[PATCH] tty: Take a 256 byte padding into account when buffering below sub-page units

[Mel Gorman]

(applies on top of linux-next, depends on "tty: Keep the default buffering to sub-page units")

The TTY layer takes some care to ensure that only sub-page allocations
are made with interrupts disabled. It does this by setting a goal of
"TTY_BUFFER_PAGE" to allocate. Unfortunately, while TTY_BUFFER_PAGE takes the
size of tty_buffer into account, it fails to account that tty_buffer_find()
rounds the buffer size out to the next 256 byte boundary before adding on
the size of the tty_buffer.

This patch adjusts the TTY_BUFFER_PAGE calculation to take into account the
size of the tty_buffer and the padding. Once applied, tty_buffer_alloc()
should not require high-order allocations.

Signed-off-by: Mel Gorman <m...@csn.ul.ie>
---
include/linux/tty.h | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/include/linux/tty.h b/include/linux/tty.h
index d96e588..8fe018b 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -70,12 +70,13 @@ struct tty_buffer {

/*
* We default to dicing tty buffer allocations to this many characters
- * in order to avoid multiple page allocations. We assume tty_buffer itself
- * is under 256 bytes. See tty_buffer_find for the allocation logic this
- * must match
+ * in order to avoid multiple page allocations. We know the size of
+ * tty_buffer itself but it must also be taken into account that the
+ * the buffer is 256 byte aligned. See tty_buffer_find for the allocation
+ * logic this must match
*/

-#define TTY_BUFFER_PAGE ((PAGE_SIZE - 256) / 2)
+#define TTY_BUFFER_PAGE (((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~0xFF)


struct tty_bufhead {
--
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[Greg KH]

On Tue, Mar 02, 2010 at 10:24:19PM +0000, Mel Gorman wrote:
> (applies on top of linux-next, depends on "tty: Keep the default buffering to sub-page units")
>
> The TTY layer takes some care to ensure that only sub-page allocations
> are made with interrupts disabled. It does this by setting a goal of
> "TTY_BUFFER_PAGE" to allocate. Unfortunately, while TTY_BUFFER_PAGE takes the
> size of tty_buffer into account, it fails to account that tty_buffer_find()
> rounds the buffer size out to the next 256 byte boundary before adding on
> the size of the tty_buffer.
>
> This patch adjusts the TTY_BUFFER_PAGE calculation to take into account the
> size of the tty_buffer and the padding. Once applied, tty_buffer_alloc()
> should not require high-order allocations.
>
> Signed-off-by: Mel Gorman <m...@csn.ul.ie>
> ---
> include/linux/tty.h | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)

Thanks, I'll queue this up after the current round of patches make it
into Linus's tree.

greg k-h