Re: [PATCH v1 2/6] mm/hwpoison: remove race consideration

[Ding Hui]

On 2021/6/16 8:11, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Tue, Jun 15, 2021 at 08:57:06PM +0800, Ding Hui wrote:
> > On 2021/6/14 10:12, Naoya Horiguchi wrote:
> > > @@ -1956,17 +1938,6 @@ int unpoison_memory(unsigned long pfn)
> > >    		goto unlock_mutex;
> > >    	}
> > > -	/*
> > > -	 * unpoison_memory() can encounter thp only when the thp is being
> > > -	 * worked by memory_failure() and the page lock is not held yet.
> > > -	 * In such case, we yield to memory_failure() and make unpoison fail.
> > > -	 */
> > > -	if (!PageHuge(page) && PageTransHuge(page)) {
> > > -		unpoison_pr_info("Unpoison: Memory failure is now running on %#lx\n",
> > > -				 pfn, &unpoison_rs);
> > > -		goto unlock_mutex;
> > > -	}
> > > -
> >
> > if a huge page is in process of alloc or free, HUGETLB_PAGE_DTOR can be set
> > after __SetPageHead() or be cleared before __ClearPageHead(), so this
> > condition may be true in racy.
>
> Hi Ding,
>
> We confirm PageHWPoison() before reaching this if-block and hwpoisoned pages
> are prohibited from allocation, so it seems to me that this check never
> races with hugetlb allocation.
>
> And according to the original patch introduced this if-block (0cea3fdc416d:
> "mm/hwpoison: fix race against poison thp"), this if-block intended to close
> the race between memory_failure() and unpoison_memory(), so that's no longer
> necessary due to mf_mutex.

I got it and thanks for your explanation.

> > Do we need the racy test for this situation?
>
> I'm not sure, but I think that we need more stress/fuzz testing focusing on
> this subsystem, and "unpoison vs allocation" race can be covered in the topic.
>
> Thank you,
> Naoya Horiguchi


--
Thanks,
- Ding Hui