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

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

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.

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.

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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