Setting and clearing mm->tlb_flush_pending can be performed by multiple
threads, since mmap_sem may only be acquired for read in task_numa_work.
If this happens, tlb_flush_pending may be cleared while one of the
threads still changes PTEs and batches TLB flushes.
As a result, TLB flushes can be skipped because the indication of
pending TLB flushes is lost, for instance due to race between
migration and change_protection_range (just as in the scenario that
caused the introduction of tlb_flush_pending).
The feasibility of such a scenario was confirmed by adding assertion to
check tlb_flush_pending is not set by two threads, adding artificial
latency in change_protection_range() and using sysctl to reduce
kernel.numa_balancing_scan_delay_ms.
Fixes: 20841405940e ("mm: fix TLB flush race between migration, and
change_protection_range")
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
include/linux/mm_types.h | 8 ++++----
kernel/fork.c | 2 +-
mm/debug.c | 2 +-
3 files changed, 6 insertions(+), 6 deletions(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 45cdb27791a3..36f4ec589544 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -493,7 +493,7 @@ struct mm_struct {
* can move process memory needs to flush the TLB when moving a
* PROT_NONE or PROT_NUMA mapped page.
*/
- bool tlb_flush_pending;
+ atomic_t tlb_flush_pending;
#endif
struct uprobes_state uprobes_state;
#ifdef CONFIG_HUGETLB_PAGE
@@ -528,11 +528,11 @@ static inline cpumask_t *mm_cpumask(struct mm_struct *mm)
static inline bool mm_tlb_flush_pending(struct mm_struct *mm)
{
barrier();
- return mm->tlb_flush_pending;
+ return atomic_read(&mm->tlb_flush_pending) > 0;
}
static inline void set_tlb_flush_pending(struct mm_struct *mm)
{
- mm->tlb_flush_pending = true;
+ atomic_inc(&mm->tlb_flush_pending);
/*
* Guarantee that the tlb_flush_pending store does not leak into the
@@ -544,7 +544,7 @@ static inline void set_tlb_flush_pending(struct mm_struct *mm)
static inline void clear_tlb_flush_pending(struct mm_struct *mm)
{
barrier();
- mm->tlb_flush_pending = false;
+ atomic_dec(&mm->tlb_flush_pending);
}
#else
static inline bool mm_tlb_flush_pending(struct mm_struct *mm)
diff --git a/kernel/fork.c b/kernel/fork.c
index e53770d2bf95..5a7ecfbb7420 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -809,7 +809,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
mm_init_aio(mm);
mm_init_owner(mm, p);
mmu_notifier_mm_init(mm);
- clear_tlb_flush_pending(mm);
+ atomic_set(&mm->tlb_flush_pending, 0);
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS
mm->pmd_huge_pte = NULL;
#endif
diff --git a/mm/debug.c b/mm/debug.c
index db1cd26d8752..d70103bb4731 100644
--- a/mm/debug.c
+++ b/mm/debug.c
@@ -159,7 +159,7 @@ void dump_mm(const struct mm_struct *mm)
mm->numa_next_scan, mm->numa_scan_offset, mm->numa_scan_seq,
#endif
#if defined(CONFIG_NUMA_BALANCING) || defined(CONFIG_COMPACTION)
- mm->tlb_flush_pending,
+ atomic_read(&mm->tlb_flush_pending),
#endif
mm->def_flags, &mm->def_flags
);
--
2.11.0
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