linux-kernel - Re: Memory management woes - order 1 allocation failures

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Date:	Tue, 2 Mar 2010 21:16:04 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Greg KH <gregkh@...e.de>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Frans Pop <elendil@...net.nl>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: Memory management woes - order 1 allocation failures

On Tue, Mar 02, 2010 at 11:29:42AM -0800, Greg KH wrote:
> On Tue, Mar 02, 2010 at 07:11:10PM +0000, Mel Gorman wrote:
> > On Tue, Mar 02, 2010 at 06:34:51PM +0000, Alan Cox wrote:
> > > > For reasons that are not particularly clear to me, tty_buffer_alloc() is
> > > > called far more frequently in 2.6.33 than in 2.6.24. I instrumented the
> > > > function to print out the size of the buffers allocated, booted under
> > > > qemu and would just "cat /bin/ls" to see what buffers were allocated.
> > > > 2.6.33 allocates loads, including high-order allocations. 2.6.24
> > > > appeared to allocate once and keep silent.
> > > 
> > > The pty layer is using them now and didn't before. That will massively
> > > distort your numhers.
> > > 
> > 
> > That makes perfect sense. It explains why only one allocation showed up
> > because it must belong to the tty attached to the serial console.
> > 
> > Thanks Alan.
> > 
> > > > While there have been snags recently with respect to high-order
> > > > allocation failures in recent kernels, this might be one of the cases
> > > > where it's due to subsystems requesting high-order allocations more.
> > > 
> > > The pty code certainly triggered more such allocations. I've sent Greg
> > > patches to make the tty buffering layer allocate sensible sizes as it
> > > doesn't need multiple page allocations in the first place.
> > > 
> > 
> > Greg, what's the story with these patches?
> 
> They are in -next and will go to Linus later on today for .34.
> 

So, Greg pointed me at the patch in question in linux-next
[c9cf55b: tty: Keep the default buffering to sub-page units]
It's attached for convenience.

However, this patch on its own does not appear to be enough. When rebased to
.33, it's still possible for the TTY layer to require order-1 allocations so
I doubt it would fix Frans's on its own. The problem is that TTY_BUFFER_PAGE
is taking struct tty_buffer into account but not the additional padding
added by tty_buffer_find().

As it's not clear why "Round the buffer size out" is required, I took a
simple approach and adjusted TTY_BUFFER_PAGE rather than being clever in
tty_buffer.c. This keeps the allocation sizes below a page but could it be done
better or did I miss another patch in linux-next that makes this unnecessary?

==== CUT HERE ===
tty: Take a 256 byte padding into account when buffering below 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 <mel@....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 {

View attachment "tty-keep-the-default-buffering-to-sub-page-units.patch" of type "text/x-diff" (2212 bytes)