From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934889AbaH0PPI (ORCPT ); Wed, 27 Aug 2014 11:15:08 -0400 Received: from mail-wg0-f49.google.com ([74.125.82.49]:50501 "EHLO mail-wg0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934344AbaH0PPF (ORCPT ); Wed, 27 Aug 2014 11:15:05 -0400 MIME-Version: 1.0 In-Reply-To: References: <1408925156-11733-1-git-send-email-minchan@kernel.org> <1408925156-11733-4-git-send-email-minchan@kernel.org> <20140826073730.GA1975@js1304-P5Q-DELUXE> <20140826075511.GI11319@bbox> <20140827012610.GA10198@js1304-P5Q-DELUXE> <20140827025132.GI32620@bbox> From: Dan Streetman Date: Wed, 27 Aug 2014 11:14:43 -0400 X-Google-Sender-Auth: 5-aACIUnQNJOe44GzoMB40eIlFo Message-ID: Subject: Re: [PATCH v5 3/4] zram: zram memory size limitation To: David Horner Cc: Minchan Kim , Joonsoo Kim , Andrew Morton , Linux-MM , linux-kernel , Sergey Senozhatsky , Jerome Marchand , juno.choi@lge.com, seungho1.park@lge.com, Luigi Semenzato , Nitin Gupta , Seth Jennings Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Aug 27, 2014 at 10:44 AM, David Horner wrote: > On Wed, Aug 27, 2014 at 10:03 AM, Dan Streetman wrote: >> On Tue, Aug 26, 2014 at 10:51 PM, Minchan Kim wrote: >>> Hey Joonsoo, >>> >>> On Wed, Aug 27, 2014 at 10:26:11AM +0900, Joonsoo Kim wrote: >>>> Hello, Minchan and David. >>>> >>>> On Tue, Aug 26, 2014 at 08:22:29AM -0400, David Horner wrote: >>>> > On Tue, Aug 26, 2014 at 3:55 AM, Minchan Kim wrote: >>>> > > Hey Joonsoo, >>>> > > >>>> > > On Tue, Aug 26, 2014 at 04:37:30PM +0900, Joonsoo Kim wrote: >>>> > >> On Mon, Aug 25, 2014 at 09:05:55AM +0900, Minchan Kim wrote: >>>> > >> > @@ -513,6 +540,14 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index, >>>> > >> > ret = -ENOMEM; >>>> > >> > goto out; >>>> > >> > } >>>> > >> > + >>>> > >> > + if (zram->limit_pages && >>>> > >> > + zs_get_total_pages(meta->mem_pool) > zram->limit_pages) { >>>> > >> > + zs_free(meta->mem_pool, handle); >>>> > >> > + ret = -ENOMEM; >>>> > >> > + goto out; >>>> > >> > + } >>>> > >> > + >>>> > >> > cmem = zs_map_object(meta->mem_pool, handle, ZS_MM_WO); >>>> > >> >>>> > >> Hello, >>>> > >> >>>> > >> I don't follow up previous discussion, so I could be wrong. >>>> > >> Why this enforcement should be here? >>>> > >> >>>> > >> I think that this has two problems. >>>> > >> 1) alloc/free happens unnecessarilly if we have used memory over the >>>> > >> limitation. >>>> > > >>>> > > True but firstly, I implemented the logic in zsmalloc, not zram but >>>> > > as I described in cover-letter, it's not a requirement of zsmalloc >>>> > > but zram so it should be in there. If every user want it in future, >>>> > > then we could move the function into zsmalloc. That's what we >>>> > > concluded in previous discussion. >>>> >>>> Hmm... >>>> Problem is that we can't avoid these unnecessary overhead in this >>>> implementation. If we can implement this feature in zram efficiently, >>>> it's okay. But, I think that current form isn't. >>> >>> >>> If we can add it in zsmalloc, it would be more clean and efficient >>> for zram but as I said, at the moment, I didn't want to put zram's >>> requirement into zsmalloc because to me, it's weird to enforce max >>> limit to allocator. It's client's role, I think. >>> >>> If current implementation is expensive and rather hard to follow, >>> It would be one reason to move the feature into zsmalloc but >>> I don't think it makes critical trobule in zram usecase. >>> See below. >>> >>> But I still open and will wait others's opinion. >>> If other guys think zsmalloc is better place, I am willing to move >>> it into zsmalloc. >> >> Moving it into zsmalloc would allow rejecting new zsmallocs before >> actually crossing the limit, since it can calculate that internally. >> However, with the current patches the limit will only be briefly >> crossed, and it should not be crossed by a large amount. Now, if this >> is happening repeatedly and quickly during extreme memory pressure, >> the constant alloc/free will clearly be worse than a simple internal >> calculation and failure. But would it ever happen repeatedly once the >> zram limit is reached? >> >> Now that I'm thinking about the limit from the perspective of the zram >> user, I wonder what really will happen. If zram is being used for >> swap space, then when swap starts getting errors trying to write >> pages, how damaging will that be to the system? I haven't checked >> what swap does when it encounters disk errors. Of course, with no >> zram limit, continually writing to zram until memory is totally >> consumed isn't good either. But in any case, I would hope that swap >> would not repeatedly hammer on a disk when it's getting write failures >> from it. >> >> Alternately, if zram was being used as a compressed ram disk for >> regular file storage, it's entirely up to the application to handle >> write failures, so it may continue to try to write to a full zram >> disk. >> >> As far as what the zsmalloc api would look like with the limit added, >> it would need a setter and getter function (adding it as a param to >> the create function would be optional i think). But more importantly, >> it would need to handle multiple ways of specifying the limit. In our >> specific current use cases, zram and zswap, each handles their >> internal limit differently - zswap currently uses a % of total ram as >> its limit (defaulting to 20), while with these patches zram will use a >> specific number of bytes as its limit (defaulting to no limit). If >> the limiting mechanism is moved into zsmalloc (and possibly zbud), >> then either both users need to use the same units (bytes or %ram), or >> zsmalloc/zbud need to be able to set their limit in either units. It >> seems to me like keeping the limit in zram/zswap is currently >> preferable, at least without both using the same limit units. >> > > zswap knows what 20% (or whatever % it currently uses , and perhaps it too > will become a tuning knob) of memory is in bytes. > > So, if the interface to establish a limit for a pool (or pool set, or whatever > zsmalloc sets up for its allocation mechanism) is stipulated in bytes > (to actually use pages internally, of visa-versa) , then both can use > that interface. > zram with its native page stipulation, and zswap with calculated % of memory). No, unless zswap monitors memory hotplug and updates the limit on each hotplug event, 20% of the *current* total ram at zswap initialization is not equal to an actual 20% of ram limit. zswap checks its size against totalram_pages for each new allocation. I don't think we would prefer adding memory hotplug monitoring to zswap just to update the zpool size limit. > > Both would need a mechanism to change the max as need change, > so the API has to handle this. > > > Or am I way off base? > > >> >>> >>>> >>>> > > >>>> > > Another idea is we could call zs_get_total_pages right before zs_malloc >>>> > > but the problem is we cannot know how many of pages are allocated >>>> > > by zsmalloc in advance. >>>> > > IOW, zram should be blind on zsmalloc's internal. >>>> > > >>>> > >>>> > We did however suggest that we could check before hand to see if >>>> > max was already exceeded as an optimization. >>>> > (possibly with a guess on usage but at least using the minimum of 1 page) >>>> > In the contested case, the max may already be exceeded transiently and >>>> > therefore we know this one _could_ fail (it could also pass, but odds >>>> > aren't good). >>>> > As Minchan mentions this was discussed before - but not into great detail. >>>> > Testing should be done to determine possible benefit. And as he also >>>> > mentions, the better place for it may be in zsmalloc, but that >>>> > requires an ABI change. >>>> >>>> Why we hesitate to change zsmalloc API? It is in-kernel API and there >>>> are just two users now, zswap and zram. We can change it easily. >>>> I think that we just need following simple API change in zsmalloc.c. >>>> >>>> zs_zpool_create(gfp_t gfp, struct zpool_ops *zpool_op) >>>> => >>>> zs_zpool_create(unsigned long limit, gfp_t gfp, struct zpool_ops >>>> *zpool_op) >>>> >>>> It's pool allocator so there is no obstacle for us to limit maximum >>>> memory usage in zsmalloc. It's a natural idea to limit memory usage >>>> for pool allocator. >>>> >>>> > Certainly a detailed suggestion could happen on this thread and I'm >>>> > also interested >>>> > in your thoughts, but this patchset should be able to go in as is. >>>> > Memory exhaustion avoidance probably trumps the possible thrashing at >>>> > threshold. >>>> > >>>> > > About alloc/free cost once if it is over the limit, >>>> > > I don't think it's important to consider. >>>> > > Do you have any scenario in your mind to consider alloc/free cost >>>> > > when the limit is over? >>>> > > >>>> > >> 2) Even if this request doesn't do new allocation, it could be failed >>>> > >> due to other's allocation. There is time gap between allocation and >>>> > >> free, so legimate user who want to use preallocated zsmalloc memory >>>> > >> could also see this condition true and then he will be failed. >>>> > > >>>> > > Yeb, we already discussed that. :) >>>> > > Such false positive shouldn't be a severe problem if we can keep a >>>> > > promise that zram user cannot exceed mem_limit. >>>> > > >>>> >>>> If we can keep such a promise, why we need to limit memory usage? >>>> I guess that this limit feature is useful for user who can't keep such promise. >>>> So, we should assume that this false positive happens frequently. >>> >>> >>> The goal is to limit memory usage within some threshold. >>> so false positive shouldn't be harmful unless it exceeds the threshold. >>> In addition, If such false positive happens frequently, it means >>> zram is very trobule so that user would see lots of write fail >>> message, sometime really slow system if zram is used for swap. >>> If we protect just one write from the race, how much does it help >>> this situation? I don't think it's critical problem. >>> >>>> >>>> > And we cannot avoid the race, nor can we avoid in a low overhead competitive >>>> > concurrent process transient inconsistent states. >>>> > Different views for different observers. >>>> > They are a consequence of the theory of "Special Computational Relativity". >>>> > I am working on a String Unification Theory of Quantum and General CR in LISP. >>>> > ;-) >>>> >>>> If we move limit logic to zsmalloc, we can avoid the race by commiting >>>> needed memory size before actual allocation attempt. This commiting makes >>>> concurrent process serialized so there is no race here. There is >>>> possibilty to fail to allocate, but I think this is better than alloc >>>> and free blindlessly depending on inconsistent states. >>> >>> Normally, zsmalloc/zsfree allocates object from existing pool so >>> it's not big overhead and if someone continue to try writing once limit is >>> full, another overhead (vfs, fs, block) would be bigger than zsmalloc >>> so it's not a problem, I think. >>> >>>> >>>> Thanks. >>>> >>>> -- >>>> To unsubscribe, send a message with 'unsubscribe linux-mm' in >>>> the body to majordomo@kvack.org. For more info on Linux MM, >>>> see: http://www.linux-mm.org/ . >>>> Don't email: email@kvack.org >>> >>> -- >>> Kind regards, >>> Minchan Kim