linux-kernel - Re: [RFC] mm: why vfree() do not free page table memory?

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Message-ID: <5A5F1C09.9040000@huawei.com>
Date:   Wed, 17 Jan 2018 17:48:57 +0800
From:   Xishi Qiu <qiuxishi@...wei.com>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        "Wujiangtao (A)" <wu.wujiangtao@...wei.com>
Subject: Re: [RFC] mm: why vfree() do not free page table memory?

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
>>
> 
> 
> .
>