linux-kernel - Re: [PATCH] mm/vmalloc: add vm_struct for vm_map

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Date:   Wed, 19 Jul 2017 13:50:14 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     zhaoyang.huang@...eadtrum.com, Michal Hocko <mhocko@...e.com>,
        Ingo Molnar <mingo@...nel.org>, zijun_hu <zijun_hu@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Thomas Garnier <thgarnie@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, zijun_hu@...o.com
Subject: Re: [PATCH] mm/vmalloc: add vm_struct for vm_map_ram area

On Wed, 19 Jul 2017 18:44:03 +0800 Zhaoyang Huang <huangzhaoyang@...il.com> wrote:

> /proc/vmallocinfo will not show the area allocated by vm_map_ram, which
> will make confusion when debug. Add vm_struct for them and show them in
> proc.
> 

Please provide sample /proc/vmallocinfo so we can better understand the
proposal.  Is there a means by which people can determine that a
particular area is from vm_map_ram()?  I don't think so.  Should there
be?

>
> ...
>
> @@ -1173,6 +1178,12 @@ void *vm_map_ram(struct page **pages, unsigned int count, int node, pgprot_t pro
>  		addr = (unsigned long)mem;
>  	} else {
>  		struct vmap_area *va;
> +		struct vm_struct *area;
> +
> +		area = kzalloc_node(sizeof(*area), GFP_KERNEL, node);
> +		if (unlikely(!area))
> +			return NULL;

Allocating a vm_struct for each vm_map_ram area is a cost.  And we're
doing this purely for /proc/vmallocinfo.  I think I'll need more
persuading to convince me that this is a good tradeoff, given that
*every* user will incur this cost, and approximately 0% of them will
ever use /proc/vmallocinfo.

So... do we *really* need this?  If so, why?