Re: [PATCH v1] mm,hwpoison: set PG_hwpoison for busy hugetlb pages

["HORIGUCHI NAOYA(堀口 直也)" <naoya.horiguchi@nec.com>]

On Wed, May 11, 2022 at 11:35:55AM -0700, Mike Kravetz wrote:
> On 5/11/22 08:19, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > 
> > If memory_failure() fails to grab page refcount on a hugetlb page
> > because it's busy, it returns without setting PG_hwpoison on it.
> > This not only loses a chance of error containment, but breaks the rule
> > that action_result() should be called only when memory_failure() do
> > any of handling work (even if that's just setting PG_hwpoison).
> > This inconsistency could harm code maintainability.
> > 
> > So set PG_hwpoison and call hugetlb_set_page_hwpoison() for such a case.
> > 
> > Fixes: 405ce051236c ("mm/hwpoison: fix race between hugetlb free/demotion and memory_failure_hugetlb()")
> > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > ---
> >  include/linux/mm.h  | 1 +
> >  mm/memory-failure.c | 8 ++++----
> >  2 files changed, 5 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index d446e834a3e5..04de0c3e4f9f 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -3187,6 +3187,7 @@ enum mf_flags {
> >  	MF_MUST_KILL = 1 << 2,
> >  	MF_SOFT_OFFLINE = 1 << 3,
> >  	MF_UNPOISON = 1 << 4,
> > +	MF_NO_RETRY = 1 << 5,
> >  };
> >  extern int memory_failure(unsigned long pfn, int flags);
> >  extern void memory_failure_queue(unsigned long pfn, int flags);
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index 6a28d020a4da..e3269b991016 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -1526,7 +1526,8 @@ int __get_huge_page_for_hwpoison(unsigned long pfn, int flags)
> >  			count_increased = true;
> >  	} else {
> >  		ret = -EBUSY;
> > -		goto out;
> > +		if (!(flags & MF_NO_RETRY))
> > +			goto out;
> >  	}
> 
> Hi Naoya,
> 
> We are in the else block because !HPageFreed() and !HPageMigratable().
> IIUC, this likely means the page is isolated.  One common reason for isolation
> is migration.  So, the page could be isolated and on a list for migration.

Yes, and I also detected this issue by testing race between hugepage allocation
and memory_failure(). 

> 
> I took a quick look at the hugetlb migration code and did not see any checks
> for PageHWPoison after a hugetlb page is isolated.  I could have missed
> something?  If there are no checks, we will read the PageHWPoison page
> in kernel mode while copying to the migration target.

Yes, that could happen.  This patch does not affect ongoing hugepage migration.
But after the migration source hugepage is freed, the PG_hwpoison should work
to prevent reusing.

> 
> Is this an issue?  Is is something we need to be concerned with?  Memory
> errors can happen at any time, and gracefully handling them is best effort.

Right, so doing nothing for this case could be OK if doing something causes
some issues or makes code too complicated.  The motivation of this patch is
that now I think memory_failure() should do something (at least setting
PG_hwpoison) unless the page is already hwpoisoned or rejected by
hwpoison_filter(), because of the effect after free as mentioned above.

This is also expected in other case too. For example, slab is a unhandlable
type of page, but we do set PG_hwpoison.  This flag should not affect any of
ongoing slab-related process, but that's OK because it becomes effective
after the slab page is freed.

So this patch is intended to align to the behavior.  Allowing hugepage
migration to do something good using PG_hwpoison seems to me an unsolved
separate issue.

Thanks,
Naoya Horiguchi