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[156.34.55.100]) by smtp.gmail.com with ESMTPSA id s25sm20315125qkm.130.2019.07.24.11.08.37 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 24 Jul 2019 11:08:37 -0700 (PDT) Received: from jgg by mlx.ziepe.ca with local (Exim 4.90_1) (envelope-from ) id 1hqLgr-0003X8-9P; Wed, 24 Jul 2019 15:08:37 -0300 Date: Wed, 24 Jul 2019 15:08:37 -0300 From: Jason Gunthorpe To: Michal Hocko Cc: Christoph Hellwig , Ralph Campbell , linux-mm@kvack.org, linux-kernel@vger.kernel.org, nouveau@lists.freedesktop.org, dri-devel@lists.freedesktop.org, =?utf-8?B?SsOpcsO0bWU=?= Glisse , Ben Skeggs Subject: Re: [PATCH] mm/hmm: replace hmm_update with mmu_notifier_range Message-ID: <20190724180837.GF28493@ziepe.ca> References: <20190723210506.25127-1-rcampbell@nvidia.com> <20190724070553.GA2523@lst.de> <20190724152858.GB28493@ziepe.ca> <20190724175858.GC6410@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190724175858.GC6410@dhcp22.suse.cz> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 24, 2019 at 07:58:58PM +0200, Michal Hocko wrote: > On Wed 24-07-19 12:28:58, Jason Gunthorpe wrote: > > On Wed, Jul 24, 2019 at 09:05:53AM +0200, Christoph Hellwig wrote: > > > Looks good: > > > > > > Reviewed-by: Christoph Hellwig > > > > > > One comment on a related cleanup: > > > > > > > list_for_each_entry(mirror, &hmm->mirrors, list) { > > > > int rc; > > > > > > > > - rc = mirror->ops->sync_cpu_device_pagetables(mirror, &update); > > > > + rc = mirror->ops->sync_cpu_device_pagetables(mirror, nrange); > > > > if (rc) { > > > > - if (WARN_ON(update.blockable || rc != -EAGAIN)) > > > > + if (WARN_ON(mmu_notifier_range_blockable(nrange) || > > > > + rc != -EAGAIN)) > > > > continue; > > > > ret = -EAGAIN; > > > > break; > > > > > > This magic handling of error seems odd. I think we should merge rc and > > > ret into one variable and just break out if any error happens instead > > > or claiming in the comments -EAGAIN is the only valid error and then > > > ignoring all others here. > > > > The WARN_ON is enforcing the rules already commented near > > mmuu_notifier_ops.invalidate_start - we could break or continue, it > > doesn't much matter how to recover from a broken driver, but since we > > did the WARN_ON this should sanitize the ret to EAGAIN or 0 > > > > Humm. Actually having looked this some more, I wonder if this is a > > problem: > > > > I see in __oom_reap_task_mm(): > > > > if (mmu_notifier_invalidate_range_start_nonblock(&range)) { > > tlb_finish_mmu(&tlb, range.start, range.end); > > ret = false; > > continue; > > } > > unmap_page_range(&tlb, vma, range.start, range.end, NULL); > > mmu_notifier_invalidate_range_end(&range); > > > > Which looks like it creates an unbalanced start/end pairing if any > > start returns EAGAIN? > > > > This does not seem OK.. Many users require start/end to be paired to > > keep track of their internal locking. Ie for instance hmm breaks > > because the hmm->notifiers counter becomes unable to get to 0. > > > > Below is the best idea I've had so far.. > > > > Michal, what do you think? > > IIRC we have discussed this with Jerome back then when I've introduced > this code and unless I misremember he said the current code was OK. Nope, it has always been broken. > Maybe new users have started relying on a new semantic in the meantime, > back then, none of the notifier has even started any action in blocking > mode on a EAGAIN bailout. Most of them simply did trylock early in the > process and bailed out so there was nothing to do for the range_end > callback. Single notifiers are not the problem. I tried to make this clear in the commit message, but lets be more explicit. We have *two* notifiers registered to the mm, A and B: A invalidate_range_start: (has no blocking) spin_lock() counter++ spin_unlock() A invalidate_range_end: spin_lock() counter-- spin_unlock() And this one: B invalidate_range_start: (has blocking) if (!try_mutex_lock()) return -EAGAIN; counter++ mutex_unlock() B invalidate_range_end: spin_lock() counter-- spin_unlock() So now the oom path does: invalidate_range_start_non_blocking: for each mn: a->invalidate_range_start b->invalidate_range_start rc = EAGAIN Now we SKIP A's invalidate_range_end even though A had no idea this would happen has state that needs to be unwound. A is broken. B survived just fine. A and B *alone* work fine, combined they fail. When the commit was landed you can use KVM as an example of A and RDMA ODP as an example of B Jason