[PATCH v3 0/6] Optimize mremap during mutual alignment within PMD

From: Joel Fernandes (Google)
Date: Wed May 24 2023 - 11:33:00 EST


Here is v3 of the mremap start address optimization / fix for exec warning.

The main changes are:
1. Care to be taken to move purely within a VMA, in other words this check
in call_align_down():
if (vma->vm_start <= addr_masked)
return false;

As an example of why this is needed:
Consider the following range which is 2MB aligned and is
a part of a larger 10MB range which is not shown. Each
character is 256KB below making the source and destination
2MB each. The lower case letters are moved (s to d) and the
upper case letters are not moved.


If we align down 'ssss' to start from the 'SSSS', we will end up destroying
SSSS. The above if statement prevents that and I verified it.

I also added a test for this in the last patch.

2. Handle the stack case separately. We do not care about #1 for stack movement
because the 'SSSS' does not matter during this move. Further we need to do this
to prevent the stack move warning.

if (!for_stack && vma->vm_start <= addr_masked)
return false;

History of patches
1. Masked address was stored in int, fixed it to unsigned long to avoid truncation.
2. We now handle moves happening purely within a VMA, a new test is added to handle this.
3. More code comments.

1. Trigger the optimization for mremaps smaller than a PMD. I tested by tracing
that it works correctly.

2. Fix issue with bogus return value found by Linus if we broke out of the
above loop for the first PMD itself.

v1: Initial RFC.

Description of patches
These patches optimizes the start addresses in move_page_tables() and tests the
changes. It addresses a warning [1] that occurs due to a downward, overlapping
move on a mutually-aligned offset within a PMD during exec. By initiating the
copy process at the PMD level when such alignment is present, we can prevent
this warning and speed up the copying process at the same time. Linus Torvalds
suggested this idea.

Please check the individual patches for more details.


- Joel

[1] https://lore.kernel.org/all/ZB2GTBD%2FLWTrkOiO@xxxxxxxxxxxxxx/

Joel Fernandes (Google) (6):
mm/mremap: Optimize the start addresses in move_page_tables()
mm/mremap: Allow moves within the same VMA
selftests: mm: Fix failure case when new remap region was not found
selftests: mm: Add a test for mutually aligned moves > PMD size
selftests: mm: Add a test for remapping to area immediately after
existing mapping
selftests: mm: Add a test for remapping within a range

fs/exec.c | 2 +-
include/linux/mm.h | 2 +-
mm/mremap.c | 69 ++++++++++-
tools/testing/selftests/mm/mremap_test.c | 148 +++++++++++++++++++++--
4 files changed, 209 insertions(+), 12 deletions(-)