Re: [PATCH v7] Add udmabuf misc device

From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Gerd Hoffmann <kraxel@redhat.com>
Cc: dri-devel@lists.freedesktop.org, David Airlie <airlied@linux.ie>,
	Tomeu Vizoso <tomeu.vizoso@collabora.com>,
	Daniel Vetter <daniel@ffwll.ch>, Jonathan Corbet <corbet@lwn.net>,
	Sumit Semwal <sumit.semwal@linaro.org>,
	Shuah Khan <shuah@kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	open list <linux-kernel@vger.kernel.org>,
	"open list:DMA BUFFER SHARING FRAMEWORK" 
	<linux-media@vger.kernel.org>,
	"moderated list:DMA BUFFER SHARING FRAMEWORK" 
	<linaro-mm-sig@lists.linaro.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>,
	linux-api@vger.kernel.org
Subject: Re: [PATCH v7] Add udmabuf misc device
Date: Tue, 11 Sep 2018 12:50:58 +0300	[thread overview]
Message-ID: <18750721.r4B5nx0M26@avalon> (raw)
In-Reply-To: <20180911065014.vo6qp6hkb7cjftdc@sirius.home.kraxel.org>

Hi Gerd,

On Tuesday, 11 September 2018 09:50:14 EEST Gerd Hoffmann wrote:
>   Hi,
> 
> >> +#define UDMABUF_CREATE       _IOW('u', 0x42, struct udmabuf_create)
> > 
> > Why do you start at 0x42 if you reserve the 0x40-0x4f range ?
> 
> No particular strong reason, just that using 42 was less boring than
> starting with 0x40.
> 
> >> +#define UDMABUF_CREATE_LIST  _IOW('u', 0x43, struct
> >> udmabuf_create_list)
> > 
> > Where's the documentation ? :-)
> 
> Isn't it simple enough?

No kernel UAPI is simple enough to get away without documenting it.

> But, well, yes, I guess I can add some kerneldoc comments.
> 
> >> +static int udmabuf_vm_fault(struct vm_fault *vmf)
> >> +{
> >> +	struct vm_area_struct *vma = vmf->vma;
> >> +	struct udmabuf *ubuf = vma->vm_private_data;
> >> +
> >> +	if (WARN_ON(vmf->pgoff >= ubuf->pagecount))
> >> +		return VM_FAULT_SIGBUS;
> > 
> > Just curious, when do you expect this to happen ?
> 
> It should not.  If it actually happens it would be a bug somewhere,
> thats why the WARN_ON.

But you seem to consider that this condition that should never happen still 
has a high enough chance of happening that it's worth a WARN_ON(). I was 
wondering why this one in particular, and not other conditions that also can't 
happen and are not checked through the code. 

> >> +	struct udmabuf *ubuf;
> >> 
> >> +	ubuf = kzalloc(sizeof(struct udmabuf), GFP_KERNEL);
> > 
> > sizeof(*ubuf)
> 
> Why?  Should not make a difference ...

Because the day we replace

	struct udmabuf *ubuf;

with

	struct udmabuf_ext *ubuf;

and forget to change the next line, we'll introduce a bug. That's why 
sizeof(variable) is preferred over sizeof(type). Another reason is that I can 
easily see that

	ubuf = kzalloc(sizeof(*ubuf), GFP_KERNEL);

is correct, while using sizeof(type) requires me to go and look up the 
declaration of the variable.

> >> +		memfd = fget(list[i].memfd);
> >> +		if (!memfd)
> >> +			goto err_put_pages;
> >> +		if (!shmem_mapping(file_inode(memfd)->i_mapping))
> >> +			goto err_put_pages;
> >> +		seals = memfd_fcntl(memfd, F_GET_SEALS, 0);
> >> +		if (seals == -EINVAL ||
> >> +		    (seals & SEALS_WANTED) != SEALS_WANTED ||
> >> +		    (seals & SEALS_DENIED) != 0)
> >> +			goto err_put_pages;
> > 
> > All these conditions will return -EINVAL. I'm not familiar with the memfd
> > API, should some error conditions return a different error code to make
> > them distinguishable by userspace ?
> 
> Hmm, I guess EBADFD would be reasonable in case the file handle isn't a
> memfd.  Other suggestions?

I'll let others comment on this as I don't feel qualified to pick proper error 
codes, not being familiar with the memfd API.

> I'll prepare a fixup patch series addressing most of the other
> review comments.

-- 
Regards,

Laurent Pinchart