On Fri, 12 Feb 2016, Will Deacon wrote: > 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@xxxxxxxxxxxxx> wrote: > > > On Thu, Feb 11, 2016 at 07:22:23PM +0100, Gerald Schaefer wrote: > > > > 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. > > 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. My testcase was compiling the kernel. Most of the time my test system didn't survive a single compile run. During bisect I did at least 20 compile runs to flag a commit as good. Sebastian -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>