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" [off-list ref] wrote:

quoted

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

quoted

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.

quoted

If yes, I'll prepare patch tomorrow (some sleep required).

We'll check if adding kick_all_cpus_sync() to pmdp_invalidate() helps.
It would also be good if Martin has a look at this, he'll return on
Monday.

Do you have a reliable way to trigger the "random kernel crashes"? We've not
seen anything reported on arm64, but I don't see why we wouldn't be affected
by the same bug and it would be good to confirm and validate a fix.

Cheers,

Will

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