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

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Date: Tue,  7 May 2024 14:33:39 +0800
From: Haiyue Wang <haiyue.wang@...el.com>
To: bpf@...r.kernel.org
Cc: Haiyue Wang <haiyue.wang@...el.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>,
	Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>,
	Stanislav Fomichev <sdf@...gle.com>,
	Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>,
	linux-kernel@...r.kernel.org (open list)
Subject: [PATCH bpf-next v1] bpf,arena: Remove redundant page mask of vmf->address

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 <haiyue.wang@...el.com>
---
 kernel/bpf/arena.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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);
-- 
2.43.2