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

[<dan.j.williams@intel.com>]

[ 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);