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Subject: Re: [PATCH v15 09/10] arm64: Call kmalloc() to allocate DTB buffer
To:     Thiago Jung Bauermann <bauerman@linux.ibm.com>
Cc:     Will Deacon <will@kernel.org>, zohar@linux.ibm.com,
        robh@kernel.org, takahiro.akashi@linaro.org,
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References: <20210115173017.30617-1-nramas@linux.microsoft.com>
 <20210115173017.30617-10-nramas@linux.microsoft.com>
 <20210127165208.GA358@willie-the-truck>
 <d3330793-6054-6e59-b727-44bf8e5653cd@linux.microsoft.com>
 <20210127184319.GA676@willie-the-truck>
 <871re5soof.fsf@manicouagan.localdomain>
 <58d3ffbf-4d80-c893-34d6-366ebfac55bd@linux.microsoft.com>
 <87y2gdr93p.fsf@manicouagan.localdomain>
From:   Lakshmi Ramasubramanian <nramas@linux.microsoft.com>
Message-ID: <a751409d-aa1c-68ea-f397-ee3bcf2f393d@linux.microsoft.com>
Date:   Wed, 27 Jan 2021 20:33:34 -0800
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On 1/27/21 8:14 PM, Thiago Jung Bauermann wrote:
> 
> 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.
> 

Sure - I'll address that.

thanks,
  -lakshmi