From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jerome Glisse Subject: Re: [PATCH v4 0/1] Use HMM for ODP v4 Date: Wed, 22 May 2019 17:49:18 -0400 Message-ID: <20190522214917.GA20179@redhat.com> References: <20190411181314.19465-1-jglisse@redhat.com> <20190506195657.GA30261@ziepe.ca> <20190521205321.GC3331@redhat.com> <20190522005225.GA30819@ziepe.ca> <20190522174852.GA23038@redhat.com> <20190522192219.GF6054@ziepe.ca> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: <20190522192219.GF6054@ziepe.ca> Sender: linux-kernel-owner@vger.kernel.org To: Jason Gunthorpe Cc: linux-kernel@vger.kernel.org, linux-rdma@vger.kernel.org, Leon Romanovsky , Doug Ledford , Artemy Kovalyov , Moni Shoua , Mike Marciniszyn , Kaike Wan , Dennis Dalessandro , linux-mm@kvack.org List-Id: linux-rdma@vger.kernel.org On Wed, May 22, 2019 at 04:22:19PM -0300, Jason Gunthorpe wrote: > On Wed, May 22, 2019 at 01:48:52PM -0400, Jerome Glisse wrote: > > > > > +long ib_umem_odp_map_dma_pages(struct ib_umem_odp *umem_odp, > > > > + struct hmm_range *range) > > > > { > > > > + struct device *device = umem_odp->umem.context->device->dma_device; > > > > + struct ib_ucontext_per_mm *per_mm = umem_odp->per_mm; > > > > struct ib_umem *umem = &umem_odp->umem; > > > > - struct task_struct *owning_process = NULL; > > > > - struct mm_struct *owning_mm = umem_odp->umem.owning_mm; > > > > - struct page **local_page_list = NULL; > > > > - u64 page_mask, off; > > > > - int j, k, ret = 0, start_idx, npages = 0, page_shift; > > > > - unsigned int flags = 0; > > > > - phys_addr_t p = 0; > > > > - > > > > - if (access_mask == 0) > > > > + struct mm_struct *mm = per_mm->mm; > > > > + unsigned long idx, npages; > > > > + long ret; > > > > + > > > > + if (mm == NULL) > > > > + return -ENOENT; > > > > + > > > > + /* Only drivers with invalidate support can use this function. */ > > > > + if (!umem->context->invalidate_range) > > > > return -EINVAL; > > > > > > > > - if (user_virt < ib_umem_start(umem) || > > > > - user_virt + bcnt > ib_umem_end(umem)) > > > > - return -EFAULT; > > > > + /* Sanity checks. */ > > > > + if (range->default_flags == 0) > > > > + return -EINVAL; > > > > > > > > - local_page_list = (struct page **)__get_free_page(GFP_KERNEL); > > > > - if (!local_page_list) > > > > - return -ENOMEM; > > > > + if (range->start < ib_umem_start(umem) || > > > > + range->end > ib_umem_end(umem)) > > > > + return -EINVAL; > > > > > > > > - page_shift = umem->page_shift; > > > > - page_mask = ~(BIT(page_shift) - 1); > > > > - off = user_virt & (~page_mask); > > > > - user_virt = user_virt & page_mask; > > > > - bcnt += off; /* Charge for the first page offset as well. */ > > > > + idx = (range->start - ib_umem_start(umem)) >> umem->page_shift; > > > > > > Is this math OK? What is supposed to happen if the range->start is not > > > page aligned to the internal page size? > > > > range->start is align on 1 << page_shift boundary within pagefault_mr > > thus the above math is ok. We can add a BUG_ON() and comments if you > > want. > > OK > > > > > + range->pfns = &umem_odp->pfns[idx]; > > > > + range->pfn_shift = ODP_FLAGS_BITS; > > > > + range->values = odp_hmm_values; > > > > + range->flags = odp_hmm_flags; > > > > > > > > /* > > > > - * owning_process is allowed to be NULL, this means somehow the mm is > > > > - * existing beyond the lifetime of the originating process.. Presumably > > > > - * mmget_not_zero will fail in this case. > > > > + * If mm is dying just bail out early without trying to take mmap_sem. > > > > + * Note that this might race with mm destruction but that is fine the > > > > + * is properly refcounted so are all HMM structure. > > > > */ > > > > - owning_process = get_pid_task(umem_odp->per_mm->tgid, PIDTYPE_PID); > > > > - if (!owning_process || !mmget_not_zero(owning_mm)) { > > > > > > But we are not in a HMM context here, and per_mm is not a HMM > > > structure. > > > > > > 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. Might have been in some patch that never went upstream. Note that right before find_vma() there is also range->valid check which will also intercept mm release. Anyway the easy fix is to get ref on mm user in range_register. > > 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. Cheers, Jérôme