Re: [PATCH v1 0/2] mremap refactor: check src address for vma boundaries first.

[Jeff Xu <jeffxu@chromium.org>]

On Wed, Aug 14, 2024 at 12:14 AM <jeffxu@chromium.org> wrote:
>
> From: Jeff Xu <jeffxu@chromium.org>
>
> mremap doesn't allow relocate, expand, shrink across VMA boundaries,
> refactor the code to check src address range before doing anything on
> the destination, i.e. destination won't be unmapped, if src address
> failed the boundaries check.
>
> This also allows us to remove can_modify_mm from mremap.c, since
> the src address must be single VMA, can_modify_vma is used.
>
> It is likely this will improve the performance on mremap, previously
> the code does sealing check using can_modify_mm for the src address range,
> and the new code removed the loop (used by can_modify_mm).
>
> In order to verify this patch doesn't regress on mremap, I added tests in
> mseal_test, the test patch can be applied before mremap refactor patch or
> checkin independently.
>
> Also this patch doesn't change mseal's existing schematic: if sealing fail,
> user can expect the src/dst address isn't updated. So this patch can be
> applied regardless if we decided to go with current out-of-loop approach
> or in-loop approach currently in discussion.
>
> Regarding the perf test report by stress-ng [1] title:
> 8be7258aad: stress-ng.pagemove.page_remaps_per_sec -4.4% regression
>
> The test is using below for testing:
> stress-ng --timeout 60 --times --verify --metrics --no-rand-seed --pagemove 64
>
> I can't repro this using ChromeOS, the pagemove test shows large value
> of stddev and stderr, and can't reasonably refect the performance impact.
>
> For example: I write a c program [2] to run the above pagemove test 10 times
> and calculate the stddev, stderr, for 3 commits:
>
> 1> before mseal feature is added:
> Ops/sec:
>   Mean     : 3564.40
>   Std Dev  : 2737.35 (76.80% of Mean)
>   Std Err  : 865.63 (24.29% of Mean)
>
> 2> after mseal feature is added:
> Ops/sec:
>   Mean     : 2703.84
>   Std Dev  : 2085.13 (77.12% of Mean)
>   Std Err  : 659.38 (24.39% of Mean)
>
> 3> after current patch (mremap refactor)
> Ops/sec:
>   Mean     : 3603.67
>   Std Dev  : 2422.22 (67.22% of Mean)
>   Std Err  : 765.97 (21.26% of Mean)
>
> The result shows 21%-24% stderr, this means whatever perf improvment/impact
> there might be won't be measured correctly by this test.
>
> This test machine has 32G memory,  Intel(R) Celeron(R) 7305, 5 CPU.
> And I reboot the machine before each test, and take the first 10 runs with
> run_stress_ng 10
>
> (I will run longer duration to see if test still shows large stdDev,StdErr)
>
I took more samples (100 run ), the stddev/stderr is smaller, however
still not at a range that can reasonably measure the perf improvement
here.

The tests were taken using the same machine as (10 times run above)
and exact the same steps: i.e. change to certain kernel commit, reboot
test device, take the first test result.

1> Before mseal feature is added:
Statistics:
Ops/sec:
  Mean     : 1733.26
  Std Dev  : 842.13 (48.59% of Mean)
  Std Err  : 84.21 (4.86% of Mean)

2> After mseal feature is added
Statistics:
Ops/sec:
  Mean     : 1701.53
  Std Dev  : 1017.29 (59.79% of Mean)
  Std Err  : 101.73 (5.98% of Mean)

3> After mremap refactor (this patch)
Statistics:
Ops/sec:
  Mean     : 1097.04
  Std Dev  : 860.67 (78.45% of Mean)
  Std Err  : 86.07 (7.85% of Mean)

Summary: even when the stderr is down to 4%-%8 percentage range, the
stddev is still too big.

Hence, there are other unknown, random variables that impact this test.

-Jeff

> [1] https://lore.kernel.org/lkml/202408041602.caa0372-oliver.sang@intel.com/
> [2] https://github.com/peaktocreek/mmperf/blob/main/run_stress_ng.c
>
>
> Jeff Xu (2):
>   mseal:selftest mremap across VMA boundaries.
>   mseal: refactor mremap to remove can_modify_mm
>
>  mm/internal.h                           |  24 ++
>  mm/mremap.c                             |  77 +++----
>  mm/mseal.c                              |  17 --
>  tools/testing/selftests/mm/mseal_test.c | 293 +++++++++++++++++++++++-
>  4 files changed, 353 insertions(+), 58 deletions(-)
>
> --
> 2.46.0.76.ge559c4bf1a-goog
>