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Subject: Re: [PATCH 1/4] mm: introduce vma_set_file function v2
To:     <christian.koenig@amd.com>, <linux-mm@kvack.org>,
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References: <20201008112342.9394-1-christian.koenig@amd.com>
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From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Fri, 9 Oct 2020 00:36:24 -0700
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On 10/9/20 12:33 AM, Christian K=C3=B6nig wrote:
> Am 08.10.20 um 23:49 schrieb John Hubbard:
>> On 10/8/20 4:23 AM, Christian K=C3=B6nig wrote:
>> ...
>>
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_mman.c b/drivers/gpu/drm=
/i915/gem/i915_gem_mman.c
>>> index 3d69e51f3e4d..c9d5f1a38af3 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
>>> @@ -893,8 +893,8 @@ int i915_gem_mmap(struct file *filp, struct vm_area=
_struct *vma)
>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * requires avoiding extraneous ref=
erences to their filp, hence why
>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * we prefer to use an anonymous fi=
le for their mmaps.
>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */
>>> -=C2=A0=C2=A0=C2=A0 fput(vma->vm_file);
>>> -=C2=A0=C2=A0=C2=A0 vma->vm_file =3D anon;
>>> +=C2=A0=C2=A0=C2=A0 vma_set_file(vma, anon);
>>> +=C2=A0=C2=A0=C2=A0 fput(anon);
>>
>> That's one fput() too many, isn't it?
>=20
> No, the other cases were replacing the vm_file with something pre-allocat=
ed and also grabbed a new=20
> reference.
>=20
> But this case here uses the freshly allocated anon file and so vma_set_fi=
le() grabs another extra=20
> reference which we need to drop.
>=20
> The alternative is to just keep it as it is. Opinions?
>=20

I think just a small comment for these cases, is probably about right.

>> ...
>>
>>> diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android=
/ashmem.c
>>> index 10b4be1f3e78..a51dc089896e 100644
>>> --- a/drivers/staging/android/ashmem.c
>>> +++ b/drivers/staging/android/ashmem.c
>>> @@ -450,9 +450,8 @@ static int ashmem_mmap(struct file *file, struct vm=
_area_struct *vma)
>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 vma_set_anonymou=
s(vma);
>>> =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 }
>>> =C2=A0 -=C2=A0=C2=A0=C2=A0 if (vma->vm_file)
>>> -=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 fput(vma->vm_file);
>>> -=C2=A0=C2=A0=C2=A0 vma->vm_file =3D asma->file;
>>> +=C2=A0=C2=A0=C2=A0 vma_set_file(vma, asma->file);
>>> +=C2=A0=C2=A0=C2=A0 fput(asma->file);
>>
>> Same here: that fput() seems wrong, as it was already done within vma_se=
t_file().
>=20
> No, that case is correct as well. The Android code here has the matching =
get_file() a few lines up,=20
> see the surrounding code.
>=20
> I didn't wanted to replace that since it does some strange error handling=
 here, so the result is=20
> that we need to drop the extra reference as again.
>=20
> We could also keep it like it is or maybe better put a TODO comment on it=
.
>=20

Yeah, I think a comment is a good way to go.


thanks,
--=20
John Hubbard
NVIDIA