From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f199.google.com (mail-pf0-f199.google.com [209.85.192.199]) by kanga.kvack.org (Postfix) with ESMTP id 6E9416B0038 for ; Thu, 20 Oct 2016 03:21:26 -0400 (EDT) Received: by mail-pf0-f199.google.com with SMTP id u84so16949574pfj.6 for ; Thu, 20 Oct 2016 00:21:26 -0700 (PDT) Received: from sender153-mail.zoho.com (sender153-mail.zoho.com. [74.201.84.153]) by mx.google.com with ESMTPS id r132si40373321pgr.231.2016.10.20.00.21.25 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Thu, 20 Oct 2016 00:21:25 -0700 (PDT) Subject: Re: [RFC PATCH 1/1] mm/vmalloc.c: correct logic errors when insert vmap_area References: <20161012144610.GN17128@dhcp22.suse.cz> <57FF2C3C.5070507@zoho.com> From: zijun_hu Message-ID: <58087049.8080106@zoho.com> Date: Thu, 20 Oct 2016 15:20:41 +0800 MIME-Version: 1.0 In-Reply-To: <57FF2C3C.5070507@zoho.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Nicholas Piggin 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, Michal Hocko On 10/13/2016 02:39 PM, zijun_hu wrote: Hi Nicholas, could you give some comments for this patch? thanks a lot > 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 >> > -- 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