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); >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, 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>