On Thu, Aug 3, 2023 at 11:15 AM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote: > > * 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. Yeah, I remember discussing that. At the time of VMA creation the mmap_lock might not be write-locked, so mmap_assert_write_locked() would trigger and mm->mm_lock_seq is not stable. Maybe we can necessitate holding mmap_lock at the time of VMA creation but that sounds like an unnecessary restriction. IIRC some drivers also create vm_are_structs without holding mmap_lock... I'll double-check. > > Anywhere else besides an address space setup would probably introduce a > race. > > Thanks, > Liam >
