[bpf-next,v1] bpf,arena: Remove redundant page mask of vmf->address

Commit Message

Wang, Haiyue May 7, 2024, 6:33 a.m. UTC

As the comment described in "struct vm_fault":
	".address"      : 'Faulting virtual address - masked'
	".real_address" : 'Faulting virtual address - unmasked'

The link [1] said: "Whatever the routes, all architectures end up to the
invocation of handle_mm_fault() which, in turn, (likely) ends up calling
__handle_mm_fault() to carry out the actual work of allocating the page
tables."

  __handle_mm_fault() does address assignment:
	.address = address & PAGE_MASK,
	.real_address = address,

This is debug dump by running `./test_progs -a "*arena*"`:

[   69.767494] arena fault: vmf->address = 10000001d000, vmf->real_address = 10000001d008
[   69.767496] arena fault: vmf->address = 10000001c000, vmf->real_address = 10000001c008
[   69.767499] arena fault: vmf->address = 10000001b000, vmf->real_address = 10000001b008
[   69.767501] arena fault: vmf->address = 10000001a000, vmf->real_address = 10000001a008
[   69.767504] arena fault: vmf->address = 100000019000, vmf->real_address = 100000019008
[   69.769388] arena fault: vmf->address = 10000001e000, vmf->real_address = 10000001e1e8

So we can use the value of 'vmf->address' to do BPF arena kernel address
space cast directly.

[1] https://docs.kernel.org/mm/page_tables.html

Signed-off-by: Haiyue Wang <[e-mail]>
---
 kernel/bpf/arena.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

[e-mail] May 7, 2024, 9:20 p.m. UTC | #1

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <[e-mail]>:

On Tue,  7 May 2024 14:33:39 +0800 you wrote:
> As the comment described in "struct vm_fault":
> 	".address"      : 'Faulting virtual address - masked'
> 	".real_address" : 'Faulting virtual address - unmasked'
> 
> The link [1] said: "Whatever the routes, all architectures end up to the
> invocation of handle_mm_fault() which, in turn, (likely) ends up calling
> __handle_mm_fault() to carry out the actual work of allocating the page
> tables."
> 
> [...]

Here is the summary with links:
  - [bpf-next,v1] bpf,arena: Remove redundant page mask of vmf->address
    https://git.kernel.org/bpf/bpf-next/c/75b0fbf15d84

You are awesome, thank you!

Patch

diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index 343c3456c8dd..1876dc7ebb57 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -251,7 +251,7 @@  static vm_fault_t arena_vm_fault(struct vm_fault *vmf)
 	int ret;
 
 	kbase = bpf_arena_get_kern_vm_start(arena);
-	kaddr = kbase + (u32)(vmf->address & PAGE_MASK);
+	kaddr = kbase + (u32)(vmf->address);
 
 	guard(mutex)(&arena->lock);
 	page = vmalloc_to_page((void *)kaddr);