linux-kernel - Re: [PATCH] mm: free reserved area's memmap if possiable

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Date:   Mon, 13 Feb 2017 12:24:07 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Will Deacon <will.deacon@....com>
Cc:     zhouxianrong@...wei.com, mark.rutland@....com,
        wangkefeng.wang@...wei.com, srikar@...ux.vnet.ibm.com,
        Mi.Sophia.Wang@...wei.com, catalin.marinas@....com,
        linux-mm@...ck.org, zhangshiming5@...wei.com,
        frowand.list@...il.com, zijun_hu@....com, jszhang@...vell.com,
        won.ho.park@...wei.com, kuleshovmail@...il.com,
        devicetree@...r.kernel.org, chengang@...ndsoft.com.cn,
        zhouxiyu@...wei.com, tj@...nel.org, robh+dt@...nel.org,
        weidu.du@...wei.com, linux-arm-kernel@...ts.infradead.org,
        ard.biesheuvel@...aro.org, steve.capper@....com,
        linux-kernel@...r.kernel.org, joe@...ches.com, dennis.chen@....com,
        akpm@...ux-foundation.org, gkulkarni@...iumnetworks.com
Subject: Re: [PATCH] mm: free reserved area's memmap if possiable

On Mon, Feb 13, 2017 at 12:17:00PM +0000, Will Deacon wrote:
> Adding linux-arm-kernel and devicetree (look for "raw-pfn"), since this
> impacts directly on those.
> 
> On Mon, Feb 13, 2017 at 08:02:29PM +0800, zhouxianrong@...wei.com wrote:
> > From: zhouxianrong <zhouxianrong@...wei.com>
> > 
> > just like freeing no-map area's memmap we could free reserved
> > area's memmap as well only when user of reserved area indicate
> > that we can do this in dts or drivers. that is, user of reserved
> > area know how to use the reserved area who could not memblock_free
> > or free_reserved_xxx the reserved area and regard the area as raw
> > pfn usage. the patch supply a way to users who want to utilize the
> > memmap memory corresponding to raw pfn reserved areas as many as
> > possible.
> 
> I don't really understand this. Can you point me at a specific use-case,
> please? Is CMA involved here?

You don't need "dt permission" to free the memmap page array for the
regions between memory areas.  In fact, adding a DT property for that
goes against the "DT describes the hardware not the implementation"
requirement, since the memmap page array is a Linux implementation
detail.

32-bit ARM has been freeing the memmap page array between memory areas
for years since pre-DT days, and continues to do so.  See
free_unused_memmap().

-- 
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