Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754208AbaJHSaH (ORCPT ); Wed, 8 Oct 2014 14:30:07 -0400 Received: from mail-yh0-f53.google.com ([209.85.213.53]:56105 "EHLO mail-yh0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753358AbaJHSaF (ORCPT ); Wed, 8 Oct 2014 14:30:05 -0400 MIME-Version: 1.0 In-Reply-To: <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> <20141006234629.GB19445@bbox> From: Dan Streetman Date: Wed, 8 Oct 2014 14:29:44 -0400 X-Google-Sender-Auth: ATcRsoYikP4TqtE9Ic562xsYod0 Message-ID: Subject: Re: [PATCH v1 4/5] zram: add swap full hint To: Minchan Kim Cc: Andrew Morton , linux-kernel , Linux-MM , Hugh Dickins , Shaohua Li , Jerome Marchand , Sergey Senozhatsky , Nitin Gupta , Luigi Semenzato , juno.choi@lge.com 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 Mon, Oct 6, 2014 at 7:46 PM, Minchan Kim wrote: > 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? yep, that looks good to me. > > 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/