[PATCH v4 0/7] Optimize mremap during mutual alignment within PMD

[Joel Fernandes (Google)]

Hello!

Here is v4 of the mremap start address optimization / fix for exec warning.  It
took me a while to write a test that catches the issue me/Linus discussed in
the last version. And I verified kernel crashes without the check. See below.

The main changes in this series is:
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.

    |DDDDddddSSSSssss|

    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.

History of patches
==================
v3->v4:
1. Make sure to check address to align is beginning of VMA
2. Add test to check this (test fails with a kernel crash if we don't do this).

v2->v3:
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.

v1->v2:
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.

thanks,

 - Joel

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

Joel Fernandes (Google) (7):
mm/mremap: Optimize the start addresses in move_page_tables()
mm/mremap: Allow moves within the same VMA for stack
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
selftests: mm: Add a test for moving from an offset from start of
mapping

fs/exec.c                                |   2 +-
include/linux/mm.h                       |   2 +-
mm/mremap.c                              |  63 ++++-
tools/testing/selftests/mm/mremap_test.c | 301 +++++++++++++++++++----
4 files changed, 319 insertions(+), 49 deletions(-)

--
2.41.0.rc2.161.g9c6817b8e7-goog