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

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Message-ID: <0ffd113e-84da-bd49-2b63-3d27d2702580@suse.cz>
Date:   Wed, 17 Jan 2018 10:16:01 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Xishi Qiu <qiuxishi@...wei.com>, Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Cc:     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 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.

> 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.

> Thanks,
> Xishi Qiu
>