On Tue, Apr 15, 2014 at 10:06:56PM -0400, Sasha Levin wrote:
> Hi all,
>
> I often see hung task triggering in khugepaged within collapse_huge_page().
>
> I've initially assumed the case may be that the guests are too loaded and
> the warning occurs because of load, but after increasing the timeout to
> 1200 sec I still see the warning.
I suspect it's race (although I didn't track down exact scenario) with
__khugepaged_exit().
Comment in __khugepaged_exit() says that khugepaged_test_exit() always
called under mmap_sem:
2045 void __khugepaged_exit(struct mm_struct *mm)
...
2063 } else if (mm_slot) {
2064 /*
2065 * This is required to serialize against
2066 * khugepaged_test_exit() (which is guaranteed to run
2067 * under mmap sem read mode). Stop here (after we
2068 * return all pagetables will be destroyed) until
2069 * khugepaged has finished working on the pagetables
2070 * under the mmap_sem.
2071 */
2072 down_write(&mm->mmap_sem);
2073 up_write(&mm->mmap_sem);
2074 }
2075 }
But this is not true. At least khugepaged_scan_mm_slot() calls it without
the sem:
2566 static unsigned int khugepaged_scan_mm_slot(unsigned int pages,
2567 struct page **hpage)
...
2046 {
2047 struct mm_slot *mm_slot;
2048 int free = 0;
2049
2050 spin_lock(&khugepaged_mm_lock);
2051 mm_slot = get_mm_slot(mm);
2052 if (mm_slot && khugepaged_scan.mm_slot != mm_slot) {
2053 hash_del(&mm_slot->hash);
2054 list_del(&mm_slot->mm_node);
2055 free = 1;
2056 }
2057 spin_unlock(&khugepaged_mm_lock);
2058
2059 if (free) {
2060 clear_bit(MMF_VM_HUGEPAGE, &mm->flags);
2061 free_mm_slot(mm_slot);
2062 mmdrop(mm);
Not sure yet if it's a real problem or not. Andrea, could you comment on
this?
Sasha, please try patch below.
Not-Yet-Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b4b1feba6472..1c6ace5207b9 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1986,6 +1986,8 @@ static void insert_to_mm_slots_hash(struct mm_struct *mm,
static inline int khugepaged_test_exit(struct mm_struct *mm)
{
+ VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem) &&
+ !spin_is_locked(&khugepaged_mm_lock));
return atomic_read(&mm->mm_users) == 0;
}
@@ -2062,14 +2064,16 @@ void __khugepaged_exit(struct mm_struct *mm)
mmdrop(mm);
} else if (mm_slot) {
/*
- * This is required to serialize against
- * khugepaged_test_exit() (which is guaranteed to run
- * under mmap sem read mode). Stop here (after we
- * return all pagetables will be destroyed) until
- * khugepaged has finished working on the pagetables
+ * This is required to serialize against khugepaged_test_exit()
+ * (which is guaranteed to run under mmap sem read mode or
+ * khugepaged_mm_lock).
+ * Stop here (after we return all pagetables will be destroyed)
+ * until khugepaged has finished working on the pagetables
* under the mmap_sem.
*/
down_write(&mm->mmap_sem);
+ spin_lock(&khugepaged_mm_lock);
+ spin_unlock(&khugepaged_mm_lock);
up_write(&mm->mmap_sem);
}
}
--
Kirill A. Shutemov
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