[BUG] random kernel crashes after THP rework on s390 (maybe also on PowerPC and ARM)

[Sebastian Ott]

On Thu, 11 Feb 2016, Kirill A. Shutemov wrote:
> On Thu, Feb 11, 2016 at 09:09:42PM +0200, Kirill A. Shutemov wrote:
> > On Thu, Feb 11, 2016 at 07:22:23PM +0100, Gerald Schaefer wrote:
> > > Hi,
> > > 
> > > Sebastian Ott reported random kernel crashes beginning with v4.5-rc1 and
> > > he also bisected this to commit 61f5d698 "mm: re-enable THP". Further
> > > review of the THP rework patches, which cannot be bisected, revealed
> > > commit fecffad "s390, thp: remove infrastructure for handling splitting PMDs"
> > > (and also similar commits for other archs).
> > > 
> > > This commit removes the THP splitting bit and also the architecture
> > > implementation of pmdp_splitting_flush(), which took care of the IPI for
> > > fast_gup serialization. The commit message says
> > > 
> > >     pmdp_splitting_flush() is not needed too: on splitting PMD we will do
> > >     pmdp_clear_flush() + set_pte_at().  pmdp_clear_flush() will do IPI as
> > >     needed for fast_gup
> > > 
> > > The assumption that a TLB flush will also produce an IPI is wrong on s390,
> > > and maybe also on other architectures, and I thought that this was actually
> > > the main reason for having an arch-specific pmdp_splitting_flush().
> > > 
> > > At least PowerPC and ARM also had an individual implementation of
> > > pmdp_splitting_flush() that used kick_all_cpus_sync() instead of a TLB
> > > flush to send the IPI, and those were also removed. Putting the arch
> > > maintainers and mailing lists on cc to verify.
> > > 
> > > On s390 this will break the IPI serialization against fast_gup, which
> > > would certainly explain the random kernel crashes, please revert or fix
> > > the pmdp_splitting_flush() removal.
> > 
> > Sorry for that.
> > 
> > I believe, the problem was already addressed for PowerPC:
> > 
> > http://lkml.kernel.org/g/454980831-16631-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com
> 
> Correct link is
> 
> http://lkml.kernel.org/g/1454980831-16631-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com
> 

Based on your suggestion Gerald provided the following patch but sadly it
didn't fix the problem.

Sebastian


---
 arch/s390/include/asm/pgtable.h |    2 ++
 1 file changed, 2 insertions(+)

--- a/arch/s390/include/asm/pgtable.h
+++ b/arch/s390/include/asm/pgtable.h
@@ -1587,6 +1587,8 @@ static inline void pmdp_invalidate(struc
 				   unsigned long address, pmd_t *pmdp)
 {
 	pmdp_flush_direct(vma->vm_mm, address, pmdp);
+	/* Serialize against fast_gup with IPI */
+	kick_all_cpus_sync();
 }

 #define __HAVE_ARCH_PMDP_SET_WRPROTECT