On Wed, Jun 30, 2021 at 5:18 PM Shakeel Butt [off-list ref] wrote:

On Mon, Jun 21, 2021 at 1:31 AM SeongJae Park [off-list ref] wrote:

quoted

From: SeongJae Park <redacted>

This commit introduces a reference implementation of the address space
specific low level primitives for the virtual address space, so that
users of DAMON can easily monitor the data accesses on virtual address
spaces of specific processes by simply configuring the implementation to
be used by DAMON.

The low level primitives for the fundamental access monitoring are
defined in two parts:

1. Identification of the monitoring target address range for the address
   space.
2. Access check of specific address range in the target space.

The reference implementation for the virtual address space does the
works as below.

PTE Accessed-bit Based Access Check
-----------------------------------

The implementation uses PTE Accessed-bit for basic access checks.  That
is, it clears the bit for the next sampling target page and checks
whether it is set again after one sampling period.  This could disturb
the reclaim logic.  DAMON uses ``PG_idle`` and ``PG_young`` page flags
to solve the conflict, as Idle page tracking does.

VMA-based Target Address Range Construction
-------------------------------------------

Only small parts in the super-huge virtual address space of the
processes are mapped to physical memory and accessed.  Thus, tracking
the unmapped address regions is just wasteful.  However, because DAMON
can deal with some level of noise using the adaptive regions adjustment
mechanism, tracking every mapping is not strictly required but could
even incur a high overhead in some cases.  That said, too huge unmapped
areas inside the monitoring target should be removed to not take the
time for the adaptive mechanism.

For the reason, this implementation converts the complex mappings to
three distinct regions that cover every mapped area of the address
space.  Also, the two gaps between the three regions are the two biggest
unmapped areas in the given address space.  The two biggest unmapped
areas would be the gap between the heap and the uppermost mmap()-ed
region, and the gap between the lowermost mmap()-ed region and the stack
in most of the cases.  Because these gaps are exceptionally huge in
usual address spaces, excluding these will be sufficient to make a
reasonable trade-off.  Below shows this in detail::

    <heap>
    <BIG UNMAPPED REGION 1>
    <uppermost mmap()-ed region>
    (small mmap()-ed regions and munmap()-ed regions)
    <lowermost mmap()-ed region>
    <BIG UNMAPPED REGION 2>
    <stack>

Signed-off-by: SeongJae Park <redacted>
Reviewed-by: Leonard Foerster <redacted>
Reviewed-by: Fernand Sieber <redacted>

Acked-by: Shakeel Butt <redacted>

Ok that was by mistake. The ACK is for v32.