Re: [PATCH] arch/x86: memory_hotplug, do not bump up max_pfn for device private pages

[Balbir Singh <balbirs@nvidia.com>]

On 12/2/25 09:11, dan.j.williams@intel.com wrote:
> [ add Gregory and linux-mm ]
> 
> [ full context for new Cc: ]
> Balbir Singh wrote:
>> Commit 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
>> exposed a bug with nokaslr and zone device
>> interaction, as seen on a system with an AMD iGPU and dGPU (see [1]).
>> The root cause of the issue is that, the gpu driver registers a zone
>> device private memory region. When kaslr is disabled or the above commit
>> is applied, the direct_map_physmem_end is set to much higher than 10 TiB
>> typically to the 64TiB address. When zone device private memory is added
>> to the system via add_pages(), it bumps up the max_pfn to the same
>> value. This causes dma_addressing_limited() to return true, since the
>> device cannot address memory all the way up to max_pfn.
>>
>> This caused a regression for games played on the iGPU, as it resulted in
>> the DMA32 zone being used for GPU allocations.
>>
>> Fix this by not bumping up max_pfn on x86 systems, when pgmap is passed
>> into add_pages(). The presence of pgmap is used to determine if device
>> private memory is being added via add_pages().
>>
>> More details:
>>
>> devm_request_mem_region() and request_free_mem_region() request for
>> device private memory. iomem_resource is passed as the base resource
>> with start and end parameters. iomem_resource's end depends on several
>> factors, including the platform and virtualization. On x86 for example
>> on bare metal, this value is set to boot_cpu_data.x86_phys_bits.
>> boot_cpu_data.x86_phys_bits can change depending on support for MKTME.
>> By default it is set to the same as log2(direct_map_physmem_end) which
>> is 46 to 52 bits depending on the number of levels in the page table.
>> The allocation routines used iomem_resource's end and
>> direct_map_physmem_end to figure out where to allocate the region.
>>
>> arch/powerpc is also impacted by this bug, this patch does not fix
>> the issue for powerpc.
>>
>> Testing:
>> 1. Tested on a virtual machine with test_hmm for zone device inseration
>> 2. A previous version of this patch was tested by Bert, please see [2]
>>
>> Link: https://lore.kernel.org/lkml/20250310112206.4168-1-spasswolf@web.de/ [1]
>> Link: https://lore.kernel.org/lkml/d87680bab997fdc9fb4e638983132af235d9a03a.camel@web.de/ [2]
>> Fixes: 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
>>
>> Cc: "Christian König" <christian.koenig@amd.com>
>> Cc: Ingo Molnar <mingo@kernel.org>
>> Cc: Kees Cook <kees@kernel.org>
>> Cc: Bjorn Helgaas <bhelgaas@google.com>
>> Cc: Linus Torvalds <torvalds@linux-foundation.org>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Andy Lutomirski <luto@kernel.org>
>> Cc: Alex Deucher <alexander.deucher@amd.com>
>> Cc: Bert Karwatzki <spasswolf@web.de>
>> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
>> Cc: Nicholas Piggin <npiggin@gmail.com>
>>
>>
>> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
>> ---
>> I've left powerpc out of this regression change due to the time required
>> to setup and test via qemu. I wanted to address the regression quickly
>>
>>
>>  arch/x86/mm/init_64.c | 15 ++++++++++++---
>>  1 file changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
>> index dce60767124f..cc60b57473a4 100644
>> --- a/arch/x86/mm/init_64.c
>> +++ b/arch/x86/mm/init_64.c
>> @@ -970,9 +970,18 @@ int add_pages(int nid, unsigned long start_pfn, unsigned long nr_pages,
>>  	ret = __add_pages(nid, start_pfn, nr_pages, params);
>>  	WARN_ON_ONCE(ret);
>>  
>> -	/* update max_pfn, max_low_pfn and high_memory */
>> -	update_end_of_memory_vars(start_pfn << PAGE_SHIFT,
>> -				  nr_pages << PAGE_SHIFT);
>> +	/*
>> +	 * add_pages() is called by memremap_pages() for adding device private
>> +	 * pages. Do not bump up max_pfn in the device private path. max_pfn
>> +	 * changes affect dma_addressing_limited. dma_addressing_limited
>> +	 * returning true when max_pfn is the device's addressable memory,
>> +	 * can force device drivers to use bounce buffers and impact their
>> +	 * performance
>> +	 */
>> +	if (!params->pgmap)
>> +		/* update max_pfn, max_low_pfn and high_memory */
>> +		update_end_of_memory_vars(start_pfn << PAGE_SHIFT,
>> +					  nr_pages << PAGE_SHIFT);
> 
> The comment says that this adjustment is only for the device-private
> case, but it applies to all driver-managed device memory.
> 
> Why not actually do what the comment says and limit this to
> DEVICE_PRIVATE? I.e.:
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index 0e4270e20fad..4cc8175f9ffd 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -989,7 +989,7 @@ int add_pages(int nid, unsigned long start_pfn, unsigned long nr_pages,
>  	 * addressable memory can force device drivers to use bounce buffers
>  	 * and impact their performance negatively:
>  	 */
> -	if (!params->pgmap)
> +	if (!params->pgmap || params->pgmap->type != MEMORY_DEVICE_PRIVATE)
>  		/* update max_pfn, max_low_pfn and high_memory */
>  		update_end_of_memory_vars(start_pfn << PAGE_SHIFT, nr_pages << PAGE_SHIFT);
>  

At that time when I audited the code, I did notice that max_pfn was already set to the upper
end of physical memory (even if it's not hotunplugged, because those regions were parsed and
added to memblock_add() and I noticed that all hotplug regions changing max_pfn were coming
from the device private).

I agree we should check for pgmap->type, so definitely the right fix

Reviewed-by: Balbir Singh <balbirs@nvidia.com>