Re: [PATCH v3 3/3] mm/hwpoison: fix unpoison_memory()

[Naoya Horiguchi <naoya.horiguchi@linux.dev>]

On Mon, Nov 08, 2021 at 03:27:55PM -0800, Yang Shi wrote:
> On Thu, Nov 4, 2021 at 10:52 PM Naoya Horiguchi
> <naoya.horiguchi@linux.dev> wrote:
...
> > @@ -1942,6 +1987,26 @@ core_initcall(memory_failure_init);
> >                 pr_info(fmt, pfn);                      \
> >  })
> >
> > +static inline int clear_page_hwpoison(struct ratelimit_state *rs, struct page *p)
> > +{
> > +       if (TestClearPageHWPoison(p)) {
> > +               unpoison_pr_info("Unpoison: Software-unpoisoned page %#lx\n",
> > +                                page_to_pfn(p), rs);
> > +               num_poisoned_pages_dec();
> > +               return 1;
> > +       }
> > +       return 0;
> > +}
> > +
> > +static inline int unpoison_taken_off_page(struct ratelimit_state *rs,
> > +                                         struct page *p)
> > +{
> > +       if (put_page_back_buddy(p) && clear_page_hwpoison(rs, p))
> 
> It seems there might be race condition between free and allocation to
> kick the page out of buddy before the hwpoisoned flag is cleared IIUC?
> 
>         CPU A                     CPU B
> Free:                            Allocation:
> acquire zone lock
> put back to buddy
> release zone lock
>                                      acquire zone lock
>                                      try to allocate this page
>                                      if it is hwpoisoned then reset
> PageBuddy (not buddy page anymore)
>                                      release zone lock
> 
> Did I miss something?

No, this is a nice catch. CPU B finds hwpoisoned page on free list then
calling bad_page().  So doing "put back to buddy" and clear_page_hwpoison
inside a single zone lock should prevent this race.

Thanks,
Naoya Horiguchi