Re: [GIT PULL] x86/mm changes for v5.9
From: Linus Torvalds
Date: Wed Aug 05 2020 - 13:07:11 EST
On Wed, Aug 5, 2020 at 4:03 AM Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
> The commit 8bb9bf242d1f ("x86/mm/64: Do not sync vmalloc/ioremap
> mappings") causes the OOPS below, in Linus' tree and in linux-next,
> unearthed by my CI on <https://www.wireguard.com/build-status/>.
> Bisecting reveals 8bb9bf242d1f, and reverting this makes the OOPS go
The oops happens early in the function, and the "Code:" line actually
gets almost the whole function prologue in it (missing first two bytes
are probably "push %rbp"):
0: 41 56 push %r14
2: 41 55 push %r13
4: 41 54 push %r12
6: 55 push %rbp
7: 48 89 f5 mov %rsi,%rbp
a: 53 push %rbx
b: 48 89 fb mov %rdi,%rbx
e: 48 83 ec 08 sub $0x8,%rsp
12: 48 8b 06 mov (%rsi),%rax
15: 4c 8b 67 40 mov 0x40(%rdi),%r12
19: 49 89 c6 mov %rax,%r14
1c: 45 30 f6 xor %r14b,%r14b
1f: a8 04 test $0x4,%al
21: b8 00 00 00 00 mov $0x0,%eax
26: 4c 0f 44 f0 cmove %rax,%r14
2a:* 49 8b 46 08 mov 0x8(%r14),%rax <-- trapping instruction
> BUG: unable to handle page fault for address: ffffe8ffffd00608
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 0 P4D 0
Yeah, missing page table because it wasn't copied.
Presumably because that kthread is using the active_mm of some random
user space process that didn't get sync'ed.
And the sync_global_pgds() may have ended up being sufficient
synchronization with whoever allocated thigns, even if it wasn't about
the TLB contents themselves.
So apparently the "the page-table pages are all pre-allocated now" is
simply not true. Joerg?
Unless somebody can figure this out fairly quickly, I think it should
just be reverted.