Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753398AbcJMGmQ (ORCPT ); Thu, 13 Oct 2016 02:42:16 -0400 Received: from sender153-mail.zoho.com ([74.201.84.153]:25386 "EHLO sender153-mail.zoho.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755685AbcJMGmF (ORCPT ); Thu, 13 Oct 2016 02:42:05 -0400 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=zapps768; d=zoho.com; h=subject:references:cc:to:from:message-id:date:user-agent:mime-version:in-reply-to:content-type; b=LDcVyhQQTx3sqDcXAMTny0x8pLWF6t/4jISR8FP0lNWypPtr2tYqmuxxwtDT0mrkEVTBWT3BkZLU ca5nMPemkJzdKX+kXAg4nQKVhgtzqTGLYkj97si7glK8ZELsmNiD Subject: Re: [RFC PATCH 1/1] mm/vmalloc.c: correct logic errors when insert vmap_area References: <20161012144610.GN17128@dhcp22.suse.cz> Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, zijun_hu@htc.com, akpm@linux-foundation.org, rientjes@google.com, tj@kernel.org, sfr@canb.auug.org.au, mingo@kernel.org, iamjoonsoo.kim@lge.com, mgorman@techsingularity.net, hannes@cmpxchg.org, chris@chris-wilson.co.uk, vdavydov.dev@gmail.com, Nicholas Piggin , Michal Hocko To: Nicholas Piggin From: zijun_hu Message-ID: <57FF2C3C.5070507@zoho.com> Date: Thu, 13 Oct 2016 14:39:56 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0 MIME-Version: 1.0 In-Reply-To: <20161012144610.GN17128@dhcp22.suse.cz> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3138 Lines: 83 Hi Nicholas, i find __insert_vmap_area() is introduced by you could you offer comments for this patch related to that funciton thanks On 10/12/2016 10:46 PM, Michal Hocko wrote: > [Let's CC Nick who has written this code] > > On Wed 12-10-16 22:30:13, zijun_hu wrote: >> From: zijun_hu >> >> the KVA allocator organizes vmap_areas allocated by rbtree. in order to >> insert a new vmap_area @i_va into the rbtree, walk around the rbtree from >> root and compare the vmap_area @t_va met on the rbtree against @i_va; walk >> toward the left branch of @t_va if @i_va is lower than @t_va, and right >> branch if higher, otherwise handle this error case since @i_va has overlay >> with @t_va; however, __insert_vmap_area() don't follow the desired >> procedure rightly, moreover, it includes a meaningless else if condition >> and a redundant else branch as shown by comments in below code segments: >> static void __insert_vmap_area(struct vmap_area *va) >> { >> as a internal interface parameter, we assume vmap_area @va has nonzero size >> ... >> if (va->va_start < tmp->va_end) >> p = &(*p)->rb_left; >> else if (va->va_end > tmp->va_start) >> p = &(*p)->rb_right; >> this else if condition is always true and meaningless due to >> va->va_end > va->va_start >= tmp_va->va_end > tmp_va->va_start normally >> else >> BUG(); >> this BUG() is meaningless too due to never be reached normally >> ... >> } >> >> it looks like the else if condition and else branch are canceled. no errors >> are caused since the vmap_area @va to insert as a internal interface >> parameter doesn't have overlay with any one on the rbtree normally. however >> __insert_vmap_area() looks weird and really has several logic errors as >> pointed out above when it is viewed as a separate function. > > I have tried to read this several times but I am completely lost to > understand what the actual bug is and how it causes vmap_area sorting to > misbehave. So is this a correctness issue, performance improvement or > theoretical fix for an incorrect input? > >> fix by walking around vmap_area rbtree as described above to insert >> a vmap_area. >> >> BTW, (va->va_end == tmp_va->va_start) is consider as legal case since it >> indicates vmap_area @va left neighbors with @tmp_va tightly. >> >> Fixes: db64fe02258f ("mm: rewrite vmap layer") >> Signed-off-by: zijun_hu >> --- >> mm/vmalloc.c | 8 ++++---- >> 1 file changed, 4 insertions(+), 4 deletions(-) >> >> diff --git a/mm/vmalloc.c b/mm/vmalloc.c >> index 5daf3211b84f..8b80931654b7 100644 >> --- a/mm/vmalloc.c >> +++ b/mm/vmalloc.c >> @@ -321,10 +321,10 @@ static void __insert_vmap_area(struct vmap_area *va) >> >> parent = *p; >> tmp_va = rb_entry(parent, struct vmap_area, rb_node); >> - if (va->va_start < tmp_va->va_end) >> - p = &(*p)->rb_left; >> - else if (va->va_end > tmp_va->va_start) >> - p = &(*p)->rb_right; >> + if (va->va_end <= tmp_va->va_start) >> + p = &parent->rb_left; >> + else if (va->va_start >= tmp_va->va_end) >> + p = &parent->rb_right; >> else >> BUG(); >> } >> -- >> 1.9.1 >