On Tue, 13 May 2014 10:45:48 +0100 Mel Gorman <mgorman@xxxxxxx> wrote:
> Discarding buffers uses a bunch of atomic operations when discarding buffers
> because ...... I can't think of a reason. Use a cmpxchg loop to clear all the
> necessary flags. In most (all?) cases this will be a single atomic operations.
>
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1485,14 +1485,18 @@ EXPORT_SYMBOL(set_bh_page);
> */
> static void discard_buffer(struct buffer_head * bh)
> {
> + unsigned long b_state, b_state_old;
> +
> lock_buffer(bh);
> clear_buffer_dirty(bh);
> bh->b_bdev = NULL;
> - clear_buffer_mapped(bh);
> - clear_buffer_req(bh);
> - clear_buffer_new(bh);
> - clear_buffer_delay(bh);
> - clear_buffer_unwritten(bh);
> + b_state = bh->b_state;
> + for (;;) {
> + b_state_old = cmpxchg(&bh->b_state, b_state, (b_state & ~BUFFER_FLAGS_DISCARD));
> + if (b_state_old == b_state)
> + break;
> + b_state = b_state_old;
> + }
> unlock_buffer(bh);
> }
>
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -77,6 +77,11 @@ struct buffer_head {
> atomic_t b_count; /* users using this buffer_head */
> };
>
> +/* Bits that are cleared during an invalidate */
> +#define BUFFER_FLAGS_DISCARD \
> + (1 << BH_Mapped | 1 << BH_New | 1 << BH_Req | \
> + 1 << BH_Delay | 1 << BH_Unwritten)
> +
There isn't much point in having this in the header file is there?
--- a/fs/buffer.c~fs-buffer-do-not-use-unnecessary-atomic-operations-when-discarding-buffers-fix
+++ a/fs/buffer.c
@@ -1483,6 +1483,12 @@ EXPORT_SYMBOL(set_bh_page);
/*
* Called when truncating a buffer on a page completely.
*/
+
+/* Bits that are cleared during an invalidate */
+#define BUFFER_FLAGS_DISCARD \
+ (1 << BH_Mapped | 1 << BH_New | 1 << BH_Req | \
+ 1 << BH_Delay | 1 << BH_Unwritten)
+
static void discard_buffer(struct buffer_head * bh)
{
unsigned long b_state, b_state_old;
@@ -1492,7 +1498,8 @@ static void discard_buffer(struct buffer
bh->b_bdev = NULL;
b_state = bh->b_state;
for (;;) {
- b_state_old = cmpxchg(&bh->b_state, b_state, (b_state & ~BUFFER_FLAGS_DISCARD));
+ b_state_old = cmpxchg(&bh->b_state, b_state,
+ (b_state & ~BUFFER_FLAGS_DISCARD));
if (b_state_old == b_state)
break;
b_state = b_state_old;
--- a/include/linux/buffer_head.h~fs-buffer-do-not-use-unnecessary-atomic-operations-when-discarding-buffers-fix
+++ a/include/linux/buffer_head.h
@@ -77,11 +77,6 @@ struct buffer_head {
atomic_t b_count; /* users using this buffer_head */
};
-/* Bits that are cleared during an invalidate */
-#define BUFFER_FLAGS_DISCARD \
- (1 << BH_Mapped | 1 << BH_New | 1 << BH_Req | \
- 1 << BH_Delay | 1 << BH_Unwritten)
-
/*
* macro tricks to expand the set_buffer_foo(), clear_buffer_foo()
* and buffer_foo() functions.
_
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