[Devel] [PATCH RHEL7 COMMIT] asm-generic/tlb: avoid potential double flush

[Vasily Averin]

The commit is pushed to "branch-rh7-3.10.0-1127.10.1.vz7.162.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1127.10.1.vz7.162.13
------>
commit f64be64b0e3d460cd44eef5371e2142e8099bdc4
Author: Peter Zijlstra <peterz at infradead.org>
Date:   Tue Jul 21 17:58:56 2020 +0300

    asm-generic/tlb: avoid potential double flush
    
    Aneesh reported that:
    
            tlb_flush_mmu()
              tlb_flush_mmu_tlbonly()
                tlb_flush()                 <-- #1
              tlb_flush_mmu_free()
                tlb_table_flush()
                  tlb_table_invalidate()
                    tlb_flush_mmu_tlbonly()
                      tlb_flush()           <-- #2
    
    does two TLBIs when tlb->fullmm, because __tlb_reset_range() will not
    clear tlb->end in that case.
    
    Observe that any caller to __tlb_adjust_range() also sets at least one of
    the tlb->freed_tables || tlb->cleared_p* bits, and those are
    unconditionally cleared by __tlb_reset_range().
    
    Change the condition for actually issuing TLBI to having one of those bits
    set, as opposed to having tlb->end != 0.
    
    Link: http://lkml.kernel.org/r/20200116064531.483522-4-aneesh.kumar@linux.ibm.com
    Signed-off-by: Peter Zijlstra (Intel) <peterz at infradead.org>
    Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
    Reported-by: "Aneesh Kumar K.V" <aneesh.kumar at linux.ibm.com>
    Cc: Michael Ellerman <mpe at ellerman.id.au>
    Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
    
    https://jira.sw.ru/browse/PSBM-101300
    (cherry picked from commit 0758cd8304942292e95a0f750c374533db378b32)
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 mm/memory.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/memory.c b/mm/memory.c
index f4874a2b8be84..4370dd4008220 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -252,8 +252,14 @@ void arch_tlb_gather_mmu(struct mmu_gather *tlb, struct mm_struct *mm,
 
 static void tlb_flush_mmu_tlbonly(struct mmu_gather *tlb)
 {
-	if (!tlb->end)
+	/*
+	 * Anything calling __tlb_adjust_range() also sets at least one of
+	 * these bits.
+	 */
+	if (!(tlb->freed_tables || tlb->cleared_ptes || tlb->cleared_pmds ||
+	    tlb->cleared_puds || tlb->cleared_p4ds))
 		return;
+
 	tlb_flush(tlb);
 	mmu_notifier_invalidate_range(tlb->mm, tlb->start, tlb->end);
 	__tlb_reset_range(tlb);