[PATCH] mm: BUG when __kmap_atomic_idx crosses boundary

__kmap_atomic_idx >= KM_TYPE_NR or < ZERO is a bug.
Report it even if CONFIG_DEBUG_HIGHMEM is not enabled.
That saves much debugging efforts.

Signed-off-by: Chintan Pandya <redacted>
---
 include/linux/highmem.h | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/include/linux/highmem.h b/include/linux/highmem.h
index 7fb31da..f42cafd 100644
--- a/include/linux/highmem.h
+++ b/include/linux/highmem.h

@@ -93,8 +93,8 @@ static inline int kmap_atomic_idx_push(void)
 
 #ifdef CONFIG_DEBUG_HIGHMEM
 	WARN_ON_ONCE(in_irq() && !irqs_disabled());
-	BUG_ON(idx > KM_TYPE_NR);
 #endif
+	BUG_ON(idx >= KM_TYPE_NR);
 	return idx;
 }
 

@@ -105,13 +105,9 @@ static inline int kmap_atomic_idx(void)
 
 static inline void kmap_atomic_idx_pop(void)
 {
-#ifdef CONFIG_DEBUG_HIGHMEM
 	int idx = __this_cpu_dec_return(__kmap_atomic_idx);
 
 	BUG_ON(idx < 0);
-#else
-	__this_cpu_dec(__kmap_atomic_idx);
-#endif
 }
 
 #endif

-- 
Chintan Pandya

QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
member of the Code Aurora Forum, hosted by The Linux Foundation

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On Wed, 30 Jul 2014 14:22:35 +0530 Chintan Pandya [off-list ref] wrote:

__kmap_atomic_idx >= KM_TYPE_NR or < ZERO is a bug.
Report it even if CONFIG_DEBUG_HIGHMEM is not enabled.
That saves much debugging efforts.

Please take considerably more care when preparing patch changelogs.

kmap_atomic() is a very commonly called function so we'll need much
more detail than this to justify adding overhead to it.

I don't think CONFIG_DEBUG_HIGHMEM really needs to exist.  We could do
s/CONFIG_DEBUG_HIGHMEM/CONFIG_DEBUG_VM/g and perhaps your secret bug
whatever it was would have been found more easily.

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On Wed, 2014-07-30 at 02:06 -0700, Andrew Morton wrote:

On Wed, 30 Jul 2014 14:22:35 +0530 Chintan Pandya [off-list ref] wrote:

quoted

__kmap_atomic_idx >= KM_TYPE_NR or < ZERO is a bug.
Report it even if CONFIG_DEBUG_HIGHMEM is not enabled.
That saves much debugging efforts.

Please take considerably more care when preparing patch changelogs.

kmap_atomic() is a very commonly called function so we'll need much
more detail than this to justify adding overhead to it.

I don't think CONFIG_DEBUG_HIGHMEM really needs to exist.  We could do
s/CONFIG_DEBUG_HIGHMEM/CONFIG_DEBUG_VM/g and perhaps your secret bug
whatever it was would have been found more easily.

Agreed, it would be nice to fold DEBUG_HIGHMEM into DEBUG_VM. However
you'd still need some kind of intermediate option as DEBUG_VM must still
exist if !HIGHMEM.

Thanks,
Davidlohr


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On Wed, 2014-07-30 at 03:37 -0700, Davidlohr Bueso wrote:

On Wed, 2014-07-30 at 02:06 -0700, Andrew Morton wrote:

quoted

On Wed, 30 Jul 2014 14:22:35 +0530 Chintan Pandya [off-list ref] wrote:

quoted

__kmap_atomic_idx >= KM_TYPE_NR or < ZERO is a bug.
Report it even if CONFIG_DEBUG_HIGHMEM is not enabled.
That saves much debugging efforts.

Please take considerably more care when preparing patch changelogs.

kmap_atomic() is a very commonly called function so we'll need much
more detail than this to justify adding overhead to it.

I don't think CONFIG_DEBUG_HIGHMEM really needs to exist.  We could do
s/CONFIG_DEBUG_HIGHMEM/CONFIG_DEBUG_VM/g and perhaps your secret bug
whatever it was would have been found more easily.

Agreed, it would be nice to fold DEBUG_HIGHMEM into DEBUG_VM. However
you'd still need some kind of intermediate option as DEBUG_VM must still
exist if !HIGHMEM.

Actually no, it's a lot more straightforward than I thought.

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On 07/30/2014 02:36 PM, Andrew Morton wrote:

On Wed, 30 Jul 2014 14:22:35 +0530 Chintan Pandya[off-list ref]  wrote:

quoted

__kmap_atomic_idx>= KM_TYPE_NR or<  ZERO is a bug.
Report it even if CONFIG_DEBUG_HIGHMEM is not enabled.
That saves much debugging efforts.

Please take considerably more care when preparing patch changelogs.

Okay. I will prepare new commit message.

kmap_atomic() is a very commonly called function so we'll need much
more detail than this to justify adding overhead to it.

I don't think CONFIG_DEBUG_HIGHMEM really needs to exist.  We could do
s/CONFIG_DEBUG_HIGHMEM/CONFIG_DEBUG_VM/g and perhaps your secret bug
whatever it was would have been found more easily.

Um, we didn't get bug directly hitting here.

__kmap_atomic_idx should not be equal to KM_TYPE_NR anyway. So, at least I will share that patch. For changing DEBUG_HIGHMEM to DEBUG_VM, I will work on it.

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