Re: [PATCH splitout] mm: memory-failure: serialize TestSetPageHWPoison with zone->lock

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jun 10, 2026 at 03:24:30PM +0800, Miaohe Lin wrote:
> On 2026/6/10 5:00, Michael S. Tsirkin wrote:
> > On Tue, Jun 09, 2026 at 04:54:01PM -0400, Zi Yan wrote:
> >> On 9 Jun 2026, at 16:34, Michael S. Tsirkin wrote:
> >>
> >>> On Tue, Jun 09, 2026 at 02:52:47PM -0400, Zi Yan wrote:
> >>>> On 9 Jun 2026, at 14:39, Zi Yan wrote:
> >>>>
> >>>>> On 9 Jun 2026, at 14:38, David Hildenbrand (Arm) wrote:
> >>>>>
> >>>>>> On 6/9/26 20:10, Andrew Morton wrote:
> >>>>>>> On Tue, 9 Jun 2026 06:12:49 -0400 "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >>>>>>>
> >>>>>>>> TestSetPageHWPoison() is called without zone->lock, so its atomic
> >>>>>>>> update to page->flags can race with non-atomic flag operations
> >>>>>>>> that run under zone->lock in the buddy allocator.
> >>>>>>>>
> >>>>>>>> In particular, __free_pages_prepare() does:
> >>>>>>>>
> >>>>>>>>     page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
> >>>>>>>>
> >>>>>>>> This non-atomic read-modify-write, while correctly excluding
> >>>>>>>> __PG_HWPOISON from the mask, can still lose a concurrent
> >>>>>>>> TestSetPageHWPoison if the read happens before the poison bit
> >>>>>>>> is set and the write happens after.  Will only get worse if/when
> >>>>>>>> we add more non-atomic flag operations.
> >>>>>>>>
> >>>>>>>> Fix by acquiring zone->lock around TestSetPageHWPoison and
> >>>>>>>> around ClearPageHWPoison in the retry path.  This
> >>>>>>>> serializes with all buddy flag manipulation.  The cost is
> >>>>>>>> negligible: one lock/unlock in an extremely rare path
> >>>>>>>> (hardware memory errors).
> >>>>>>>>
> >>>>>>>> Note: SetPageHWPoison and TestClearPageHWPoison calls elsewhere
> >>>>>>>> in this file operate on pages already removed from the buddy
> >>>>>>>> allocator or on non-buddy pages (DAX, hugetlb), so they do not
> >>>>>>>> need zone->lock protection.
> >>>>>>>
> >>>>>>> Sashiko is saying this doesn't do anything "Because
> >>>>>>> __free_pages_prepare() executes entirely locklessly".  Did it goof?
> >>>>>>>
> >>>>>>> https://sashiko.dev/#/patchset/df06b66fe4ff8e925ee0714955abc2183a727b90.1780998980.git.mst@redhat.com
> >>>>>>
> >>>>>> Battle of the bots: it's right.
> >>>>>
> >>>>> Yep, __free_pages_prepare() changes the page flag without holding
> >>>>> zone->lock.
> >>>>
> >>>> __free_pages_prepare() works on frozen pages and assumes no one else
> >>>> touches the input page. To avoid this race, memory_failure() might
> >>>> want to try_get_page() before TestClearPageHWPoison(), but I am not
> >>>> sure if that works along with memory failure flow.
> >>>>
> >>>> Best Regards,
> >>>> Yan, Zi
> >>>
> >>>
> >>>
> >>> Actually memory failure already plays with this down the road no?
> >>>
> >>> So maybe it's enough to just SetPageHWPoison afterwards again?
> >>>
> >>>
> >>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> >>> index ee42d4361309..4758fea94a96 100644
> >>> --- a/mm/memory-failure.c
> >>> +++ b/mm/memory-failure.c
> >>> @@ -2415,6 +2415,7 @@ int memory_failure(unsigned long pfn, int flags)
> >>>  	if (!res) {
> >>>  		if (is_free_buddy_page(p)) {
> >>>  			if (take_page_off_buddy(p)) {
> >>> +				SetPageHWPoison(p);
> >>>  				page_ref_inc(p);
> >>>  				res = MF_RECOVERED;
> >>>  			} else {
> >>>
> >>>
> >>> and maybe in a bunch of other places in there?
> >>
> >> You mean for fear of losing HWPoison flag in the earlier TestSetPageHWPoison(),
> >> just set it again here?
> > 
> > Yea.
> > 
> >> Why not do it after get_hwpoison_page(), since that
> >> is the expected page flag?
> > 
> > It's still in the buddy at that point right? I'm worried buddy might
> > poke at flags.
> 
> Since __free_pages_prepare() executes entirely locklessly, the only way to ensure
> HWPoison flag won't be lost might be only set hwpoison flag iff we can make sure
> pages are not on the way to buddy...
> 
> Thanks.
> .


To clarify do you not agree repeating SetPageHWPoison is enough for
this? And if not, do you have suggestions on how to fix this race?

Thanks a lot,

-- 
MST