mm, locking: Rework {set,clear,mm}_tlb_flush_pending()

Commit:

  af2c1401e6f9 ("mm: numa: guarantee that tlb_flush_pending updates are visible before page table updates")

added smp_mb__before_spinlock() to set_tlb_flush_pending(). I think we
can solve the same problem without this barrier.

If instead we mandate that mm_tlb_flush_pending() is used while
holding the PTL we're guaranteed to observe prior
set_tlb_flush_pending() instances.

For this to work we need to rework migrate_misplaced_transhuge_page()
a little and move the test up into do_huge_pmd_numa_page().

NOTE: this relies on flush_tlb_range() to guarantee:

   (1) it ensures that prior page table updates are visible to the
       page table walker and
   (2) it ensures that subsequent memory accesses are only made
       visible after the invalidation has completed

This is required for architectures that implement TRANSPARENT_HUGEPAGE
(arc, arm, arm64, mips, powerpc, s390, sparc, x86) or otherwise use
mm_tlb_flush_pending() in their page-table operations (arm, arm64,
x86).

This appears true for:

 - arm (DSB ISB before and after),
 - arm64 (DSB ISHST before, and DSB ISH after),
 - powerpc (PTESYNC before and after),
 - s390 and x86 TLB invalidate are serializing instructions

But I failed to understand the situation for:

 - arc, mips, sparc

Now SPARC64 is a wee bit special in that flush_tlb_range() is a no-op
and it flushes the TLBs using arch_{enter,leave}_lazy_mmu_mode()
inside the PTL. It still needs to guarantee the PTL unlock happens
_after_ the invalidate completes.

Vineet, Ralf and Dave could you guys please have a look?

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 20 insertions, 6 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 86975dec0ba1..c76a720b936b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1410,6 +1410,7 @@ int do_huge_pmd_numa_page(struct vm_fault *vmf, pmd_t pmd)
 	unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
 	int page_nid = -1, this_nid = numa_node_id();
 	int target_nid, last_cpupid = -1;
+	bool need_flush = false;
 	bool page_locked;
 	bool migrated = false;
 	bool was_writable;
@@ -1496,10 +1497,29 @@ int do_huge_pmd_numa_page(struct vm_fault *vmf, pmd_t pmd)
 	}
 
 	/*
+	 * Since we took the NUMA fault, we must have observed the !accessible
+	 * bit. Make sure all other CPUs agree with that, to avoid them
+	 * modifying the page we're about to migrate.
+	 *
+	 * Must be done under PTL such that we'll observe the relevant
+	 * set_tlb_flush_pending().
+	 */
+	if (mm_tlb_flush_pending(vma->vm_mm))
+		need_flush = true;
+
+	/*
 	 * Migrate the THP to the requested node, returns with page unlocked
 	 * and access rights restored.
 	 */
 	spin_unlock(vmf->ptl);
+
+	/*
+	 * We are not sure a pending tlb flush here is for a huge page
+	 * mapping or not. Hence use the tlb range variant
+	 */
+	if (need_flush)
+		flush_tlb_range(vma, haddr, haddr + HPAGE_PMD_SIZE);
+
 	migrated = migrate_misplaced_transhuge_page(vma->vm_mm, vma,
 				vmf->pmd, pmd, vmf->address, page, target_nid);
 	if (migrated) {
diff --git a/mm/migrate.c b/mm/migrate.c
index 627671551873..d68a41da6abb 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1937,12 +1937,6 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
 		put_page(new_page);
 		goto out_fail;
 	}
-	/*
-	 * We are not sure a pending tlb flush here is for a huge page
-	 * mapping or not. Hence use the tlb range variant
-	 */
-	if (mm_tlb_flush_pending(mm))
-		flush_tlb_range(vma, mmun_start, mmun_end);
 
 	/* Prepare a page as a migration target */
 	__SetPageLocked(new_page);