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>]

On Mon, Aug 26, 2013 at 08:46:54PM -0400, Naoya Horiguchi wrote:
>On Tue, Aug 27, 2013 at 08:21:05AM +0800, Wanpeng Li wrote:
>> Hi Naoya,
>> On Mon, Aug 26, 2013 at 08:12:29PM -0400, Naoya Horiguchi wrote:
>> >Hi Wanpeng,
>> >
>> >On Tue, Aug 27, 2013 at 07:26:04AM +0800, Wanpeng Li wrote:
>> >> 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. 
>> >
>> >OK. I agree.
>> >
>> >> 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! ;-)
>> >
>> >No objection if you (and Andrew) decide to go with current approach.
>> 
>> Andrew prefer your method, I will resend the patch w/ your suggested-by. ;-)
>
>Thanks you :)
>
>> >But I think that if we shift to fix this problem in madvise(),
>> >we don't need 10/10 any more. So it looks simpler to me.
>> 
>> I don't think it's same issue. There is just one page in my test case.
>> #define PAGES_TO_TEST 1
>> If I miss something?
>
>Ah, OK.

I complete do it in madvise codes, however, the bug mentioned in patch
10/10 is still there. ;-)

>
>BTW, in my understanding, zero pages are not exist physically (I mean that
>no real page is allocated to store 4096 bytes of 0.) So there can't happen
>any real MCE SRAO on zero page. So one possible solution might be that we
>completely ignore all of madvise(MADV_HWPOISON) over zero pages.

What's the userland visible difference against mmap w/o write access before poison 
you expect?

Regards,
Wanpeng Li 

>
>Thanks,
>Naoya Horiguchi

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