linux-kernel - Re: [PATCH v2] resource: Merge resources on a node when hot-adding memory

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Date:   Tue, 7 Aug 2018 13:52:07 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Rashmica Gupta <rashmica.g@...il.com>, toshi.kani@....com,
        tglx@...utronix.de, akpm@...ux-foundation.org, 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,
        malat@...ian.org, pasha.tatashin@...cle.com, bhelgaas@...gle.com,
        osalvador@...hadventures.net, yasu.isimatu@...il.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] resource: Merge resources on a node when hot-adding
 memory

On 08/06/2018 08:52 AM, Rashmica Gupta 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 a section of 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.

Hi,

it's the first time I see the resource code, so I might be easily wrong.
How can it happen that the second remove is section aligned but the
first one not?

> 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.
> 
> Signed-off-by: Rashmica Gupta <rashmica.g@...il.com>
...
> --- a/kernel/resource.c
> +++ b/kernel/resource.c
...
> +/*
> + * Attempt to merge resources on the node
> + */
> +static void merge_node_resources(int nid, struct resource *parent)
> +{
> +	struct resource *res;
> +	uint64_t start_addr;
> +	uint64_t end_addr;
> +	int ret;
> +
> +	start_addr = node_start_pfn(nid) << PAGE_SHIFT;
> +	end_addr = node_end_pfn(nid) << PAGE_SHIFT;
> +
> +	write_lock(&resource_lock);
> +
> +	/* Get the first resource */
> +	res = parent->child;
> +
> +	while (res) {
> +		/* Check that the resource is within the node */
> +		if (res->start < start_addr) {
> +			res = res->sibling;
> +			continue;
> +		}
> +		/* Exit if resource is past end of node */
> +		if (res->sibling->end > end_addr)
> +			break;

IIUC, resource end is closed, so adjacent resources's start is end+1.
But node_end_pfn is open, so the comparison above should use '>='
instead of '>'?

> +
> +		ret = merge_resources(res);
> +		if (!ret)
> +			continue;
> +		res = res->sibling;

Should this rather use next_resource() to merge at all levels of the
hierarchy? Although memory seems to be flat under &iomem_resource so it
would be just future-proofing.

Thanks,
Vlastimil