From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751515AbdARGAs (ORCPT ); Wed, 18 Jan 2017 01:00:48 -0500 Received: from hqemgate16.nvidia.com ([216.228.121.65]:6954 "EHLO hqemgate16.nvidia.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751150AbdARGAp (ORCPT ); Wed, 18 Jan 2017 01:00:45 -0500 X-PGP-Universal: processed; by hqnvupgp07.nvidia.com on Tue, 17 Jan 2017 09:54:55 -0800 Subject: Re: [PATCH 1/6] mm: introduce kv[mz]alloc helpers To: Michal Hocko References: <20170112153717.28943-1-mhocko@kernel.org> <20170112153717.28943-2-mhocko@kernel.org> <20170116084717.GA13641@dhcp22.suse.cz> <0ca8a212-c651-7915-af25-23925e1c1cc3@nvidia.com> <20170116194052.GA9382@dhcp22.suse.cz> <1979f5e1-a335-65d8-8f9a-0aef17898ca1@nvidia.com> <20170116214822.GB9382@dhcp22.suse.cz> <20170117075100.GB19699@dhcp22.suse.cz> CC: Andrew Morton , Vlastimil Babka , David Rientjes , Mel Gorman , Johannes Weiner , Al Viro , , LKML , Anatoly Stepanov , Paolo Bonzini , Mike Snitzer , "Michael S. Tsirkin" , "Theodore Ts'o" X-Nvconfidentiality: public From: John Hubbard Message-ID: Date: Tue, 17 Jan 2017 21:59:13 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0 MIME-Version: 1.0 In-Reply-To: <20170117075100.GB19699@dhcp22.suse.cz> X-Originating-IP: [172.17.160.221] X-ClientProxiedBy: HQMAIL102.nvidia.com (172.18.146.10) To HQMAIL107.nvidia.com (172.20.187.13) Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/16/2017 11:51 PM, Michal Hocko wrote: > On Mon 16-01-17 13:57:43, John Hubbard wrote: >> >> >> On 01/16/2017 01:48 PM, Michal Hocko wrote: >>> On Mon 16-01-17 13:15:08, John Hubbard wrote: >>>> >>>> >>>> On 01/16/2017 11:40 AM, Michal Hocko wrote: >>>>> On Mon 16-01-17 11:09:37, John Hubbard wrote: >>>>>> >>>>>> >>>>>> On 01/16/2017 12:47 AM, Michal Hocko wrote: >>>>>>> On Sun 15-01-17 20:34:13, John Hubbard wrote: >>>>> [...] >>>>>>>> Is that "Reclaim modifiers" line still true, or is it a leftover from an >>>>>>>> earlier approach? I am having trouble reconciling it with rest of the >>>>>>>> patchset, because: >>>>>>>> >>>>>>>> a) the flags argument below is effectively passed on to either kmalloc_node >>>>>>>> (possibly adding, but not removing flags), or to __vmalloc_node_flags. >>>>>>> >>>>>>> The above only says thos are _unsupported_ - in other words the behavior >>>>>>> is not defined. Even if flags are passed down to kmalloc resp. vmalloc >>>>>>> it doesn't mean they are used that way. Remember that vmalloc uses >>>>>>> some hardcoded GFP_KERNEL allocations. So while I could be really >>>>>>> strict about this and mask away these flags I doubt this is worth the >>>>>>> additional code. >>>>>> >>>>>> I do wonder about passing those flags through to kmalloc. Maybe it is worth >>>>>> stripping out __GFP_NORETRY and __GFP_NOFAIL, after all. It provides some >>>>>> insulation from any future changes to the implementation of kmalloc, and it >>>>>> also makes the documentation more believable. >>>>> >>>>> I am not really convinced that we should take an extra steps for these >>>>> flags. There are no existing users for those flags and new users should >>>>> follow the documentation. >>>> >>>> OK, let's just fortify the documentation ever so slightly, then, so that >>>> users are more likely to do the right thing. How's this sound: >>>> >>>> * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL are not supported. (Even >>>> * though the current implementation passes the flags on through to kmalloc and >>>> * vmalloc, that is done for efficiency and to avoid unnecessary code. The caller >>>> * should not pass in these flags.) >>>> * >>>> * __GFP_REPEAT is supported, but only for large (>64kB) allocations. >>>> >>>> >>>> ? Or is that documentation overkill? >>> >>> Dunno, it sounds like an overkill to me. It is telling more than >>> necessary. If we want to be so vocal about gfp flags then we would have >>> to say much more I suspect. E.g. what about __GFP_HIGHMEM? This flag is >>> supported for vmalloc while unsupported for kmalloc. I am pretty sure >>> there would be other gfp flags to consider and then this would grow >>> borringly large and uninteresting to the point when people simply stop >>> reading it. Let's just be as simple as possible. >> >> Agreed, on the simplicity point: simple and clear is ideal. But here, it's >> merely short, and not quite simple. :) People will look at that short bit >> of documentation, and then notice that the flags are, in fact, all passed >> right on through down to both kmalloc_node and __vmalloc_node_flags. >> >> If you don't want too much documentation, then I'd be inclined to say >> something higher-level, about the intent, rather than mentioning those two >> flags directly. Because as it stands, the documentation contradicts what the >> code does. > > Feel free to suggest a better wording. I am, of course, open to any > changes. OK, here's the best I've got, I tried to keep it concise, but (as you suspected) I'm not sure it's actually any better than the original: * Reclaim modifiers - __GFP_NORETRY and __GFP_NOFAIL should not be passed in. * Passing in __GFP_REPEAT is supported, but note that it is ignored for small * (<=64KB) allocations, during the kmalloc attempt. __GFP_REPEAT is fully * honored for all allocation sizes during the second part: the vmalloc attempt. > > -- > Michal Hocko > SUSE Labs >