Re: [PATCH v15 09/10] arm64: Call kmalloc() to allocate DTB buffer

[Thiago Jung Bauermann <bauerman@linux.ibm.com>]

Lakshmi Ramasubramanian <nramas@linux.microsoft.com> writes:

> On 1/27/21 7:52 PM, Thiago Jung Bauermann wrote:
>> Will Deacon <will@kernel.org> writes:
>> 
>>> On Wed, Jan 27, 2021 at 09:59:38AM -0800, Lakshmi Ramasubramanian wrote:
>>>> On 1/27/21 8:52 AM, Will Deacon wrote:
>>>>
>>>> Hi Will,
>>>>
>>>>> On Fri, Jan 15, 2021 at 09:30:16AM -0800, Lakshmi Ramasubramanian wrote:
>>>>>> create_dtb() function allocates kernel virtual memory for
>>>>>> the device tree blob (DTB).  This is not consistent with other
>>>>>> architectures, such as powerpc, which calls kmalloc() for allocating
>>>>>> memory for the DTB.
>>>>>>
>>>>>> Call kmalloc() to allocate memory for the DTB, and kfree() to free
>>>>>> the allocated memory.
>>>>>>
>>>>>> Co-developed-by: Prakhar Srivastava <prsriva@linux.microsoft.com>
>>>>>> Signed-off-by: Prakhar Srivastava <prsriva@linux.microsoft.com>
>>>>>> Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
>>>>>> ---
>>>>>>    arch/arm64/kernel/machine_kexec_file.c | 12 +++++++-----
>>>>>>    1 file changed, 7 insertions(+), 5 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/arm64/kernel/machine_kexec_file.c b/arch/arm64/kernel/machine_kexec_file.c
>>>>>> index 7de9c47dee7c..51c40143d6fa 100644
>>>>>> --- a/arch/arm64/kernel/machine_kexec_file.c
>>>>>> +++ b/arch/arm64/kernel/machine_kexec_file.c
>>>>>> @@ -29,7 +29,7 @@ const struct kexec_file_ops * const kexec_file_loaders[] = {
>>>>>>    int arch_kimage_file_post_load_cleanup(struct kimage *image)
>>>>>>    {
>>>>>> -	vfree(image->arch.dtb);
>>>>>> +	kfree(image->arch.dtb);
>>>>>>    	image->arch.dtb = NULL;
>>>>>>    	vfree(image->arch.elf_headers);
>>>>>> @@ -59,19 +59,21 @@ static int create_dtb(struct kimage *image,
>>>>>>    			+ cmdline_len + DTB_EXTRA_SPACE;
>>>>>>    	for (;;) {
>>>>>> -		buf = vmalloc(buf_size);
>>>>>> +		buf = kmalloc(buf_size, GFP_KERNEL);
>>>>>
>>>>> Is there a functional need for this patch? I build the 'dtbs' target just
>>>>> now and sdm845-db845c.dtb is approaching 100K, which feels quite large
>>>>> for kmalloc().
>>>>
>>>> Changing the allocation from vmalloc() to kmalloc() would help us further
>>>> consolidate the DTB setup code for powerpc and arm64.
>>>
>>> Ok, but at the risk of allocation failure. Can powerpc use vmalloc()
>>> instead?
>> I believe this patch stems from this suggestion by Rob Herring:
>> 
>>> This could be taken a step further and do the allocation of the new
>>> FDT. The difference is arm64 uses vmalloc and powerpc uses kmalloc. The
>>> arm64 version also retries with a bigger allocation. That seems
>>> unnecessary.
>> in
>> https://lore.kernel.org/linux-integrity/20201211221006.1052453-3-robh@kernel.org/
>> The problem is that this patch implements only part of the suggestion,
>> which isn't useful in itself. So the patch series should either drop
>> this patch or consolidate the FDT allocation between the arches.
>> I just tested on powernv and pseries platforms and powerpc can use
>> vmalloc for the FDT buffer.
>> 
>
> Thanks for verifying on powerpc platform Thiago.
>
> I'll update the patch to do the following:
>
> => Use vmalloc for FDT buffer allocation on powerpc
> => Keep vmalloc for arm64, but remove the retry on allocation.
> => Also, there was a memory leak of FDT buffer in the error code path on arm64,
> which I'll fix as well.
>
> Did I miss anything?

Yes, you missed the second part of Rob's suggestion I was mentioning,
which is factoring out the code which allocates the new FDT from both
arm64 and powerpc.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center