Re: [PATCH 1/4] mm/memory-failure.c: fix race with changing page compound again

[Miaohe Lin]

On 2022/3/5 3:32, Mike Kravetz wrote:
> On 3/4/22 00:26, HORIGUCHI NAOYA(堀口 直也) wrote:
>> On Mon, Feb 28, 2022 at 10:02:42PM +0800, Miaohe Lin wrote:
>>> There is a race window where we got the compound_head, the hugetlb page
>>> could be freed to buddy, or even changed to another compound page just
>>> before we try to get hwpoison page. If this happens, just bail out.
>>
>> I think that when some hugetlb page is about to change into other type/size
>> of compound page, it has to go through buddy allocator because hugetlb pages
>> are maintained in separate memory allocator and they never change into other
>> normal state directly.  memory_failure_hugetlb() takes refcount before
>> lock_page(), so the hugetlb page seems not change between get_hwpoison_page()
>> and lock_page(). So it this new check really necessary?
> 
> A hugetlb page could change size without going through buddy via the new
> demote functionality [1].  Only hugetlb pages on the hugetlb free list can
> be demoted.  
> 
> We should not demote a page if poison is set.  However, there is no check in
> the demote code.  IIUC, poison is set early in the memory error handling
> process, even before taking ref on page.  Demote code needs to be fixed so
> that poisoned pages are not demoted.  I can do that.
> 
> With this change in place, then I think Naoya's statement that hugetlb pages
> can not change state is correct and this patch is not necessary.
> 

Sorry for my confusing commit words. What I mean to tell is indeed the below race:
  CPU 1							CPU 2
  memory_failure_hugetlb
  struct page *head = compound_head(p);
							hugetlb page is freed to buddy, or
							even changed to another compound page
							as we haven't held the page refcnt now
  get_hwpoison_page -- page is not what we want now...

Does this make sense for both of you? Many thanks for comment and reply! :)

> Does that sound reasonable?
> 
> [1] https://lore.kernel.org/linux-mm/20211007181918.136982-1-mike.kravetz@xxxxxxxxxx/

This is really a nice feature. Thanks.

>