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From:   Jaewon Kim <jaewon31.kim@gmail.com>
Date:   Tue, 7 Nov 2017 07:30:05 +0900
Message-ID: <CAJrd-UtBcnvZqu77LuRTzc2u8X+qL_kWC5xaYsA-8BHVRLBaBg@mail.gmail.com>
Subject: Re: [PATCH] mm: page_ext: allocate page extension though first PFN is invalid
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Jaewon Kim <jaewon31.kim@samsung.com>,
        Andrew Morton <akpm@linux-foundation.org>, vbabka@suse.cz,
        minchan@kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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2017-11-02 17:02 GMT+09:00 Michal Hocko <mhocko@kernel.org>:
> 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.
Hello Michal Hocko
Thank you for your comment.
I think checking Null by erasing #if defined(CONFIG_DEBUG_VM) is the
stable material.
I wonder if you want me to split and resend the 2 patches, or if you
will use this mail thread for the further discussion.

Thank you
Jaewon Kim
>
>> 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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