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Date:   Thu, 9 Aug 2018 18:12:24 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Rashmica Gupta <rashmica.g@...il.com>
Cc:     toshi.kani@....com, tglx@...utronix.de, bp@...e.de,
        brijesh.singh@....com, thomas.lendacky@....com, jglisse@...hat.com,
        gregkh@...uxfoundation.org, baiyaowei@...s.chinamobile.com,
        dan.j.williams@...el.com, mhocko@...e.com, iamjoonsoo.kim@....com,
        vbabka@...e.cz, malat@...ian.org, bhelgaas@...gle.com,
        osalvador@...hadventures.net, yasu.isimatu@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        rppt@...ux.vnet.ibm.com
Subject: Re: [PATCH v3] resource: Merge resources on a node when hot-adding
 memory

On Thu,  9 Aug 2018 12:54:09 +1000 Rashmica Gupta <rashmica.g@...il.com> wrote:

> When hot-removing memory release_mem_region_adjustable() splits
> iomem resources if they are not the exact size of the memory being
> hot-deleted. Adding this memory back to the kernel adds a new
> resource.
> 
> Eg a node has memory 0x0 - 0xfffffffff. Offlining and hot-removing
> 1GB from 0xf40000000 results in the single resource 0x0-0xfffffffff being
> split into two resources: 0x0-0xf3fffffff and 0xf80000000-0xfffffffff.
> 
> When we hot-add the memory back we now have three resources:
> 0x0-0xf3fffffff, 0xf40000000-0xf7fffffff, and 0xf80000000-0xfffffffff.
> 
> Now if we try to remove some memory that overlaps these resources,
> like 2GB from 0xf40000000, release_mem_region_adjustable() fails as it
> expects the chunk of memory to be within the boundaries of a single
> resource.
> 
> This patch adds a function request_resource_and_merge(). This is called
> instead of request_resource_conflict() when registering a resource in
> add_memory(). It calls request_resource_conflict() and if hot-removing is
> enabled (if it isn't we won't get resource fragmentation) we attempt to
> merge contiguous resources on the node.

What is the end-user impact of this patch?

Do you believe the fix should be merged into 4.18?  Backporting into
-stable kernels?  If so, why?

Thanks.

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