linux-kernel - Re: [PATCH v1 11/11] mm/memory_hotplug: mark pageblocks MIGRATE

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Date:   Wed, 19 Aug 2020 15:16:15 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Baoquan He <bhe@...hat.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Charan Teja Reddy <charante@...eaurora.org>
Subject: Re: [PATCH v1 11/11] mm/memory_hotplug: mark pageblocks
 MIGRATE_ISOLATE while onlining memory

On Wed 19-08-20 12:11:57, David Hildenbrand wrote:
> Currently, it can happen that pages are allocated (and freed) via the buddy
> before we finished basic memory onlining.
> 
> For example, pages are exposed to the buddy and can be allocated before
> we actually mark the sections online. Allocated pages could suddenly
> fail pfn_to_online_page() checks. We had similar issues with pcp
> handling, when pages are allocated+freed before we reach
> zone_pcp_update() in online_pages() [1].
> 
> Instead, mark all pageblocks MIGRATE_ISOLATE, such that allocations are
> impossible. Once done with the heavy lifting, use
> undo_isolate_page_range() to move the pages to the MIGRATE_MOVABLE
> freelist, marking them ready for allocation. Similar to offline_pages(),
> we have to manually adjust zone->nr_isolate_pageblock.
> 
> [1] https://lkml.kernel.org/r/1597150703-19003-1-git-send-email-charante@codeaurora.org
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Wei Yang <richard.weiyang@...ux.alibaba.com>
> Cc: Baoquan He <bhe@...hat.com>
> Cc: Pankaj Gupta <pankaj.gupta.linux@...il.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Charan Teja Reddy <charante@...eaurora.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Acked-by: Michal Hocko <mhocko@...e.com>

Yes this looks very sensible and we should have done that from the
beginning. I just have one minor comment below
> @@ -816,6 +816,14 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
>  	if (ret)
>  		goto failed_addition;
>  
> +	/*
> +	 * Fixup the number of isolated pageblocks before marking the sections
> +	 * onlining, such that undo_isolate_page_range() works correctly.
> +	 */
> +	spin_lock_irqsave(&zone->lock, flags);
> +	zone->nr_isolate_pageblock += nr_pages / pageblock_nr_pages;
> +	spin_unlock_irqrestore(&zone->lock, flags);
> +

I am not entirely happy about this. I am wondering whether it would make
more sense to keep the counter in sync already in memmap_init_zone. Sure
we add a branch to the boot time initialization - and it always fails
there - but the code would be cleaner and we wouldn't have to do tricks
like this in caller(s).
-- 
Michal Hocko
SUSE Labs