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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Feb 11, 2016 at 08:57:02PM +0100, Gerald Schaefer wrote:
> On Thu, 11 Feb 2016 21:09:42 +0200
> "Kirill A. Shutemov" <kirill@shutemov.name> 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
> > 
> > I think kick_all_cpus_sync() in arch-specific pmdp_invalidate() would do
> > the trick, right?
> 
> Hmm, not sure about that. After pmdp_invalidate(), a pmd_none() check in
> fast_gup will still return false, because the pmd is not empty (at least
> on s390). So I don't see spontaneously how it will help fast_gup to break
> out to the slow path in case of THP splitting.

What pmdp_flush_direct() does in pmdp_invalidate()? It's hard to unwrap for me :-/
Does it make the pmd !pmd_present()?

I'm also confused by pmd_none() is equal to !pmd_present() on s390. Hm?

-- 
 Kirill A. Shutemov