linux-kernel - Re: [Xen-devel] [PATCH] xen/gntdev: Don't allocate struct gntdev_copy

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Date:	Tue, 19 Jan 2016 14:31:03 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	<konrad.wilk@...cle.com>
CC:	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen/gntdev: Don't allocate struct
 gntdev_copy_batch on stack

On 19/01/16 14:26, Boris Ostrovsky wrote:
> On 01/18/2016 06:11 AM, David Vrabel wrote:
>> On 15/01/16 19:43, Boris Ostrovsky wrote:
>>> struct gntdev_copy_batch is over 1300 bytes in size, we shouldn't
>>> put it on stack.
>>>
>>> Some compilers (e.g. 5.2.1) complain:
>>>   drivers/xen/gntdev.c: In function ‘gntdev_ioctl_grant_copy.isra.5’:
>>>   drivers/xen/gntdev.c:949:1: warning: the frame size of 1416 bytes
>>>    is larger than 1024 bytes [-Wframe-larger-than=]
>> I thought I'd already reduced the size of this enough (from a batch size
>> of 32 to 24) but this obviously isn't enough for 64-bit platforms.
>>
>> In the absence of any performance data on the best approach I would
>> prefer just reducing the batch size to 16.
> 
> That would still leave us with over 900 bytes on the stack which I think
> is rather high.

The stack depth to here isn't very deep, so I think even the ~1500 byte
frame was fine.

> Do we expect this ioctl to be on some sort of a hot path?

Yes.

David