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Message-ID: <174349849358.14745.17615225659857776438.tip-bot2@tip-bot2>
Date: Tue, 01 Apr 2025 09:08:13 -0000
From: "tip-bot2 for Balbir Singh" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Bert Karwatzki <spasswolf@....de>, Balbir Singh <balbirs@...dia.com>,
Ingo Molnar <mingo@...nel.org>, Brian Gerst <brgerst@...il.com>,
Juergen Gross <jgross@...e.com>, "H. Peter Anvin" <hpa@...or.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>, Christoph Hellwig <hch@....de>,
"Pierre-Eric Pelloux-Prayer" <pierre-eric.pelloux-prayer@....com>,
Alex Deucher <alexander.deucher@....com>, christian.koenig@....com,
David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86/mm/init: Handle the special case of device
private pages in add_pages(), to not increase max_pfn and trigger
dma_addressing_limited() bounce buffers
The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 7170130e4c72ce0caa0cb42a1627c635cc262821
Gitweb: https://git.kernel.org/tip/7170130e4c72ce0caa0cb42a1627c635cc262821
Author: Balbir Singh <balbirs@...dia.com>
AuthorDate: Tue, 01 Apr 2025 11:07:52 +11:00
Committer: Ingo Molnar <mingo@...nel.org>
CommitterDate: Tue, 01 Apr 2025 10:52:38 +02:00
x86/mm/init: Handle the special case of device private pages in add_pages(), to not increase max_pfn and trigger dma_addressing_limited() bounce buffers
As Bert Karwatzki reported, the following recent commit causes a
performance regression on AMD iGPU and dGPU systems:
7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
It exposed a bug with nokaslr and zone device interaction.
The root cause of the bug 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 problem, but 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:
https://lore.kernel.org/lkml/d87680bab997fdc9fb4e638983132af235d9a03a.camel@web.de/
[ mingo: Clarified the comments and the changelog. ]
Reported-by: Bert Karwatzki <spasswolf@....de>
Tested-by: Bert Karwatzki <spasswolf@....de>
Fixes: 7ffb791423c7 ("x86/kaslr: Reduce KASLR entropy on most x86 systems")
Signed-off-by: Balbir Singh <balbirs@...dia.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Christoph Hellwig <hch@....de>
Cc: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@....com>
Cc: Alex Deucher <alexander.deucher@....com>
Cc: Christian König <christian.koenig@....com>
Cc: David Airlie <airlied@...il.com>
Cc: Simona Vetter <simona@...ll.ch>
Link: https://lore.kernel.org/r/20250401000752.249348-1-balbirs@nvidia.com
---
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 519aa53..821a0b5 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -959,9 +959,18 @@ 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);
+ /*
+ * Special case: add_pages() is called by memremap_pages() for adding device
+ * private pages. Do not bump up max_pfn in the device private path,
+ * because 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 negatively:
+ */
+ 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);
return ret;
}
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