[patch V2 00/10] x86/mm/cpa: Improve large page preservation handling

Thomas Gleixner
2018-09-14T13:21:04Z

Bin reported that try_preserve_large_page() in the page attribute code
consumes an large amount of CPU time. His initial attempts of addressing
this made me look deeper into the code.

The logic in this code is not really intelligent. It requires to check a
large page in 4k steps for conflicts. That's insane as most operations do
not conflict at all.

The code also lacks sanity checks which allow to detect whether the
existing mapping is incorrect vs. the static protections.

Any form of debugging or statistics is missing as well.

The following series addresses this:

- Clean up the code so it becomes extensible

- Provide the ability to check a full range for conflicts

- Add debug output and statistics to quantify the changes and to allow
observation of the mechanism in the future.

- Add a sanity check for existing mappings with a fixup for the 2M case
and a warning for the 1G case. The 2M case is trivial to address, the
1G case requires larger changes and is just warned about for now.

- Avoid conflict checks for operations which clear the PRESENT bit

- Utilize the range checks to detect conflicts in one operation

- Drop the 4k wise checking which turned out to provide no extra large
page preservation in testing. There might be corner cases where a page
would be preserved, but that's overkill for the common cases.

Before:
1G pages checked: 2
1G pages sameprot: 0
1G pages preserved: 0
2M pages checked: 540
2M pages sameprot: 466
2M pages preserved: 47
4K pages checked: 800770
4K pages set-checked: 7668

After:
1G pages checked: 2
1G pages sameprot: 0
1G pages preserved: 0
2M pages checked: 538
2M pages sameprot: 466
2M pages preserved: 47
4K pages set-checked: 7668

This gets rid of ~800000 checks whether a particular address is with a
static protection region. Each check tests against 4 different regions,
which adds up to several million instructions.

Changes since V1: Fix patch 1/10 extra argument issue which breaks
bisectability.

Thanks,

tglx

8<---------------------
Kconfig | 8
mm/pageattr.c | 515 +++++++++++++++++++++++++++++++++++++++++++---------------
2 files changed, 391 insertions(+), 132 deletions(-)