[PATCH] mm: debugfs: move rounddown_pow_of_two() out from do_fault path

[Andrey Ryabinin]

do_fault_around expects fault_around_bytes rounded down to nearest
page order. Instead of calling rounddown_pow_of_two every time
in fault_around_pages()/fault_around_mask() we could do round down
when user changes fault_around_bytes via debugfs interface.

This also fixes bug when user set fault_around_bytes to 0.
Result of rounddown_pow_of_two(0) is not defined, therefore
fault_around_bytes == 0 doesn't work without this patch.

Let's set fault_around_bytes to PAGE_SIZE if user sets to something
less than PAGE_SIZE

Fixes: a9b0f861("mm: nominate faultaround area in bytes rather than page order")
Signed-off-by: Andrey Ryabinin <a.ryabinin@xxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # 3.15.x
---
 mm/memory.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 7e8d820..e0c6fd6 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2758,20 +2758,16 @@ void do_set_pte(struct vm_area_struct *vma, unsigned long address,
 	update_mmu_cache(vma, address, pte);
 }

-static unsigned long fault_around_bytes = 65536;
+static unsigned long fault_around_bytes = rounddown_pow_of_two(65536);

-/*
- * fault_around_pages() and fault_around_mask() round down fault_around_bytes
- * to nearest page order. It's what do_fault_around() expects to see.
- */
 static inline unsigned long fault_around_pages(void)
 {
-	return rounddown_pow_of_two(fault_around_bytes) / PAGE_SIZE;
+	return fault_around_bytes >> PAGE_SHIFT;
 }

 static inline unsigned long fault_around_mask(void)
 {
-	return ~(rounddown_pow_of_two(fault_around_bytes) - 1) & PAGE_MASK;
+	return ~(fault_around_bytes - 1) & PAGE_MASK;
 }


@@ -2782,11 +2778,18 @@ static int fault_around_bytes_get(void *data, u64 *val)
 	return 0;
 }

+/*
+ * fault_around_pages() and fault_around_mask() expects fault_around_bytes
+ * rounded down to nearest page order. It's what do_fault_around() expects to see.
+ */
 static int fault_around_bytes_set(void *data, u64 val)
 {
 	if (val / PAGE_SIZE > PTRS_PER_PTE)
 		return -EINVAL;
-	fault_around_bytes = val;
+	if (val > PAGE_SIZE)
+		fault_around_bytes = rounddown_pow_of_two(val);
+	else
+		fault_around_bytes = PAGE_SIZE; /* rounddown_pow_of_two(0) is undefined */
 	return 0;
 }
 DEFINE_SIMPLE_ATTRIBUTE(fault_around_bytes_fops,
-- 
1.8.5.5







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