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Date:   Thu, 2 Nov 2017 09:02:49 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Jaewon Kim <jaewon31.kim@samsung.com>
Cc:     akpm@linux-foundation.org, vbabka@suse.cz, minchan@kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        jaewon31.kim@gmail.com
Subject: Re: [PATCH] mm: page_ext: allocate page extension though first PFN
 is invalid
Message-ID: <20171102080249.uxxq4ko3cc2wgnbz@dhcp22.suse.cz>
References: <CGME20171102063347epcas2p2ce3e91597de3bf68e818130ea44ac769@epcas2p2.samsung.com>
 <20171102063507.25671-1-jaewon31.kim@samsung.com>
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On Thu 02-11-17 15:35:07, Jaewon Kim wrote:
> online_page_ext and page_ext_init allocate page_ext for each section, but
> they do not allocate if the first PFN is !pfn_present(pfn) or
> !pfn_valid(pfn). Then section->page_ext remains as NULL. lookup_page_ext
> checks NULL only if CONFIG_DEBUG_VM is enabled. For a valid PFN,
> __set_page_owner will try to get page_ext through lookup_page_ext.
> Without CONFIG_DEBUG_VM lookup_page_ext will misuse NULL pointer as value
> 0. This incurrs invalid address access.
> 
> This is the panic example when PFN 0x100000 is not valid but PFN 0x13FC00
> is being used for page_ext. section->page_ext is NULL, get_entry returned
> invalid page_ext address as 0x1DFA000 for a PFN 0x13FC00.
> 
> <1>[   11.618085] Unable to handle kernel paging request at virtual address 01dfa014
> <1>[   11.618140] pgd = ffffffc0c6dc9000
> <1>[   11.618174] [01dfa014] *pgd=0000000000000000, *pud=0000000000000000
> <4>[   11.618240] ------------[ cut here ]------------
> <2>[   11.618278] Kernel BUG at ffffff80082371e0 [verbose debug info unavailable]
> <0>[   11.618338] Internal error: Oops: 96000045 [#1] PREEMPT SMP
> <4>[   11.618381] Modules linked in:
> <4>[   11.618524] task: ffffffc0c6ec9180 task.stack: ffffffc0c6f40000
> <4>[   11.618569] PC is at __set_page_owner+0x48/0x78
> <4>[   11.618607] LR is at __set_page_owner+0x44/0x78
> <4>[   11.626025] [<ffffff80082371e0>] __set_page_owner+0x48/0x78
> <4>[   11.626071] [<ffffff80081df9f0>] get_page_from_freelist+0x880/0x8e8
> <4>[   11.626118] [<ffffff80081e00a4>] __alloc_pages_nodemask+0x14c/0xc48
> <4>[   11.626165] [<ffffff80081e610c>] __do_page_cache_readahead+0xdc/0x264
> <4>[   11.626214] [<ffffff80081d8824>] filemap_fault+0x2ac/0x550
> <4>[   11.626259] [<ffffff80082e5cf8>] ext4_filemap_fault+0x3c/0x58
> <4>[   11.626305] [<ffffff800820a2f8>] __do_fault+0x80/0x120
> <4>[   11.626347] [<ffffff800820eb4c>] handle_mm_fault+0x704/0xbb0
> <4>[   11.626393] [<ffffff800809ba70>] do_page_fault+0x2e8/0x394
> <4>[   11.626437] [<ffffff8008080be4>] do_mem_abort+0x88/0x124
> 
> Though the first page is not valid, page_ext could be useful for other
> pages in the section. But checking all PFNs in a section may be time
> consuming job. Let's check each (section count / 16) PFN, then prepare
> page_ext if any PFN is present or valid. And remove the CONFIG_DEBUG_VM in
> lookup_page_ext to avoid panic.

So I would split this patch into two. First one to address the panic
which sounds like a stable material and then the enhancement which will
most likely need a further discussion.

> Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com>
> ---
>  mm/page_ext.c | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 32f18911deda..bf9c99beb312 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -124,7 +124,6 @@ struct page_ext *lookup_page_ext(struct page *page)
>  	struct page_ext *base;
>  
>  	base = NODE_DATA(page_to_nid(page))->node_page_ext;
> -#if defined(CONFIG_DEBUG_VM)
>  	/*
>  	 * The sanity checks the page allocator does upon freeing a
>  	 * page can reach here before the page_ext arrays are
> @@ -133,7 +132,6 @@ struct page_ext *lookup_page_ext(struct page *page)
>  	 */
>  	if (unlikely(!base))
>  		return NULL;
> -#endif
>  	index = pfn - round_down(node_start_pfn(page_to_nid(page)),
>  					MAX_ORDER_NR_PAGES);
>  	return get_entry(base, index);
> @@ -198,7 +196,6 @@ struct page_ext *lookup_page_ext(struct page *page)
>  {
>  	unsigned long pfn = page_to_pfn(page);
>  	struct mem_section *section = __pfn_to_section(pfn);
> -#if defined(CONFIG_DEBUG_VM)
>  	/*
>  	 * The sanity checks the page allocator does upon freeing a
>  	 * page can reach here before the page_ext arrays are
> @@ -207,7 +204,6 @@ struct page_ext *lookup_page_ext(struct page *page)
>  	 */
>  	if (!section->page_ext)
>  		return NULL;
> -#endif
>  	return get_entry(section->page_ext, pfn);
>  }
>  
> @@ -312,7 +308,17 @@ static int __meminit online_page_ext(unsigned long start_pfn,
>  	}
>  
>  	for (pfn = start; !fail && pfn < end; pfn += PAGES_PER_SECTION) {
> -		if (!pfn_present(pfn))
> +		unsigned long t_pfn = pfn;
> +		bool present = false;
> +
> +		while (t_pfn <	ALIGN(pfn + 1, PAGES_PER_SECTION)) {
> +			if (pfn_present(t_pfn)) {
> +				present = true;
> +				break;
> +			}
> +			t_pfn = ALIGN(pfn + 1, PAGES_PER_SECTION >> 4);
> +		}
> +		if (!present)
>  			continue;
>  		fail = init_section_page_ext(pfn, nid);
>  	}
> @@ -391,8 +397,17 @@ void __init page_ext_init(void)
>  		 */
>  		for (pfn = start_pfn; pfn < end_pfn;
>  			pfn = ALIGN(pfn + 1, PAGES_PER_SECTION)) {
> -
> -			if (!pfn_valid(pfn))
> +			unsigned long t_pfn = pfn;
> +			bool valid = false;
> +
> +			while (t_pfn <	ALIGN(pfn + 1, PAGES_PER_SECTION)) {
> +				if (pfn_valid(t_pfn)) {
> +					valid = true;
> +					break;
> +				}
> +				t_pfn = ALIGN(pfn + 1, PAGES_PER_SECTION >> 4);
> +			}
> +			if (!valid)
>  				continue;
>  			/*
>  			 * Nodes's pfns can be overlapping.
> -- 
> 2.13.0

-- 
Michal Hocko
SUSE Labs

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