easy that might be to achieve. -- Thanks, David / dhildenb
