[RFC 0/6]mm: add new LRU list for MADV_FREE pages

From: Shaohua Li
Date: Mon Jan 30 2017 - 00:52:10 EST


We are trying to use MADV_FREE in jemalloc. Several issues are found. Without
solving the issues, jemalloc can't use the MADV_FREE feature.
- Doesn't support system without swap enabled. Because if swap is off, we can't
or can't efficiently age anonymous pages. And since MADV_FREE pages are mixed
with other anonymous pages, we can't reclaim MADV_FREE pages. In current
implementation, MADV_FREE will fallback to MADV_DONTNEED without swap enabled.
But in our environment, a lot of machines don't enable swap. This will prevent
our setup using MADV_FREE.
- Increases memory pressure. page reclaim bias file pages reclaim against
anonymous pages. This doesn't make sense for MADV_FREE pages, because those
pages could be freed easily and refilled with very slight penality. Even page
reclaim doesn't bias file pages, there is still an issue, because MADV_FREE
pages and other anonymous pages are mixed together. To reclaim a MADV_FREE
page, we probably must scan a lot of other anonymous pages, which is
inefficient. In our test, we usually see oom with MADV_FREE enabled and nothing
without it.
- RSS accounting. MADV_FREE pages are accounted as normal anon pages and
reclaimed lazily, so application's RSS becomes bigger. This confuses our
workloads. We have monitoring daemon running and if it finds applications' RSS
becomes abnormal, the daemon will kill the applications even kernel can reclaim
the memory easily. Currently we don't export separate RSS accounting for
MADV_FREE pages. This will prevent our setup using MADV_FREE too.

For the first two issues, introducing a new LRU list for MADV_FREE pages could
solve the issues. We can directly reclaim MADV_FREE pages without writting them
out to swap, so the first issue could be fixed. If only MADV_FREE pages are in
the new list, page reclaim can easily reclaim such pages without interference
of file or anonymous pages. The memory pressure issue will disappear.

Actually Minchan posted patches to add the LRU list before, but he didn't
pursue. So I picked up them and the patches are based on Minchan's previous
patches. The main difference between my patches and Minchan previous patches is
page reclaim policy. Minchan's patches introduces a knob to balance the reclaim
of MADV_FREE pages and anon/file pages, while the patches always reclaim
MADV_FREE pages first if there are. I described the reason in patch 5.

For the third issue, we can add a separate RSS count for MADV_FREE pages. The
count will be increased in madvise syscall and decreased in page reclaim (eg,
unmap). One issue is activate_page(). A MADV_FREE page can be promoted to
active page there. But there isn't mm_struct context at that place. Iterating
vma there sounds too silly. The patchset don't fix this issue yet. Hopefully
somebody can share a hint how to fix this issue.


Minchan previous patches:

Shaohua Li (6):
mm: add wrap for page accouting index
mm: add lazyfree page flag
mm: add LRU_LAZYFREE lru list
mm: move MADV_FREE pages into LRU_LAZYFREE list
mm: reclaim lazyfree pages
mm: enable MADV_FREE for swapless system

drivers/base/node.c | 2 +
drivers/staging/android/lowmemorykiller.c | 3 +-
fs/proc/meminfo.c | 1 +
fs/proc/task_mmu.c | 8 ++-
include/linux/mm_inline.h | 41 +++++++++++++
include/linux/mmzone.h | 9 +++
include/linux/page-flags.h | 6 ++
include/linux/swap.h | 2 +-
include/linux/vm_event_item.h | 2 +-
include/trace/events/mmflags.h | 1 +
include/trace/events/vmscan.h | 31 +++++-----
kernel/power/snapshot.c | 1 +
mm/compaction.c | 11 ++--
mm/huge_memory.c | 6 +-
mm/khugepaged.c | 6 +-
mm/madvise.c | 11 +---
mm/memcontrol.c | 4 ++
mm/memory-failure.c | 3 +-
mm/memory_hotplug.c | 3 +-
mm/mempolicy.c | 3 +-
mm/migrate.c | 29 ++++------
mm/page_alloc.c | 10 ++++
mm/rmap.c | 7 ++-
mm/swap.c | 51 +++++++++-------
mm/vmscan.c | 96 +++++++++++++++++++++++--------
mm/vmstat.c | 4 ++
26 files changed, 242 insertions(+), 109 deletions(-)