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

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Date:	Tue, 19 Jan 2016 09:26:12 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	David Vrabel <david.vrabel@...rix.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 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.

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


-boris