From mboxrd@z Thu Jan 1 00:00:00 1970 From: Wei Wang Subject: Re: [PULL] vhost: cleanups and fixes Date: Tue, 12 Jun 2018 19:05:19 +0800 Message-ID: <5B1FA8EF.4030409@intel.com> References: <20180611192353-mutt-send-email-mst@kernel.org> <20180612042600-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Cc: KVM list , Network Development , Linux Kernel Mailing List , Bjorn Andersson , Andrew Morton , virtualization To: Linus Torvalds , "Michael S. Tsirkin" Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org List-Id: netdev.vger.kernel.org On 06/12/2018 09:59 AM, Linus Torvalds wrote: > On Mon, Jun 11, 2018 at 6:36 PM Michael S. Tsirkin wrote: >> Maybe it will help to have GFP_NONE which will make any allocation >> fail if attempted. Linus, would this address your comment? > It would definitely have helped me initially overlook that call chain. > > But then when I started looking at the whole dma_map_page() thing, it > just raised my hackles again. > > I would seriously suggest having a much simpler version for the "no > allocation, no dma mapping" case, so that it's *obvious* that that > never happens. > > So instead of having virtio_balloon_send_free_pages() call a really > generic complex chain of functions that in _some_ cases can do memory > allocation, why isn't there a short-circuited "vitruque_add_datum()" > that is guaranteed to never do anything like that? > > Honestly, I look at "add_one_sg()" and it really doesn't make me > happy. It looks hacky as hell. If I read the code right, you're really > trying to just queue up a simple tuple of , except you encode > it as a page pointer in order to play games with the SG logic, and > then you hmap that to the ring, except in this case it's all a fake > ring that just adds the cpu-physical address instead. > > And to figuer that out, it's like five layers of indirection through > different helper functions that *can* do more generic things but in > this case don't. > > And you do all of this from a core VM callback function with some > _really_ core VM locks held. > > That makes no sense to me. > > How about this: > > - get rid of all that code > > - make the core VM callback save the "these are the free memory > regions" in a fixed and limited array. One that DOES JUST THAT. No > crazy "SG IO dma-mapping function crap". Just a plain array of a fixed > size, pre-allocated for that virtio instance. > > - make it obvious that what you do in that sequence is ten > instructions and no allocations ("Look ma, I wrote a value to an array > and incremented the array idex, and I'M DONE") > > - then in that workqueue entry that you start *anyway*, you empty the > array and do all the crazy virtio stuff. > > In fact, while at it, just simplify the VM interface too. Instead of > traversing a random number of buddy lists, just trraverse *one* - the > top-level one. Are you seriously ever going to shrink or mark > read-only anythin *but* something big enough to be in the maximum > order? > > MAX_ORDER is what, 11? So we're talking 8MB blocks. Do you *really* > want the balloon code to work on smaller things, particularly since > the whole interface is fundamentally racy and opportunistic to begin > with? OK, I will implement a new version based on the suggestions. Thanks. Best, Wei