Re: [PATCH V5] x86/mm: Tracking linear mapping split events

Saravanan D
2021-01-28T21:02:05Z

Hi Mathew,

> Is this tracing of userspace programs causing splits, or is it kernel
> tracing? Also, we have lots of kinds of tracing these days; are you
> referring to kprobes? tracepoints? ftrace? Something else?

It has to be kernel tracing (kprobes, tracepoints) as we are dealing with
direct mapping splits.

Kernel's direct mapping
`` ffff888000000000 | -119.5 TB | ffffc87fffffffff | 64 TB | direct
mapping of all physical memory (page_offset_base)``

The kernel text range
``ffffffff80000000 | -2 GB | ffffffff9fffffff | 512 MB | kernel
text mapping, mapped to physical address 0``

Source : Documentation/x86/x86_64/mm.rst

Kernel code segment points to the same physical addresses already mapped
in the direct mapping range (0x20000000 = 512 MB)

When we enable kernel tracing, we would have to modify attributes/permissions
of the text segment pages that are direct mapped causing them to split.

When we track the direct_pages_count[] in arch/x86/mm/pat/set_memory.c
There are only splits from higher levels. They never coalesce back.

Splits when we turn on dynamic tracing
....
cat /proc/vmstat | grep -i direct_map_level
direct_map_level2_splits 784
direct_map_level3_splits 12
bpftrace -e 'tracepoint:raw_syscalls:sys_enter { @ [pid, comm] = count(); }'
cat /proc/vmstat | grep -i
direct_map_level
direct_map_level2_splits 789
direct_map_level3_splits 12
....

Thanks,
Saravanan D