On Fri, 12 Feb 2016 16:57:27 +0100
Christian Borntraeger [off-list ref] wrote:

On 02/12/2016 04:41 PM, Kirill A. Shutemov wrote:

quoted

On Thu, Feb 11, 2016 at 08:57:02PM +0100, Gerald Schaefer wrote:

quoted

On Thu, 11 Feb 2016 21:09:42 +0200
"Kirill A. Shutemov" [off-list ref] wrote:

quoted

On Thu, Feb 11, 2016 at 07:22:23PM +0100, Gerald Schaefer wrote:

quoted

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()?

It uses the idte instruction, which in an atomic fashion flushes the associated
TLB entry and changes the value of the pmd entry to invalid. This comes from the
HW requirement to not  change a PTE/PMD that might be still in use, other than 
with special instructions that does the tlb handling and the invalidation together.

Correct, and it does _not_ make the pmd !pmd_present(), that would only be the
case after a _clear_flush(). It only marks the pmd as invalid and flushes,
so that it cannot generate a new TLB entry before the following pmd_populate(),
but it keeps its other content. This is to fulfill the requirements outlined in
the comment in mm/huge_memory.c before the call to pmdp_invalidate(). And
independent from that comment, we would need such an _invalidate() or
_clear_flush() on s390 before the pmd_populate() because of the HW details
that Christian described.

Reading the comment again, I do now notice that it also says "mark the current
pmd notpresent", which we cannot do w/o losing the huge and (formerly) splitting
bits, but it also shouldn't be needed to provide the "single TLB guarantee" that
is required from the comment. So, a pmd_present() check on s390 in this state
would still return true. Not sure yet if this is a problem, need more thinking,
this behavior was already present before the THP rework but maybe it was OK
before and is not OK now.

At least for fast_gup this should not be a problem though.

(It also does some some other magic to the attach_count, which might hold off
finish_arch_post_lock_switch while some flushing is happening, but this should
be unrelated here)

quoted

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

Don't know, Gerald or Martin?

The implementation frequently changes depending on how many new bits Martin
needs to squeeze out :-)
We don't have a _PAGE_PRESENT bit for pmds, so pmd_present() just checks if the
entry is not empty. pmd_none() of course does the opposite, it checks if it is
empty.

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