[00/12] mm: userspace hugepage collapse

Message

Zach O'Keefe April 10, 2022, 1:54 p.m. UTC

Introduction
--------------------------------

This series provides a mechanism for userspace to induce a collapse of
eligible ranges of memory into transparent hugepages in process context,
thus permitting users to more tightly control their own hugepage
utilization policy at their own expense.

This idea was introduced by David Rientjes[1], and the semantics and
implementation were introduced and discussed in a previous PATCH RFC[2].

Interface
--------------------------------

The proposed interface adds a new madvise(2) mode, MADV_COLLAPSE, and
leverages the new process_madvise(2) call.

(*) process_madvise(2)

	Performs a synchronous collapse of the native pages
	mapped by the list of iovecs into transparent hugepages.

	Allocation semantics are the same as khugepaged, and depend on
	(1) the active sysfs settings
	/sys/kernel/mm/transparent_hugepage/enabled and
	/sys/kernel/mm/transparent_hugepage/khugepaged/defrag, and (2)
	the VMA flags of the memory range being collapsed.

	Collapse eligibility criteria differs from khugepaged in that
	the sysfs files
	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_[none|swap|shared]
	are ignored.

	When a range spans multiple hugepage-aligned/sized regions, the
	semantics of the collapse of each region is independent from the
	others.

	Caller must have CAP_SYS_ADMIN if not acting on self.

	Return value follows existing process_madvise(2) conventions.  A
	“success” indicates that all hugepage-sized/aligned regions
	covered by the provided range were either successfully
	collapsed, or were already pmd-mapped THPs.

(*) madvise(2)

	Equivalent to process_madvise(2) on self, with 0 returned on
	“success”.

Future work
--------------------------------

Only private anonymous memory is supported by this series. File and
shmem memory support will be added later.

One possible user of this functionality is a userspace agent that
attempts to optimize THP utilization system-wide by allocating THPs
based on, for example, task priority, task performance requirements, or
heatmaps.  For the latter, one idea that has already surfaced is using
DAMON to identify hot regions, and driving THP collapse through a new
DAMOS_COLLAPSE scheme[3].

Sequence of Patches
--------------------------------

Patches 1-4 perform refactoring of collapse logic within khugepaged.c
and introduce the notion of a collapse context.

Patches 5-9 introduces MADV_COLLAPSE, does some renaming, adds support
so that MADV_COLLAPSE context has the eligibility and allocation
semantics referenced above, and adds process_madivse(2) support.

Patches 10-12 add selftests to test the new functionality.

Applies against next-20220408.

[1] https://lore.kernel.org/all/[e-mail]/
[2] https://lore.kernel.org/linux-mm/[e-mail]/
[3] https://lore.kernel.org/lkml/[e-mail]/T/

Zach O'Keefe (13):
  mm/khugepaged: separate hugepage preallocation and cleanup
  mm/khugepaged: record SCAN_PMD_MAPPED when scan_pmd() finds THP
  mm/khugepaged: add struct collapse_control
  mm/khugepaged: make hugepage allocation context-specific
  mm/khugepaged: add struct collapse_result
  mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse
  mm/khugepaged: remove khugepaged prefix from shared collapse functions
  mm/khugepaged: add flag to ignore khugepaged_max_ptes_*
  mm/khugepaged: add flag to ignore page young/referenced requirement
  mm/madvise: add MADV_COLLAPSE to process_madvise()
  selftests/vm: modularize collapse selftests
  selftests/vm: add MADV_COLLAPSE collapse context to selftests
  selftests/vm: add test to verify recollapse of THPs

 include/linux/huge_mm.h                 |  12 +
 include/trace/events/huge_memory.h      |   5 +-
 include/uapi/asm-generic/mman-common.h  |   2 +
 mm/internal.h                           |   1 +
 mm/khugepaged.c                         | 598 ++++++++++++++++--------
 mm/madvise.c                            |  11 +-
 mm/rmap.c                               |  15 +-
 tools/testing/selftests/vm/khugepaged.c | 417 +++++++++++------
 8 files changed, 702 insertions(+), 359 deletions(-)