linux-kernel - Re: [Xen-devel] [PATCH 1/1] xen/gntdev: kmalloc structure gntdev_copy

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Message-ID: <573066D0.1030603@citrix.com>
Date:	Mon, 9 May 2016 11:30:40 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Heinrich Schuchardt <xypron.glpk@....de>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"Juergen Gross" <jgross@...e.com>
CC:	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 1/1] xen/gntdev: kmalloc structure
 gntdev_copy_batch

On 07/05/16 09:17, Heinrich Schuchardt wrote:
> Commit a4cdb556cae0 ("xen/gntdev: add ioctl for grant copy")
> leads to a warning
> xen/gntdev.c: In function ‘gntdev_ioctl_grant_copy’:
> xen/gntdev.c:949:1: warning: the frame size of 1248 bytes
> is larger than 1024 bytes [-Wframe-larger-than=]
> 
> This can be avoided by using kmalloc instead of the stack.
> 
> Testing requires CONFIG_XEN_GNTDEV.

Thanks, but I've applied the following patch instead.

David

8<-------------
xen/gntdev: reduce copy batch size to 16

IOCTL_GNTDEV_GRANT_COPY batches copy operations to reduce the number
of hypercalls.  The stack is used to avoid a memory allocation in a
hot path. However, a batch size of 24 requires more than 1024 bytes of
stack which in some configurations causes a compiler warning.

    xen/gntdev.c: In function ‘gntdev_ioctl_grant_copy’:
    xen/gntdev.c:949:1: warning: the frame size of 1248 bytes is
    larger than 1024 bytes [-Wframe-larger-than=]

This is a harmless warning as there is still plenty of stack spare,
but people keep trying to "fix" it.  Reduce the batch size to 16 to
reduce stack usage to less than 1024 bytes.  This should have minimal
impact on performance.

Signed-off-by: David Vrabel <david.vrabel@...rix.com>
---
 drivers/xen/gntdev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index dc49538..6793957 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -748,7 +748,7 @@ static long gntdev_ioctl_notify(struct gntdev_priv
*priv, void __user *u)
 	return rc;
 }

-#define GNTDEV_COPY_BATCH 24
+#define GNTDEV_COPY_BATCH 16

 struct gntdev_copy_batch {
 	struct gnttab_copy ops[GNTDEV_COPY_BATCH];
-- 
2.1.4