Re: [PATCH v4 8/10] mm/hwpoison: fix memory failure still hold reference count after unpoison empty zero page

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

Hi Naoya,
On Mon, Aug 26, 2013 at 11:45:37AM -0400, Naoya Horiguchi wrote:
>On Mon, Aug 26, 2013 at 04:46:12PM +0800, Wanpeng Li wrote:
>> madvise hwpoison inject will poison the read-only empty zero page if there is 
>> no write access before poison. Empty zero page reference count will be increased 
>> for hwpoison, subsequent poison zero page will return directly since page has
>> already been set PG_hwpoison, however, page reference count is still increased 
>> by get_user_pages_fast. The unpoison process will unpoison the empty zero page 
>> and decrease the reference count successfully for the fist time, however, 
>> subsequent unpoison empty zero page will return directly since page has already 
>> been unpoisoned and without decrease the page reference count of empty zero page.
>> This patch fix it by decrease page reference count for empty zero page which has 
>> already been unpoisoned and page count > 1.
>
>I guess that fixing on the madvise side looks reasonable to me, because this
>refcount mismatch happens only when we poison with madvise(). The root cause
>is that we can get refcount multiple times on a page, even if memory_failure()
>or soft_offline_page() can do its work only once.
>

I think this just happen in read-only before poison case against empty
zero page. 

Hi Andrew,

I see you have already merged the patch, which method you prefer? 

>How about making madvise_hwpoison() put a page and return immediately
>(without calling memory_failure() or soft_offline_page()) when the page
>is already hwpoisoned? 
>I hope it also helps us avoid meaningless printk flood.
>

Btw, Naoya, how about patch 10/10, any input are welcome! ;-)

Regards,
Wanpeng Li 

>Thanks,
>Naoya Horiguchi
>
>> Testcase:
>> 
>> #define _GNU_SOURCE
>> #include <stdlib.h>
>> #include <stdio.h>
>> #include <sys/mman.h>
>> #include <unistd.h>
>> #include <fcntl.h>
>> #include <sys/types.h>
>> #include <errno.h>
>> 
>> #define PAGES_TO_TEST 3
>> #define PAGE_SIZE	4096
>> 
>> int main(void)
>> {
>> 	char *mem;
>> 	int i;
>> 
>> 	mem = mmap(NULL, PAGES_TO_TEST * PAGE_SIZE,
>> 			PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
>> 
>> 	if (madvise(mem, PAGES_TO_TEST * PAGE_SIZE, MADV_HWPOISON) == -1)
>> 		return -1;
>> 	
>> 	munmap(mem, PAGES_TO_TEST * PAGE_SIZE);
>> 
>> 	return 0;
>> }
>> 
>> Add printk to dump page reference count:
>> 
>> [   93.075959] Injecting memory failure for page 0x19d0 at 0xb77d8000
>> [   93.076207] MCE 0x19d0: non LRU page recovery: Ignored
>> [   93.076209] pfn 0x19d0, page count = 1 after memory failure
>> [   93.076220] Injecting memory failure for page 0x19d0 at 0xb77d9000
>> [   93.076221] MCE 0x19d0: already hardware poisoned
>> [   93.076222] pfn 0x19d0, page count = 2 after memory failure
>> [   93.076224] Injecting memory failure for page 0x19d0 at 0xb77da000
>> [   93.076224] MCE 0x19d0: already hardware poisoned
>> [   93.076225] pfn 0x19d0, page count = 3 after memory failure
>> 
>> Before patch:
>> 
>> [  139.197474] MCE: Software-unpoisoned page 0x19d0
>> [  139.197479] pfn 0x19d0, page count = 2 after unpoison memory
>> [  150.478130] MCE: Page was already unpoisoned 0x19d0
>> [  150.478135] pfn 0x19d0, page count = 2 after unpoison memory
>> [  151.548288] MCE: Page was already unpoisoned 0x19d0
>> [  151.548292] pfn 0x19d0, page count = 2 after unpoison memory
>> 
>> After patch:
>> 
>> [  116.022122] MCE: Software-unpoisoned page 0x19d0
>> [  116.022127] pfn 0x19d0, page count = 2 after unpoison memory
>> [  117.256163] MCE: Page was already unpoisoned 0x19d0
>> [  117.256167] pfn 0x19d0, page count = 1 after unpoison memory
>> [  117.917772] MCE: Page was already unpoisoned 0x19d0
>> [  117.917777] pfn 0x19d0, page count = 1 after unpoison memory
>> 
>> Signed-off-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
>> ---
>>  mm/memory-failure.c | 2 ++
>>  1 file changed, 2 insertions(+)
>> 
>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>> index ca714ac..fb687fd 100644
>> --- a/mm/memory-failure.c
>> +++ b/mm/memory-failure.c
>> @@ -1335,6 +1335,8 @@ int unpoison_memory(unsigned long pfn)
>>  	page = compound_head(p);
>>  
>>  	if (!PageHWPoison(p)) {
>> +		if (pfn == my_zero_pfn(0) && page_count(p) > 1)
>> +			put_page(p);
>>  		pr_info("MCE: Page was already unpoisoned %#lx\n", pfn);
>>  		return 0;
>>  	}
>> -- 
>> 1.8.1.2
>>
>
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