Re: [PATCH] gpu: drm: use struct_size() in kmalloc()

[Christian KÃnig]

Am 21.05.19 um 09:23 schrieb Daniel Vetter:
> On Tue, May 21, 2019 at 4:33 AM Alex Deucher <alexdeucher@xxxxxxxxx> wrote:
> > On Mon, May 20, 2019 at 7:19 PM Pan, Xinhui <Xinhui.Pan@xxxxxxx> wrote:
> > > Daniel, what you are talking about is totally wrong.
> > > 1) AFAIK, only one zero-size array can be in the end of a struct.
> > > 2) two struct_size will add up struct itself twice. the sum is wrong then.
> > >
> > > No offense. I can't help feeling lucky that you are in intel.
> > Xinhui,
> >
> > Please keep things civil.  There is no need for comments like this.
> Yeah, this was over the line, thanks Alex for already taking care of
> this. Please note that fd.o mailing lists operate under a CoC:
>
> https://www.freedesktop.org/wiki/CodeOfConduct/

Seconded. I also enjoy the humiliation of other in email, but it doesn't 
helps us getting code written and problems solved in a professional 
environment.

> Wrt the technical comment: I know that you can only do one variable
> sized array, and it must be at the end. But you can put multiple
> structures all within the same allocation. Which is what I thought you
> wanted to do. And my sketch would allow you to do that even if you
> have multiple variable length structures you want to allocate. There's
> plenty examples of this (but open-coded ones) in the kernel.

BTW: Is there actually good documentation how to correctly do the 
variable length array at end of structure thing in the kernel?

I do know that I've seen a lot of different variants like array[] 
array[0] or array[1] and I have also seen a bunch of gcc versions 
failing to generate correct code for some of them.

So we should probably nail down how to do things correctly.

> Except in really hot paths I personally think that that kind of
> trickery isn't worth it.

Well for kmalloc() it's not that much overhead, but with vmalloc that is 
a completely different picture.

Christian.

> Cheers, Daniel
>
> > Alex
> >
> > > åää: Daniel Vetter <daniel.vetter@xxxxxxxx> äè Daniel Vetter <daniel@xxxxxxxx>
> > > åéæé: 2019å5æ21æ 0:28
> > > æää: Pan, Xinhui
> > > æé: Deucher, Alexander; Koenig, Christian; Zhou, David(ChunMing); airlied@xxxxxxxx; daniel@xxxxxxxx; Quan, Evan; xiaolinkui; amd-gfx@xxxxxxxxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > > äé: Re: [PATCH] gpu: drm: use struct_size() in kmalloc()
> > >
> > > [CAUTION: External Email]
> > >
> > > On Fri, May 17, 2019 at 04:44:30PM +0000, Pan, Xinhui wrote:
> > > > I am going to put more members which are also array after this struct,
> > > > not only obj[].  Looks like this struct_size did not help on multiple
> > > > array case. Thanks anyway.  ________________________________
> > > You can then add them up, e.g. kmalloc(struct_size()+struct_size(),
> > > GFP_KERNEL), so this patch here still looks like a good idea.
> > >
> > > Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > >
> > > Cheers, Daniel
> > >
> > > > From: xiaolinkui <xiaolinkui@xxxxxxxxxx>
> > > > Sent: Friday, May 17, 2019 4:46:00 PM
> > > > To: Deucher, Alexander; Koenig, Christian; Zhou, David(ChunMing); airlied@xxxxxxxx; daniel@xxxxxxxx; Pan, Xinhui; Quan, Evan
> > > > Cc: amd-gfx@xxxxxxxxxxxxxxxxxxxxx; dri-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; xiaolinkui@xxxxxxxxxx
> > > > Subject: [PATCH] gpu: drm: use struct_size() in kmalloc()
> > > >
> > > > [CAUTION: External Email]
> > > >
> > > > Use struct_size() helper to keep code simple.
> > > >
> > > > Signed-off-by: xiaolinkui <xiaolinkui@xxxxxxxxxx>
> > > > ---
> > > >   drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c | 3 +--
> > > >   1 file changed, 1 insertion(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c
> > > > index 22bd21e..4717a64 100644
> > > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c
> > > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ras.c
> > > > @@ -1375,8 +1375,7 @@ int amdgpu_ras_init(struct amdgpu_device *adev)
> > > >          if (con)
> > > >                  return 0;
> > > >
> > > > -       con = kmalloc(sizeof(struct amdgpu_ras) +
> > > > -                       sizeof(struct ras_manager) * AMDGPU_RAS_BLOCK_COUNT,
> > > > +       con = kmalloc(struct_size(con, objs, AMDGPU_RAS_BLOCK_COUNT),
> > > >                          GFP_KERNEL|__GFP_ZERO);
> > > >          if (!con)
> > > >                  return -ENOMEM;
> > > > --
> > > > 2.7.4
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
> > > _______________________________________________
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> > > amd-gfx@xxxxxxxxxxxxxxxxxxxxx
> > > https://lists.freedesktop.org/mailman/listinfo/amd-gfx