On Thu, 11 Feb 2016 21:09:42 +0200 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx > > 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. > > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
