On Fri, 12 Feb 2016 09:34:33 +0530 "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote: > Gerald Schaefer <gerald.schaefer@xxxxxxxxxx> writes: > > > 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). > > Why can't we do this ? I did this for ppc64. > > void pmdp_invalidate(struct vm_area_struct *vma, unsigned long address, > pmd_t *pmdp) > { > - pmd_hugepage_update(vma->vm_mm, address, pmdp, _PAGE_PRESENT, 0); > + pmd_hugepage_update(vma->vm_mm, address, pmdp, ~0UL, 0); > Wouldn't that semantically change what pmdp_invalidate() was supposed to do? The comment before the call says "the pmd_trans_huge and pmd_trans_splitting must remain set at all times on the pmd". So, after removing pmd_trans_splitting, it seems to be necessary to at least keep pmd_trans_huge set. In your case, the pmd would be completely cleared, which may help to find it in fast_gup with pmd_none(), but I'm not sure if this would open up other problems, e.g. with concurrent page faults. But I must also admit that my THP overview got a little rusty. > >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. > > -aneesh > > -- > 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 > -- 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>