On Wed, May 22, 2019 at 05:49:18PM -0400, Jerome Glisse wrote: > > > > So why is mm suddenly guarenteed valid? It was a bug report that > > > > triggered the race the mmget_not_zero is fixing, so I need a better > > > > explanation why it is now safe. From what I see the hmm_range_fault > > > > is doing stuff like find_vma without an active mmget?? > > > > > > So the mm struct can not go away as long as we hold a reference on > > > the hmm struct and we hold a reference on it through both hmm_mirror > > > and hmm_range struct. So struct mm can not go away and thus it is > > > safe to try to take its mmap_sem. > > > > This was always true here, though, so long as the umem_odp exists the > > the mm has a grab on it. But a grab is not a get.. > > > > The point here was the old code needed an mmget() in order to do > > get_user_pages_remote() > > > > If hmm does not need an external mmget() then fine, we delete this > > stuff and rely on hmm. > > > > But I don't think that is true as we have: > > > > CPU 0 CPU1 > > mmput() > > __mmput() > > exit_mmap() > > down_read(&mm->mmap_sem); > > hmm_range_dma_map(range, device,.. > > ret = hmm_range_fault(range, block); > > if (hmm->mm == NULL || hmm->dead) > > > > mmu_notifier_release() > > hmm->dead = true > > vma = find_vma(hmm->mm, start); > > .. rb traversal .. while (vma) > > remove_vma() > > > > *goes boom* > > > > I think this is violating the basic constraint of the mm by acting on > > a mm's VMA's without holding a mmget() to prevent concurrent > > destruction. > > > > In other words, mmput() destruction does not respect the mmap_sem - so > > holding the mmap sem alone is not enough locking. > > > > The unlucked hmm->dead simply can't save this. Frankly every time I > > look a struct with 'dead' in it, I find races like this. > > > > Thus we should put the mmget_notzero back in. > > So for some reason i thought exit_mmap() was setting the mm_rb > to empty node and flushing vmacache so that find_vma() would > fail.
It would still be racy without locks. > Note that right before find_vma() there is also range->valid > check which will also intercept mm release. There is no locking on range->valid so it is just moves the race around. You can't solve races with unlocked/non-atomic variables. > Anyway the easy fix is to get ref on mm user in range_register. Yes a mmget_not_zero inside range_register would be fine. How do you want to handle that patch? > > I saw some other funky looking stuff in hmm as well.. > > > > > Hence it is safe to take mmap_sem and it is safe to call in hmm, if > > > mm have been kill it will return EFAULT and this will propagate to > > > RDMA. > > > > > As per_mm i removed the per_mm->mm = NULL from release so that it is > > > always safe to use that field even in face of racing mm "killing". > > > > Yes, that certainly wasn't good. > > > > > > > - * An array of the pages included in the on-demand paging umem. > > > > > - * Indices of pages that are currently not mapped into the > > > > > device will > > > > > - * contain NULL. > > > > > + * An array of the pages included in the on-demand paging umem. > > > > > Indices > > > > > + * of pages that are currently not mapped into the device will > > > > > contain > > > > > + * 0. > > > > > */ > > > > > - struct page **page_list; > > > > > + uint64_t *pfns; > > > > > > > > Are these actually pfns, or are they mangled with some shift? (what is > > > > range->pfn_shift?) > > > > > > They are not pfns they have flags (hence range->pfn_shift) at the > > > bottoms i just do not have a better name for this. > > > > I think you need to have a better name then > > Suggestion ? i have no idea for a better name, it has pfn value > in it. pfn_flags? Jason
