linux-kernel - Re: [PATCH v1 2/4] mm/memory_hotplug: Handle unaligned start and nr_pages in online_pages

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Date:   Wed, 14 Aug 2019 16:08:05 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Arun KS <arunks@...eaurora.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oscar Salvador <osalvador@...e.de>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v1 2/4] mm/memory_hotplug: Handle unaligned start and
 nr_pages in online_pages_blocks()

On Fri 09-08-19 14:56:59, David Hildenbrand wrote:
> Take care of nr_pages not being a power of two and start not being
> properly aligned. Essentially, what walk_system_ram_range() could provide
> to us. get_order() will round-up in case it's not a power of two.
> 
> This should only apply to memory blocks that contain strange memory
> resources (especially with holes), not to ordinary DIMMs.

I would really like to see an example of such setup before making the
code hard to read. Because I am not really sure something like that
exists at all.

> Fixes: a9cd410a3d29 ("mm/page_alloc.c: memory hotplug: free pages as higher order")
> Cc: Arun KS <arunks@...eaurora.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Pavel Tatashin <pasha.tatashin@...een.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/memory_hotplug.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 3706a137d880..2abd938c8c45 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -640,6 +640,10 @@ static int online_pages_blocks(unsigned long start, unsigned long nr_pages)
>  	while (start < end) {
>  		order = min(MAX_ORDER - 1,
>  			get_order(PFN_PHYS(end) - PFN_PHYS(start)));
> +		/* make sure the PFN is aligned and we don't exceed the range */
> +		while (!IS_ALIGNED(start, 1ul << order) ||
> +		       (1ul << order) > end - start)
> +			order--;
>  		(*online_page_callback)(pfn_to_page(start), order);
>  
>  		onlined_pages += (1UL << order);
> -- 
> 2.21.0

-- 
Michal Hocko
SUSE Labs