Gerald Schaefer <gerald.schaefer@xxxxxxxxxx> writes: > 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. Thinking about this more, I guess, I should not be doing this. Because this bring in the exit_mmap race that I outlined in the patch even though the window now is small. I guess we should fix this in the gup path by checking for what ever trick we are using to mark the pmd splitting. For ppc64 we clear the _PAGE_USER. We are ok as long as autonuma is enabled because pmd_protnone() check will check against _PAGE_USER. But that may not be sufficient. -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>