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[213.151.95.130]) by smtp.gmail.com with ESMTPSA id b128sm7032226wmb.25.2020.01.24.06.46.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Jan 2020 06:46:44 -0800 (PST) Date: Fri, 24 Jan 2020 15:46:43 +0100 From: Michal Hocko To: Wei Yang Cc: akpm@linux-foundation.org, yang.shi@linux.alibaba.com, jhubbard@nvidia.com, vbabka@suse.cz, cl@linux.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [Patch v2] mm/migrate.c: also overwrite error when it is bigger than zero Message-ID: <20200124144643.GV29276@dhcp22.suse.cz> References: <20200119065753.21694-1-richardw.yang@linux.intel.com> <20200124072127.GO29276@dhcp22.suse.cz> <20200124141538.GA12509@richard> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200124141538.GA12509@richard> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri 24-01-20 22:15:38, Wei Yang wrote: > On Fri, Jan 24, 2020 at 08:21:27AM +0100, Michal Hocko wrote: > >[Sorry I have missed this patch previously] > > > > No problem, thanks for your comment. > > >On Sun 19-01-20 14:57:53, Wei Yang wrote: > >> If we get here after successfully adding page to list, err would be > >> 1 to indicate the page is queued in the list. > >> > >> Current code has two problems: > >> > >> * on success, 0 is not returned > >> * on error, if add_page_for_migratioin() return 1, and the following err1 > >> from do_move_pages_to_node() is set, the err1 is not returned since err > >> is 1 > > > >This made my really scratch my head to grasp. So essentially err > 0 > >will happen when we reach the end of the loop and rely on the > >out_flush flushing to migrate the batch. Then err contains the > >add_page_for_migratioin return value. And that would leak to the > >userspace. > > > >What would you say about the following wording instead? > >" > >out_flush part of do_pages_move is responsible for migrating the last > >batch that accumulated while processing the input in the loop. > >do_move_pages_to_node return value is supposed to override any > >preexisting error (e.g. when the user input is garbage) but the current > > I am afraid I have a different understanding here. > > If we jump to out_flush on the test of node_isset(), err is -EACCESS. Current > logic would return this instead of the error from do_move_pages_to_node(). > Seems we don't override -EACCESS. And this is the expected logic. The unexpected behavior is the one you have fixed by this patch because err = 1 wouldn't get overriden and that should have been. -- Michal Hocko SUSE Labs