linux-kernel - Re: [PATCH] mm/hotplug: fix offline undo_isolate_page

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Date:   Wed, 13 Mar 2019 08:52:16 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Qian Cai <cai@....pw>
Cc:     akpm@...ux-foundation.org, mhocko@...nel.org, vbabka@...e.cz,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/hotplug: fix offline undo_isolate_page_range()

On Tue, Mar 12, 2019 at 09:42:16PM -0400, Qian Cai wrote:
> +
> +	/*
> +	 * Onlining will reset pagetype flags and makes migrate type
> +	 * MOVABLE, so just need to decrease the number of isolated
> +	 * pageblocks zone counter here.
> +	 */
> +	for (pfn = start_pfn; pfn < end_pfn; pfn += pageblock_nr_pages) {
> +		int i;
> +
> +		for (i = 0; i < pageblock_nr_pages; i++)
> +			if (pfn_valid_within(pfn + i)) {
> +				zone->nr_isolate_pageblock--;
> +				break;
> +			}
> +	}
> +

I do not really like this.

I first thought about saving the value before entering start_isolate_page_range,
but that could race with alloc_contig_range for instance.
So, why not make start_isolate_page_range to return the actual number of isolated
pageblocks?
Sure, that would mean to change a bit how we threat its return code, but
I think that it is pretty simple.
In that way, we would only have to substract the value start_isolate_page_range
gave us at the end of __offline__pages() to set nr_isolate_pageblock back to
its original value.

-- 
Oscar Salvador
SUSE L3