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[209.85.220.65]) by mx.google.com with SMTPS id r27sor67294803pfg.42.2019.08.05.18.38.52 for (Google Transport Security); Mon, 05 Aug 2019 18:38:52 -0700 (PDT) Received-SPF: pass (google.com: domain of minchan.kim@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=LWHFsZXQ; spf=pass (google.com: domain of minchan.kim@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=minchan.kim@gmail.com; dmarc=fail (p=NONE sp=NONE dis=NONE) header.from=kernel.org DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=pl8UsF4L2ZgN2cVoUatHU9qNDymPWthAuYy0ZpUojvc=; b=LWHFsZXQeOtaAf0vU5ZLc/uArDplP5t2VWo/9efFUT1IM0+JY3EiW/byzmVHAkZxRd IAZBSUHc4bKMvF0PjTiQN6WDOgaa69V8P6AmKSOsNnmGu+gnDKQHbRrDLBNSiui/s49E aDXuIerm8fGsMSv7mOfIFaSVfvZAb+VXtfUBtCvOYAVwmzsKjGLnV0hkzr7Wi+eUHWXc 9VuCF0PqMmZGa2T4HrHrkFj2TEcOMQ6/Iq1jkQmNtpQyYhBbHwyIB6A1xaoakgpxqdIu zgPPlIGpfrdUi/8CzM+k6XC7mMcJa7+6ZZO1LW5zvkpUQVjyFdkD/tj6tr5SJQbGqTzK JwOA== X-Google-Smtp-Source: APXvYqzfJAxA1OnUEHw9LW/32P82SS68sMs2IhsEUGFClYXA8MiJ6dqBpGqNgm00eqeVW9d9O6FQvg== X-Received: by 2002:aa7:8a97:: with SMTP id a23mr936671pfc.117.1565055531996; Mon, 05 Aug 2019 18:38:51 -0700 (PDT) Received: from google.com ([2401:fa00:d:0:98f1:8b3d:1f37:3e8]) by smtp.gmail.com with ESMTPSA id f19sm125035106pfk.180.2019.08.05.18.38.48 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Mon, 05 Aug 2019 18:38:50 -0700 (PDT) Date: Tue, 6 Aug 2019 10:38:46 +0900 From: Minchan Kim To: Henry Burns Cc: Nitin Gupta , Sergey Senozhatsky , Shakeel Butt , Jonathan Adams , Linux MM , LKML Subject: Re: [PATCH 2/2] mm/zsmalloc.c: Fix race condition in zs_destroy_pool Message-ID: <20190806013846.GA71899@google.com> References: <20190802015332.229322-1-henryburns@google.com> <20190802015332.229322-2-henryburns@google.com> <20190805042821.GA102749@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Aug 05, 2019 at 10:34:41AM -0700, Henry Burns wrote: > On Sun, Aug 4, 2019 at 9:28 PM Minchan Kim wrote: > > > > Hi Henry, > > > > On Thu, Aug 01, 2019 at 06:53:32PM -0700, Henry Burns wrote: > > > In zs_destroy_pool() we call flush_work(&pool->free_work). However, we > > > have no guarantee that migration isn't happening in the background > > > at that time. > > > > > > Since migration can't directly free pages, it relies on free_work > > > being scheduled to free the pages. But there's nothing preventing an > > > in-progress migrate from queuing the work *after* > > > zs_unregister_migration() has called flush_work(). Which would mean > > > pages still pointing at the inode when we free it. > > > > We already unregister shrinker so there is no upcoming async free call > > via shrinker so the only concern is zs_compact API direct call from > > the user. Is that what what you desribe from the description? > > What I am describing is a call to zsmalloc_aops->migratepage() by > kcompactd (which can call schedule work in either > zs_page_migrate() or zs_page_putback should the zspage become empty). > > While we are migrating a page, we remove it from the class. Suppose > zs_free() loses a race with migration. We would schedule > async_free_zspage() to handle freeing that zspage, however we have no > guarantee that migration has finished > by the time we finish flush_work(&pool->work). In that case we then > call iput(inode), and now we have a page > pointing to a non-existent inode. (At which point something like > kcompactd would potentially BUG() if it tries to get a page > (from the inode) that doesn't exist anymore) > True. I totally got mixed up internal migration and external migration. :-/ > > > > > If so, can't we add a flag to indicate destroy of the pool and > > global counter to indicate how many of zs_compact was nested? > > > > So, zs_unregister_migration in zs_destroy_pool can set the flag to > > prevent upcoming zs_compact call and wait until the global counter > > will be zero. Once it's done, finally flush the work. > > > > My point is it's not a per-class granuarity but global. > > We could have a pool level counter of isolated pages, and wait for > that to finish before starting flush_work(&pool->work); However, > that would require an atomic_long in zs_pool, and we would have to eat > the cost of any contention over that lock. Still, it might be > preferable to a per-class granularity. That would be better for performance-wise but how it's significant? Migration is not already hot path so adding a atomic variable in that path wouldn't make noticible slow. Rather than performance, my worry is maintainance so prefer simple and not fragile. > > > > > Thanks. > > > > > > > > Since we know at destroy time all objects should be free, no new > > > migrations can come in (since zs_page_isolate() fails for fully-free > > > zspages). This means it is sufficient to track a "# isolated zspages" > > > count by class, and have the destroy logic ensure all such pages have > > > drained before proceeding. Keeping that state under the class > > > spinlock keeps the logic straightforward. > > > > > > Signed-off-by: Henry Burns > > > --- > > > mm/zsmalloc.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++--- > > > 1 file changed, 65 insertions(+), 3 deletions(-) > > > > > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c > > > index efa660a87787..1f16ed4d6a13 100644 > > > --- a/mm/zsmalloc.c > > > +++ b/mm/zsmalloc.c > > > @@ -53,6 +53,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > #include > > > #include > > > > > > @@ -206,6 +207,10 @@ struct size_class { > > > int objs_per_zspage; > > > /* Number of PAGE_SIZE sized pages to combine to form a 'zspage' */ > > > int pages_per_zspage; > > > +#ifdef CONFIG_COMPACTION > > > + /* Number of zspages currently isolated by compaction */ > > > + int isolated; > > > +#endif > > > > > > unsigned int index; > > > struct zs_size_stat stats; > > > @@ -267,6 +272,8 @@ struct zs_pool { > > > #ifdef CONFIG_COMPACTION > > > struct inode *inode; > > > struct work_struct free_work; > > > + /* A workqueue for when migration races with async_free_zspage() */ > > > + struct wait_queue_head migration_wait; > > > #endif > > > }; > > > > > > @@ -1917,6 +1924,21 @@ static void putback_zspage_deferred(struct zs_pool *pool, > > > > > > } > > > > > > +static inline void zs_class_dec_isolated(struct zs_pool *pool, > > > + struct size_class *class) > > > +{ > > > + assert_spin_locked(&class->lock); > > > + VM_BUG_ON(class->isolated <= 0); > > > + class->isolated--; > > > + /* > > > + * There's no possibility of racing, since wait_for_isolated_drain() > > > + * checks the isolated count under &class->lock after enqueuing > > > + * on migration_wait. > > > + */ > > > + if (class->isolated == 0 && waitqueue_active(&pool->migration_wait)) > > > + wake_up_all(&pool->migration_wait); > > > +} > > > + > > > static void replace_sub_page(struct size_class *class, struct zspage *zspage, > > > struct page *newpage, struct page *oldpage) > > > { > > > @@ -1986,6 +2008,7 @@ static bool zs_page_isolate(struct page *page, isolate_mode_t mode) > > > */ > > > if (!list_empty(&zspage->list) && !is_zspage_isolated(zspage)) { > > > get_zspage_mapping(zspage, &class_idx, &fullness); > > > + class->isolated++; > > > remove_zspage(class, zspage, fullness); > > > } > > > > > > @@ -2085,8 +2108,14 @@ static int zs_page_migrate(struct address_space *mapping, struct page *newpage, > > > * Page migration is done so let's putback isolated zspage to > > > * the list if @page is final isolated subpage in the zspage. > > > */ > > > - if (!is_zspage_isolated(zspage)) > > > + if (!is_zspage_isolated(zspage)) { > > > + /* > > > + * We still hold the class lock while all of this is happening, > > > + * so we cannot race with zs_destroy_pool() > > > + */ > > > putback_zspage_deferred(pool, class, zspage); > > > + zs_class_dec_isolated(pool, class); > > > + } > > > > > > reset_page(page); > > > put_page(page); > > > @@ -2131,9 +2160,11 @@ static void zs_page_putback(struct page *page) > > > > > > spin_lock(&class->lock); > > > dec_zspage_isolation(zspage); > > > - if (!is_zspage_isolated(zspage)) > > > - putback_zspage_deferred(pool, class, zspage); > > > > > > + if (!is_zspage_isolated(zspage)) { > > > + putback_zspage_deferred(pool, class, zspage); > > > + zs_class_dec_isolated(pool, class); > > > + } > > > spin_unlock(&class->lock); > > > } > > > > > > @@ -2156,8 +2187,36 @@ static int zs_register_migration(struct zs_pool *pool) > > > return 0; > > > } > > > > > > +static bool class_isolated_are_drained(struct size_class *class) > > > +{ > > > + bool ret; > > > + > > > + spin_lock(&class->lock); > > > + ret = class->isolated == 0; > > > + spin_unlock(&class->lock); > > > + return ret; > > > +} > > > + > > > +/* Function for resolving migration */ > > > +static void wait_for_isolated_drain(struct zs_pool *pool) > > > +{ > > > + int i; > > > + > > > + /* > > > + * We're in the process of destroying the pool, so there are no > > > + * active allocations. zs_page_isolate() fails for completely free > > > + * zspages, so we need only wait for each size_class's isolated > > > + * count to hit zero. > > > + */ > > > + for (i = 0; i < ZS_SIZE_CLASSES; i++) { > > > + wait_event(pool->migration_wait, > > > + class_isolated_are_drained(pool->size_class[i])); > > > + } > > > +} > > > + > > > static void zs_unregister_migration(struct zs_pool *pool) > > > { > > > + wait_for_isolated_drain(pool); /* This can block */ > > > flush_work(&pool->free_work); > > > iput(pool->inode); > > > } > > > @@ -2401,6 +2460,8 @@ struct zs_pool *zs_create_pool(const char *name) > > > if (!pool->name) > > > goto err; > > > > > > + init_waitqueue_head(&pool->migration_wait); > > > + > > > if (create_cache(pool)) > > > goto err; > > > > > > @@ -2466,6 +2527,7 @@ struct zs_pool *zs_create_pool(const char *name) > > > class->index = i; > > > class->pages_per_zspage = pages_per_zspage; > > > class->objs_per_zspage = objs_per_zspage; > > > + class->isolated = 0; > > > spin_lock_init(&class->lock); > > > pool->size_class[i] = class; > > > for (fullness = ZS_EMPTY; fullness < NR_ZS_FULLNESS; > > > -- > > > 2.22.0.770.g0f2c4a37fd-goog > > >