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 Andrew,
On Mon, Aug 26, 2013 at 04:31:50PM -0700, Andrew Morton wrote:
>On Tue, 27 Aug 2013 07:26:04 +0800 Wanpeng Li <liwanp@linux.vnet.ibm.com> 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. 
>> 
>> Hi Andrew,
>> 
>> I see you have already merged the patch, which method you prefer? 
>> 
>
>Addressing it within the madvise code does sound more appropriate.  The
>change which
>mm-hwpoison-fix-memory-failure-still-holding-reference-count-after-unpoisoning-empty-zero-page.patch
>makes is pretty darn strange-looking at at least needs a comment
>telling people what it's doing, and why.

Thanks for your great point out. ;-)
I will address it within the madvise code and resend patch 8/10 ~ patch 10/10.

Regards,
Wanpeng Li 

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