On 09.08.22 22:14, Linus Torvalds wrote: > On Tue, Aug 9, 2022 at 1:07 PM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >>> /* But FOLL_FORCE has no effect on shared mappings */ >>> if (vma->vm_flags & MAP_SHARED) >>> return false; >> >> I'd actually rather check for MAP_MAYSHARE here, which is even stronger. >> Thoughts? > > Hmm. Adding the test for both is basically free (all those vm_flags > checks end up being a bit mask and compare), so no objections. > > For some reason I though VM_SHARED and VM_MAYSHARE end up always being > the same bits, and it was a mistake to make them two bits in the first > place (unlike the read/write/exec bits that can are about mprotect), > > But as there are two bits, I'm sure somebody ends up touching one and > not the other. > > So yeah, I don't see any downside to just checking both bits. > > [ That said, is_cow_mapping() only checks VM_SHARED, so if they are > ever different, that's a potential source of confusion ] 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 (and we can set VM_MAYWRITE or even VM_WRITE). [don't ask me why, it's a steady source of confusion] That's why is_cow_mapping() works even due to the weird MAP_SHARED vs. VM_MAYSHARE logic. I'd actually only check for VM_MAYSHARE here, which implies MAP_SHARED. If someone would ever mess that up, I guess we'd be in bigger trouble. But whatever you prefer. -- Thanks, David / dhildenb
