On Mon, Jan 23, 2023 at 8:55 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Mon 23-01-23 08:22:53, Suren Baghdasaryan wrote: > > On Mon, Jan 23, 2023 at 1:56 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Fri 20-01-23 09:50:01, Suren Baghdasaryan wrote: > > > > On Fri, Jan 20, 2023 at 9:32 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > [...] > > > > > The page fault handler (or whatever other reader -- ptrace, proc, etc) > > > > > should have a refcount on the mm_struct, so we can't be in this path > > > > > trying to free VMAs. Right? > > > > > > > > Hmm. That sounds right. I checked process_mrelease() as well, which > > > > operated on mm with only mmgrab()+mmap_read_lock() but it only unmaps > > > > VMAs without freeing them, so we are still good. Michal, do you agree > > > > this is ok? > > > > > > Don't we need RCU procetions for the vma life time assurance? Jann has > > > already shown how rwsem is not safe wrt to unlock and free without RCU. > > > > Jann's case requires a thread freeing the VMA to be blocked on vma > > write lock waiting for the vma real lock to be released by a page > > fault handler. However exit_mmap() means mm->mm_users==0, which in > > turn suggests that there are no racing page fault handlers and no new > > page fault handlers will appear. Is that a correct assumption? If so, > > then races with page fault handlers can't happen while in exit_mmap(). > > Any other path (other than page fault handlers), accesses vma->lock > > under protection of mmap_lock (for read or write, does not matter). > > One exception is when we operate on an isolated VMA, then we don't > > need mmap_lock protection, but exit_mmap() does not deal with isolated > > VMAs, so out of scope here. exit_mmap() frees vm_area_structs under > > protection of mmap_lock in write mode, so races with anything other > > than page fault handler should be safe as they are today. > > I do not see you talking about #PF (RCU + vma read lock protected) with > munmap. It is my understanding that the latter will synchronize over per > vma lock (along with mmap_lock exclusive locking). But then we are back > to the lifetime guarantees, or do I miss anything. munmap() or any VMA-freeing operation other than exit_mmap() will free using call_rcu(), as implemented today. The suggestion is to free VMAs directly, without RCU grace period only when done from exit_mmap(). That' because VMA freeing flood has been seen so far only in the case of exit_mmap() and we assume other cases are not that heavy to cause call_rcu() flood to cause regressions. That assumption might prove false but we can deal with that once we know it needs fixing. > -- > Michal Hocko > SUSE Labs
