* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> [230803 14:02]: > On Thu, 3 Aug 2023 at 10:27, Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > While it's not strictly necessary to lock a newly created vma before > > adding it into the vma tree (as long as no further changes are performed > > to it), it seems like a good policy to lock it and prevent accidental > > changes after it becomes visible to the page faults. Lock the vma before > > adding it into the vma tree. > > So my main reaction here is that I started to wonder about the vma allocation. > > Why doesn't vma_init() do something like > > mmap_assert_write_locked(mm); > vma->vm_lock_seq = mm->mm_lock_seq; > > and instead we seem to expect vma_lock_alloc() to do this (and do it > very badly indeed). > > Strange. > > Anyway, this observation was just a reaction to that "not strictly > necessary to lock a newly created vma" part of the commentary. I feel > like we could/should just make sure that all newly created vma's are > always simply created write-locked. > I thought the same thing initially, but Suren pointed out that it's not necessary to hold the vma lock to allocate a vma object. And it seems there is at least one user (arch/ia64/mm/init.c) which does allocate outside the lock during ia64_init_addr_space(), which is fine but I'm not sure it gains much to do it this way - the insert needs to take the lock anyways and it is hardly going to be contended. Anywhere else besides an address space setup would probably introduce a race. Thanks, Liam
