x86/mm: Enable RCU based page table freeing (CONFIG_HAVE_RCU_TABLE_FREE=y)

There's a subtle bug in how some of the paravirt guest code handles
page table freeing on x86:

On x86 software page table walkers depend on the fact that remote TLB flush
does an IPI: walk is performed lockless but with interrupts disabled and in
case the page table is freed the freeing CPU will get blocked as remote TLB
flush is required. On other architectures which don't require an IPI to do
remote TLB flush we have an RCU-based mechanism (see
include/asm-generic/tlb.h for more details).

In virtualized environments we may want to override the ->flush_tlb_others
callback in pv_mmu_ops and use a hypercall asking the hypervisor to do a
remote TLB flush for us. This breaks the assumption about IPIs. Xen PV has
been doing this for years and the upcoming remote TLB flush for Hyper-V will
do it too.

This is not safe, as software page table walkers may step on an already
freed page.

Fix the bug by enabling the RCU-based page table freeing mechanism,
CONFIG_HAVE_RCU_TABLE_FREE=y.

Testing with kernbench and mmap/munmap microbenchmarks, and neither showed
any noticeable performance impact.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Jork Loeser <Jork.Loeser@microsoft.com>
Cc: KY Srinivasan <kys@microsoft.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/20170828082251.5562-1-vkuznets@redhat.com
[ Rewrote/fixed/clarified the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e4844e934728c58f054eefaf1375e6b018dc982a..87e447286c377f730c3d94ed267e136b741b01e9 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -167,6 +167,7 @@ config X86
        select HAVE_HARDLOCKUP_DETECTOR_PERF    if PERF_EVENTS && HAVE_PERF_EVENTS_NMI
        select HAVE_PERF_REGS
        select HAVE_PERF_USER_STACK_DUMP
+       select HAVE_RCU_TABLE_FREE
        select HAVE_REGS_AND_STACK_ACCESS_API
        select HAVE_RELIABLE_STACKTRACE         if X86_64 && FRAME_POINTER && STACK_VALIDATION
        select HAVE_STACK_VALIDATION            if X86_64

diff --git a/arch/x86/include/asm/tlb.h b/arch/x86/include/asm/tlb.h
index c7797307fc2ba5b24adacced83c9587bc5d2a8de..79a4ca6a96063529aafd883bfc69b40febf1d2f1 100644 (file)
--- a/arch/x86/include/asm/tlb.h
+++ b/arch/x86/include/asm/tlb.h
@@ -15,4 +15,18 @@
 
 #include <asm-generic/tlb.h>
 
+/*
+ * While x86 architecture in general requires an IPI to perform TLB
+ * shootdown, enablement code for several hypervisors overrides
+ * .flush_tlb_others hook in pv_mmu_ops and implements it by issuing
+ * a hypercall. To keep software pagetable walkers safe in this case we
+ * switch to RCU based table free (HAVE_RCU_TABLE_FREE). See the comment
+ * below 'ifdef CONFIG_HAVE_RCU_TABLE_FREE' in include/asm-generic/tlb.h
+ * for more details.
+ */
+static inline void __tlb_remove_table(void *table)
+{
+       free_page_and_swap_cache(table);
+}
+
 #endif /* _ASM_X86_TLB_H */

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 508a708eb9a6533adbfc1003c802d6d781285367..218834a3e9adde25ea3ddf3ef2b3f513846ce1c7 100644 (file)
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -56,7 +56,7 @@ void ___pte_free_tlb(struct mmu_gather *tlb, struct page *pte)
 {
        pgtable_page_dtor(pte);
        paravirt_release_pte(page_to_pfn(pte));
-       tlb_remove_page(tlb, pte);
+       tlb_remove_table(tlb, pte);
 }
 
 #if CONFIG_PGTABLE_LEVELS > 2
@@ -72,21 +72,21 @@ void ___pmd_free_tlb(struct mmu_gather *tlb, pmd_t *pmd)
        tlb->need_flush_all = 1;
 #endif
        pgtable_pmd_page_dtor(page);
-       tlb_remove_page(tlb, page);
+       tlb_remove_table(tlb, page);
 }
 
 #if CONFIG_PGTABLE_LEVELS > 3
 void ___pud_free_tlb(struct mmu_gather *tlb, pud_t *pud)
 {
        paravirt_release_pud(__pa(pud) >> PAGE_SHIFT);
-       tlb_remove_page(tlb, virt_to_page(pud));
+       tlb_remove_table(tlb, virt_to_page(pud));
 }
 
 #if CONFIG_PGTABLE_LEVELS > 4
 void ___p4d_free_tlb(struct mmu_gather *tlb, p4d_t *p4d)
 {
        paravirt_release_p4d(__pa(p4d) >> PAGE_SHIFT);
-       tlb_remove_page(tlb, virt_to_page(p4d));
+       tlb_remove_table(tlb, virt_to_page(p4d));
 }
 #endif /* CONFIG_PGTABLE_LEVELS > 4 */
 #endif /* CONFIG_PGTABLE_LEVELS > 3 */