On Mon 23-01-23 09:07:34, Suren Baghdasaryan wrote: > 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(). OK, I have clearly missed that. This makes more sense but it also adds some more complexity and assumptions - a harder to maintain code in the end. Whoever wants to touch this scheme in future would have to re-evaluate all of them. So, I would just avoid that special casing if that is feasible. Dealing with the flood of call_rcu during exit_mmap is a trivial thing to deal with as proposed elsewhere (just batch all of them in a single run). This will surely add some more code but at least the locking would consistent. -- Michal Hocko SUSE Labs
