Re: [PATCH v2] dma-buf: remove restriction of IOCTL:DMA_BUF_SET_NAME

["Christian König" <christian.koenig@amd.com>]

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Am 13.10.21 um 01:56 schrieb Sumit Semwal:
> Hello Guangming, Christian,
>
>
>
> On Tue, 12 Oct 2021, 14:09 , <guangming.cao@mediatek.com 
> <mailto:guangming.cao@mediatek.com>> wrote:
>
>     From: Guangming Cao <Guangming.Cao@mediatek.com
>     <mailto:Guangming.Cao@mediatek.com>>
>
>     > Am 09.10.21 um 07:55 schrieb guangming.cao@mediatek.com
>     <mailto:guangming.cao@mediatek.com>:
>     > From: Guangming Cao <Guangming.Cao@mediatek.com
>     <mailto:Guangming.Cao@mediatek.com>>
>     > >
>     > > If dma-buf don't want userspace users to touch the dmabuf buffer,
>     > > it seems we should add this restriction into dma_buf_ops.mmap,
>     > > not in this IOCTL:DMA_BUF_SET_NAME.
>     > >
>     > > With this restriction, we can only know the kernel users of
>     the dmabuf
>     > > by attachments.
>     > > However, for many userspace users, such as userpsace users of
>     dma_heap,
>     > > they also need to mark the usage of dma-buf, and they don't
>     care about
>     > > who attached to this dmabuf, and seems it's no meaning to be
>     waiting for
>     > > IOCTL:DMA_BUF_SET_NAME rather than mmap.
>     >
>     > Sounds valid to me, but I have no idea why this restriction was
>     added in
>     > the first place.
>     >
>     > Can you double check the git history and maybe identify when
>     that was
>     > added? Mentioning this change in the commit message then might make
>     > things a bit easier to understand.
>     >
>     > Thanks,
>     > Christian.
>     It was add in this patch:
>     https://patchwork.freedesktop.org/patch/310349/
>     <https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpatchwork.freedesktop.org%2Fpatch%2F310349%2F&data=04%7C01%7Cchristian.koenig%40amd.com%7C4149923e2b0646de82ce08d98ddbf2c2%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637696798278342557%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=N49RVF4s%2BGQ4D%2Ft1MOwRsCnslFnwobSB3G86pvP9m7A%3D&reserved=0>.
>     However, there is no illustration about it.
>     I guess it wants users to set_name when no attachments on the dmabuf,
>     for case with attachments, we can find owner by device in attachments.
>     But just I said in commit message, this is might not a good idea.
>
>     Do you have any idea?
>
>
> For the original series, the idea was that allowing name change 
> mid-use could confuse the users about the dma-buf. However, the rest 
> of the series also makes sure each dma-buf have a unique inode, and 
> any accounting should probably use that, without relying on the name 
> as much.
> So I don't have an objection to this change.

I suggest to add that explanation and the original commit id into the 
commit message.

With that changed the patch has my rb as well.

Regards,
Christian.

>
> Best,
> Sumit.
>
>     >
>     > >
>     > > Signed-off-by: Guangming Cao <Guangming.Cao@mediatek.com
>     <mailto:Guangming.Cao@mediatek.com>>
>     > > ---
>     > >   drivers/dma-buf/dma-buf.c | 14 ++------------
>     > >   1 file changed, 2 insertions(+), 12 deletions(-)
>     > >
>     > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>     > > index 511fe0d217a0..db2f4efdec32 100644
>     > > --- a/drivers/dma-buf/dma-buf.c
>     > > +++ b/drivers/dma-buf/dma-buf.c
>     > > @@ -325,10 +325,8 @@ static __poll_t dma_buf_poll(struct file
>     *file, poll_table *poll)
>     > >
>     > >   /**
>     > >    * dma_buf_set_name - Set a name to a specific dma_buf to
>     track the usage.
>     > > - * The name of the dma-buf buffer can only be set when the
>     dma-buf is not
>     > > - * attached to any devices. It could theoritically support
>     changing the
>     > > - * name of the dma-buf if the same piece of memory is used
>     for multiple
>     > > - * purpose between different devices.
>     > > + * It could theoretically support changing the name of the
>     dma-buf if the same
>     > > + * piece of memory is used for multiple purpose between
>     different devices.
>     > >    *
>     > >    * @dmabuf: [in]     dmabuf buffer that will be renamed.
>     > >    * @buf:    [in]     A piece of userspace memory that
>     contains the name of
>     > > @@ -346,19 +344,11 @@ static long dma_buf_set_name(struct
>     dma_buf *dmabuf, const char __user *buf)
>     > >     if (IS_ERR(name))
>     > >             return PTR_ERR(name);
>     > >
>     > > -   dma_resv_lock(dmabuf->resv, NULL);
>     > > -   if (!list_empty(&dmabuf->attachments)) {
>     > > -           ret = -EBUSY;
>     > > -           kfree(name);
>     > > -           goto out_unlock;
>     > > -   }
>     > >     spin_lock(&dmabuf->name_lock);
>     > >     kfree(dmabuf->name);
>     > >     dmabuf->name = name;
>     > >     spin_unlock(&dmabuf->name_lock);
>     > >
>     > > -out_unlock:
>     > > -   dma_resv_unlock(dmabuf->resv);
>     > >     return ret;
>     > >   }
>     > >
>


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