From mboxrd@z Thu Jan  1 00:00:00 1970
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752700AbeAQJt0 (ORCPT <rfc822;ralf@linux-mips.org> + 1 other);
        Wed, 17 Jan 2018 04:49:26 -0500
Received: from szxga05-in.huawei.com ([45.249.212.191]:4236 "EHLO huawei.com"
        rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP
        id S1752268AbeAQJtU (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 17 Jan 2018 04:49:20 -0500
Message-ID: <5A5F1C09.9040000@huawei.com>
Date: Wed, 17 Jan 2018 17:48:57 +0800
From: Xishi Qiu <qiuxishi@huawei.com>
User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:12.0) Gecko/20120428 Thunderbird/12.0.1
MIME-Version: 1.0
To: Vlastimil Babka <vbabka@suse.cz>
CC: Michal Hocko <mhocko@kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        "Wujiangtao (A)" <wu.wujiangtao@huawei.com>
Subject: Re: [RFC] mm: why vfree() do not free page table memory?
References: <5A4603AB.8060809@huawei.com> <0ffd113e-84da-bd49-2b63-3d27d2702580@suse.cz>
In-Reply-To: <0ffd113e-84da-bd49-2b63-3d27d2702580@suse.cz>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit
X-Originating-IP: [10.177.25.179]
X-CFilter-Loop: Reflected
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Return-Path: <linux-kernel-owner@vger.kernel.org>

On 2018/1/17 17:16, Vlastimil Babka wrote:

> On 12/29/2017 09:58 AM, Xishi Qiu wrote:
>> When calling vfree(), it calls unmap_vmap_area() to clear page table,
>> but do not free the memory of page table, why? just for performance?
> 
> I guess it's expected that the free virtual range and associated page
> tables it might be reused later.
> 

Hi Vlastimil，

If use vmalloc/vfree different size, then there will be some hols during 
VMALLOC_START to VMALLOC_END, and this holes takes page table memory, right?

>> If a driver use vmalloc() and vfree() frequently, we will lost much
>> page table memory, maybe oom later.
> 
> If it's reused, then not really.
> 
> Did you notice an actual issue, or is this just theoretical concern.
> 

Yes, we have this problem on our production line.
I find the page table memory takes 200-300M.

Thanks,
Xishi Qiu

>> Thanks,
>> Xishi Qiu
>>
> 
> 
> .
>