Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752689AbaJFXqN (ORCPT ); Mon, 6 Oct 2014 19:46:13 -0400 Received: from LGEMRELSE7Q.lge.com ([156.147.1.151]:38089 "EHLO lgemrelse7q.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751380AbaJFXqL (ORCPT ); Mon, 6 Oct 2014 19:46:11 -0400 X-Original-SENDERIP: 10.177.220.156 X-Original-MAILFROM: minchan@kernel.org Date: Tue, 7 Oct 2014 08:46:29 +0900 From: Minchan Kim To: Dan Streetman Cc: Andrew Morton , linux-kernel , Linux-MM , Hugh Dickins , Shaohua Li , Jerome Marchand , Sergey Senozhatsky , Nitin Gupta , Luigi Semenzato , juno.choi@lge.com Subject: Re: [PATCH v1 4/5] zram: add swap full hint Message-ID: <20141006234629.GB19445@bbox> References: <1411344191-2842-1-git-send-email-minchan@kernel.org> <1411344191-2842-5-git-send-email-minchan@kernel.org> <20140925010229.GA17364@bbox> <20141006233608.GA19445@bbox> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20141006233608.GA19445@bbox> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 07, 2014 at 08:36:08AM +0900, Minchan Kim wrote: > Hello Dan, > > Sorry for the delay. I had internal works which should be handled > urgent. I hope you don't lose your interest due to my bad response > latency. > > On Thu, Sep 25, 2014 at 11:52:22AM -0400, Dan Streetman wrote: > > On Wed, Sep 24, 2014 at 9:02 PM, Minchan Kim wrote: > > > On Wed, Sep 24, 2014 at 10:01:03AM -0400, Dan Streetman wrote: > > >> On Sun, Sep 21, 2014 at 8:03 PM, Minchan Kim wrote: > > >> > This patch implement SWAP_FULL handler in zram so that VM can > > >> > know whether zram is full or not and use it to stop anonymous > > >> > page reclaim. > > >> > > > >> > How to judge fullness is below, > > >> > > > >> > fullness = (100 * used space / total space) > > >> > > > >> > It means the higher fullness is, the slower we reach zram full. > > >> > Now, default of fullness is 80 so that it biased more momory > > >> > consumption rather than early OOM kill. > > >> > > > >> > Above logic works only when used space of zram hit over the limit > > >> > but zram also pretend to be full once 32 consecutive allocation > > >> > fail happens. It's safe guard to prevent system hang caused by > > >> > fragment uncertainty. > > >> > > > >> > Signed-off-by: Minchan Kim > > >> > --- > > >> > drivers/block/zram/zram_drv.c | 60 ++++++++++++++++++++++++++++++++++++++++--- > > >> > drivers/block/zram/zram_drv.h | 1 + > > >> > 2 files changed, 57 insertions(+), 4 deletions(-) > > >> > > > >> > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c > > >> > index 22a37764c409..649cad9d0b1c 100644 > > >> > --- a/drivers/block/zram/zram_drv.c > > >> > +++ b/drivers/block/zram/zram_drv.c > > >> > @@ -43,6 +43,20 @@ static const char *default_compressor = "lzo"; > > >> > /* Module params (documentation at end) */ > > >> > static unsigned int num_devices = 1; > > >> > > > >> > +/* > > >> > + * If (100 * used_pages / total_pages) >= ZRAM_FULLNESS_PERCENT), > > >> > + * we regards it as zram-full. It means that the higher > > >> > + * ZRAM_FULLNESS_PERCENT is, the slower we reach zram full. > > >> > + */ > > >> > +#define ZRAM_FULLNESS_PERCENT 80 > > >> > > >> As Andrew said, this (or the user-configurable fullness param from the > > >> next patch) should have more detail about exactly why it's needed and > > >> what it does. The details of how zram considers itself "full" should > > >> be clear, which probably includes explaining zsmalloc fragmentation. > > >> It should be also clear this param only matters when limit_pages is > > >> set, and this param is only checked when zsmalloc's total size has > > >> reached that limit. > > > > > > Sure, How about this? > > > > > > The fullness file is read/write and specifies how easily > > > zram become full state. Normally, we can think "full" > > > once all of memory is consumed but it's not simple with > > > zram because zsmalloc has some issue by internal design > > > so that write could fail once consumed *page* by zram > > > reaches the mem_limit and zsmalloc cannot have a empty > > > slot for the compressed object's size on fragmenet space > > > although it has more empty slots for other sizes. > > > > I understand that, but it might be confusing or unclear to anyone > > who's not familiar with how zsmalloc works. > > > > Maybe it could be explained by referencing the existing > > compr_data_size and mem_used_total? In addition to some or all of the > > above, you could add something like: > > > > This controls when zram decides that it is "full". It is a percent > > value, checked against compr_data_size / mem_used_total. When > > mem_used_total is equal to mem_limit, the fullness is checked and if > > the compr_data_size / mem_used_total percentage is higher than this > > specified fullness value, zram is considered "full". > > Better than my verbose version. > > > > > > > > > > > We regard zram as full once consumed *page* reaches the > > > mem_limit and consumed memory until now is higher the value > > > resulted from the knob. So, if you set the value high, > > > you can squeeze more pages into fragment space so you could > > > avoid early OOM while you could see more write-fail warning, > > > overhead to fail-write recovering by VM and reclaim latency. > > > If you set the value low, you can see OOM kill easily > > > even though there are memory space in zram but you could > > > avoid shortcomings mentioned above. > > > > You should clarify also that this is currently only used by > > swap-on-zram, and this value prevents swap from writing to zram once > > it is "full". This setting has no effect when using zram for a > > mounted filesystem. > > Sure. > > > > > > > > > This knobs is valid ony if you set mem_limit. > > > Currently, initial value is 80% but it could be changed. > > > > > > I didn't decide how to change it from percent. > > > Decimal fraction Jerome mentioned does make sense to me so please ignore > > > percent part in above. > > > > > >> > > >> Also, since the next patch changes it to be used only as a default, > > >> shouldn't it be DEFAULT_ZRAM_FULLNESS_PERCENT or similar? > > > > > > Okay, I will do it in 5/5. > > > > > >> > > >> > + > > >> > +/* > > >> > + * If zram fails to allocate memory consecutively up to this, > > >> > + * we regard it as zram-full. It's safe guard to prevent too > > >> > + * many swap write fail due to lack of fragmentation uncertainty. > > >> > + */ > > >> > +#define ALLOC_FAIL_MAX 32 > > >> > + > > >> > #define ZRAM_ATTR_RO(name) \ > > >> > static ssize_t zram_attr_##name##_show(struct device *d, \ > > >> > struct device_attribute *attr, char *b) \ > > >> > @@ -148,6 +162,7 @@ static ssize_t mem_limit_store(struct device *dev, > > >> > > > >> > down_write(&zram->init_lock); > > >> > zram->limit_pages = PAGE_ALIGN(limit) >> PAGE_SHIFT; > > >> > + atomic_set(&zram->alloc_fail, 0); > > >> > up_write(&zram->init_lock); > > >> > > > >> > return len; > > >> > @@ -410,6 +425,7 @@ static void zram_free_page(struct zram *zram, size_t index) > > >> > atomic64_sub(zram_get_obj_size(meta, index), > > >> > &zram->stats.compr_data_size); > > >> > atomic64_dec(&zram->stats.pages_stored); > > >> > + atomic_set(&zram->alloc_fail, 0); > > >> > > > >> > meta->table[index].handle = 0; > > >> > zram_set_obj_size(meta, index, 0); > > >> > @@ -597,10 +613,15 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index, > > >> > } > > >> > > > >> > alloced_pages = zs_get_total_pages(meta->mem_pool); > > >> > - if (zram->limit_pages && alloced_pages > zram->limit_pages) { > > >> > - zs_free(meta->mem_pool, handle); > > >> > - ret = -ENOMEM; > > >> > - goto out; > > >> > + if (zram->limit_pages) { > > >> > + if (alloced_pages > zram->limit_pages) { > > >> > + zs_free(meta->mem_pool, handle); > > >> > + atomic_inc(&zram->alloc_fail); > > >> > + ret = -ENOMEM; > > >> > + goto out; > > >> > + } else { > > >> > + atomic_set(&zram->alloc_fail, 0); > > >> > + } > > >> > > >> So, with zram_full() checking for alloced_pages >= limit_pages, this > > >> will need to be changed; the way it is now it prevents that from ever > > >> being true. > > >> > > >> Instead I believe this check has to be moved to before zs_malloc(), so > > >> that alloced_pages > limit_pages is true. > > > > > > I don't get it why you said "it prevents that from ever being true". > > > Now, zram can use up until limit_pages (ie, used memory == zram->limit_pages) > > > and trying to get more is failed. so zram_full checks it as > > > toal_pages >= zram->limit_pages so what is problem? > > > If I miss your point, could you explain more? > > > > ok, that's true, it's possible for alloc_pages == limit_pages, but > > since zsmalloc will increase its size by a full zspage, and those can > > be anywhere between 1 and 4 pages in size, it's only a (very roughly) > > 25% chance that an alloc will cause alloc_pages == limit_pages, it's > > more likely that an alloc will cause alloc_pages > limit_pages. Now, > > after some number of write failures, that 25% (-ish) probability will > > be met, and alloc_pages == limit_pages will happen, but there's a > > rather high chance that there will be some number of write failures > > first. > > > > To summarize or restate that, I guess what I'm saying is that for > > users who don't care about some write failures and/or users with no > > other swap devices except zram, it probably does not matter. However > > for them, they probably will rely on the 32 write failure limit, and > > not the fullness limit. For users where zram is only the primary swap > > device, and there is a backup swap device, they probably will want > > zram to fail over to the backup fairly quickly, with as few write > > failures as possible (preferably, none, I would think). And this > > situation makes that highly unlikely - since there's only about a 25% > > chance of alloc_pages == limit_pages with no previous write failures, > > it's almost a certainty that there will be write failures before zram > > is decided to be "full", even if "fullness" is set to 0. > > > > With that said, you're right that it will eventually work, and those > > few write failures while trying to get to alloc_pages == limit_pages > > would probably not be noticable. However, do remember that zram won't > > stay full forever, so if it is only the primary swap device, it's > > likely it will move between "full" and "not full" quite a lot, and > > those few write failures may start adding up. > > Fair enough. > > But it is possible to see write-failure even though we correct > it because there is potential chance for zram to fail to allocate > order-0 page by a few reason which one of them is CMA I got several > reports because zRAM cannot allocate a movable page due to lack of > migration while usersapce goes with it well. I have a plan to fix it > with zsmalloc migration work but there are another chances to make > fail order-0 page by serval ways so I don't think we cannot prevent > write-failure completely unless we have reserved memory for zram. > > Having said that, I agree it would be better to reduce such fails > with small code piece so I will check zram_full as follows, > > /* > * XXX: zsmalloc_maxpages check should be removed when zsmalloc > * implement using of fragmented spaces in last page of zspage. > */ > if (total_pages >= zram->limit_pages - zsmalloc_maxpages()) { > ... > } > How about this? diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c index 19da34aaf4f5..f03a94d7aa17 100644 --- a/drivers/block/zram/zram_drv.c +++ b/drivers/block/zram/zram_drv.c @@ -978,7 +978,7 @@ static void zram_full(struct block_device *bdev, bool *full) meta = zram->meta; total_pages = zs_get_total_pages(meta->mem_pool); - if (total_pages >= zram->limit_pages) { + if (total_pages > zram->limit_pages - zs_get_maxpages_per_zspage()) { compr_pages = atomic64_read(&zram->stats.compr_data_size) >> PAGE_SHIFT; diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h index 05c214760977..73eb87bc5a4e 100644 --- a/include/linux/zsmalloc.h +++ b/include/linux/zsmalloc.h @@ -48,4 +48,5 @@ void zs_unmap_object(struct zs_pool *pool, unsigned long handle); unsigned long zs_get_total_pages(struct zs_pool *pool); +int zs_get_maxpages_per_zspage(void); #endif diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c index 839a48c3ca27..6b6653455573 100644 --- a/mm/zsmalloc.c +++ b/mm/zsmalloc.c @@ -316,6 +316,12 @@ static struct zpool_driver zs_zpool_driver = { MODULE_ALIAS("zpool-zsmalloc"); #endif /* CONFIG_ZPOOL */ +int zs_get_maxpages_per_zspage(void) +{ + return ZS_MAX_PAGES_PER_ZSPAGE; +} +EXPORT_SYMBOL_GPL(zs_get_maxpages_per_zspage); + /* per-cpu VM mapping areas for zspage accesses that cross page boundaries */ static DEFINE_PER_CPU(struct mapping_area, zs_map_area); -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/