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Date:   Mon, 10 Sep 2018 15:17:54 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Mikhail Zaslonko <zaslonko@linux.ibm.com>
Cc:     akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Pavel.Tatashin@microsoft.com,
        osalvador@suse.de, gerald.schaefer@de.ibm.com
Subject: Re: [PATCH] memory_hotplug: fix the panic when memory end is not on
 the section boundary
Message-ID: <20180910131754.GG10951@dhcp22.suse.cz>
References: <20180910123527.71209-1-zaslonko@linux.ibm.com>
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[Cc Pavel]

On Mon 10-09-18 14:35:27, Mikhail Zaslonko wrote:
> If memory end is not aligned with the linux memory section boundary, such
> a section is only partly initialized. This may lead to VM_BUG_ON due to
> uninitialized struct pages access from is_mem_section_removable() or
> test_pages_in_a_zone() function.
> 
> Here is one of the panic examples:
>  CONFIG_DEBUG_VM_PGFLAGS=y
>  kernel parameter mem=3075M

OK, so the last memory section is not full and we have a partial memory
block right?

>  page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))

OK, this means that the struct page is not fully initialized. Do you
have a specific place which has triggered this assert?

>  ------------[ cut here ]------------
>  Call Trace:
>  ([<000000000039b8a4>] is_mem_section_removable+0xcc/0x1c0)
>   [<00000000009558ba>] show_mem_removable+0xda/0xe0
>   [<00000000009325fc>] dev_attr_show+0x3c/0x80
>   [<000000000047e7ea>] sysfs_kf_seq_show+0xda/0x160
>   [<00000000003fc4e0>] seq_read+0x208/0x4c8
>   [<00000000003cb80e>] __vfs_read+0x46/0x180
>   [<00000000003cb9ce>] vfs_read+0x86/0x148
>   [<00000000003cc06a>] ksys_read+0x62/0xc0
>   [<0000000000c001c0>] system_call+0xdc/0x2d8
> 
> This fix checks if the page lies within the zone boundaries before
> accessing the struct page data. The check is added to both functions.
> Actually similar check has already been present in
> is_pageblock_removable_nolock() function but only after the struct page
> is accessed.
> 

Well, I am afraid this is not the proper solution. We are relying on the
full pageblock worth of initialized struct pages at many other place. We
used to do that in the past because we have initialized the full
section but this has been changed recently. Pavel, do you have any ideas
how to deal with this partial mem sections now?

> Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
> Cc: <stable@vger.kernel.org>
> ---
>  mm/memory_hotplug.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index 9eea6e809a4e..8e20e8fcc3b0 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -1229,9 +1229,8 @@ static struct page *next_active_pageblock(struct page *page)
>  	return page + pageblock_nr_pages;
>  }
>  
> -static bool is_pageblock_removable_nolock(struct page *page)
> +static bool is_pageblock_removable_nolock(struct page *page, struct zone **zone)
>  {
> -	struct zone *zone;
>  	unsigned long pfn;
>  
>  	/*
> @@ -1241,15 +1240,14 @@ static bool is_pageblock_removable_nolock(struct page *page)
>  	 * We have to take care about the node as well. If the node is offline
>  	 * its NODE_DATA will be NULL - see page_zone.
>  	 */
> -	if (!node_online(page_to_nid(page)))
> -		return false;
> -
> -	zone = page_zone(page);
>  	pfn = page_to_pfn(page);
> -	if (!zone_spans_pfn(zone, pfn))
> +	if (*zone && !zone_spans_pfn(*zone, pfn))
>  		return false;
> +	if (!node_online(page_to_nid(page)))
> +		return false;
> +	*zone = page_zone(page);
>  
> -	return !has_unmovable_pages(zone, page, 0, MIGRATE_MOVABLE, true);
> +	return !has_unmovable_pages(*zone, page, 0, MIGRATE_MOVABLE, true);
>  }
>  
>  /* Checks if this range of memory is likely to be hot-removable. */
> @@ -1257,10 +1255,11 @@ bool is_mem_section_removable(unsigned long start_pfn, unsigned long nr_pages)
>  {
>  	struct page *page = pfn_to_page(start_pfn);
>  	struct page *end_page = page + nr_pages;
> +	struct zone *zone = NULL;
>  
>  	/* Check the starting page of each pageblock within the range */
>  	for (; page < end_page; page = next_active_pageblock(page)) {
> -		if (!is_pageblock_removable_nolock(page))
> +		if (!is_pageblock_removable_nolock(page, &zone))
>  			return false;
>  		cond_resched();
>  	}
> @@ -1296,6 +1295,9 @@ int test_pages_in_a_zone(unsigned long start_pfn, unsigned long end_pfn,
>  				i++;
>  			if (i == MAX_ORDER_NR_PAGES || pfn + i >= end_pfn)
>  				continue;
> +			/* Check if we got outside of the zone */
> +			if (zone && !zone_spans_pfn(zone, pfn))
> +				return 0;
>  			page = pfn_to_page(pfn + i);
>  			if (zone && page_zone(page) != zone)
>  				return 0;
> -- 
> 2.16.4

-- 
Michal Hocko
SUSE Labs