From: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> The H_REGISTER_VPA hcall implementation in HV Power KVM needs to pin some guest memory pages into host memory so that they can be safely accessed from usermode. It does this used get_user_pages_fast(). When the VPA is unregistered, or the VCPUs are cleaned up, these pages are released using put_page(). However, the get_user_pages() is invoked on the specific memory are of the VPA which could lie within hugepages. In case the pinned page is huge, we explicitly find the head page of the compound page before calling put_page() on it. At least with the latest kernel, this is not correct. put_page() already handles finding the correct head page of a compound, and also deals with various counts on the individual tail page which are important for transparent huge pages. We don't support transparent hugepages on Power, but even so, bypassing this count maintenance can lead (when the VM ends) to a hugepage being released back to the pool with a non-zero mapcount on one of the tail pages. This can then lead to a bad_page() when the page is released from the hugepage pool. This removes the explicit compound_head() call to correct this bug. Signed-off-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> Signed-off-by: Paul Mackerras <paulus@xxxxxxxxx> --- This needs to go upstream into 3.4-rc since it is a regression fix. The regression is that after using a Book3S HV guest backed by 16MB large pages, if the large page pool is reduced in size the system will hit a BUG_ON. That didn't happen with 3.3. arch/powerpc/kvm/book3s_64_mmu_hv.c | 22 +++++++++++++--------- arch/powerpc/kvm/book3s_hv.c | 2 -- 3 files changed, 13 insertions(+), 11 deletions(-) diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c index ddc485a..c3beaee 100644 --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c @@ -258,6 +258,8 @@ static long kvmppc_get_guest_page(struct kvm *kvm, unsigned long gfn, !(memslot->userspace_addr & (s - 1))) { start &= ~(s - 1); pgsize = s; + get_page(hpage); + put_page(page); page = hpage; } } @@ -281,11 +283,8 @@ static long kvmppc_get_guest_page(struct kvm *kvm, unsigned long gfn, err = 0; out: - if (got) { - if (PageHuge(page)) - page = compound_head(page); + if (got) put_page(page); - } return err; up_err: @@ -678,8 +677,15 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu, SetPageDirty(page); out_put: - if (page) - put_page(page); + if (page) { + /* + * We drop pages[0] here, not page because page might + * have been set to the head page of a compound, but + * we have to drop the reference on the correct tail + * page to match the get inside gup() + */ + put_page(pages[0]); + } return ret; out_unlock: @@ -979,6 +985,7 @@ void *kvmppc_pin_guest_page(struct kvm *kvm, unsigned long gpa, pa = *physp; } page = pfn_to_page(pa >> PAGE_SHIFT); + get_page(page); } else { hva = gfn_to_hva_memslot(memslot, gfn); npages = get_user_pages_fast(hva, 1, 1, pages); @@ -991,8 +998,6 @@ void *kvmppc_pin_guest_page(struct kvm *kvm, unsigned long gpa, page = compound_head(page); psize <<= compound_order(page); } - if (!kvm->arch.using_mmu_notifiers) - get_page(page); offset = gpa & (psize - 1); if (nb_ret) *nb_ret = psize - offset; @@ -1003,7 +1008,6 @@ void kvmppc_unpin_guest_page(struct kvm *kvm, void *va) { struct page *page = virt_to_page(va); - page = compound_head(page); put_page(page); } diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c index d0bc79b..146c17c 100644 --- a/arch/powerpc/kvm/book3s_hv.c +++ b/arch/powerpc/kvm/book3s_hv.c @@ -1368,8 +1368,6 @@ static void unpin_slot(struct kvm *kvm, int slot_id) continue; pfn = physp[j] >> PAGE_SHIFT; page = pfn_to_page(pfn); - if (PageHuge(page)) - page = compound_head(page); SetPageDirty(page); put_page(page); } -- 1.7.10 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html