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

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

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?

-- 
MST