mm: add vma_has_recency()

Add vma_has_recency() to indicate whether a VMA may exhibit temporal
locality that the LRU algorithm relies on.

This function returns false for VMAs marked by VM_SEQ_READ or
VM_RAND_READ.  While the former flag indicates linear access, i.e., a
special case of spatial locality, both flags indicate a lack of temporal
locality, i.e., the reuse of an area within a relatively small duration.

"Recency" is chosen over "locality" to avoid confusion between temporal
and spatial localities.

Before this patch, the active/inactive LRU only ignored the accessed bit
from VMAs marked by VM_SEQ_READ.  After this patch, the active/inactive
LRU and MGLRU share the same logic: they both ignore the accessed bit if
vma_has_recency() returns false.

For the active/inactive LRU, the following fio test showed a [6, 8]%
increase in IOPS when randomly accessing mapped files under memory
pressure.

  kb=$(awk '/MemTotal/ { print $2 }' /proc/meminfo)
  kb=$((kb - 8*1024*1024))

  modprobe brd rd_nr=1 rd_size=$kb
  dd if=/dev/zero of=/dev/ram0 bs=1M

  mkfs.ext4 /dev/ram0
  mount /dev/ram0 /mnt/
  swapoff -a

  fio --name=test --directory=/mnt/ --ioengine=mmap --numjobs=8 \
      --size=8G --rw=randrw --time_based --runtime=10m \
      --group_reporting

The discussion that led to this patch is here [1].  Additional test
results are available in that thread.

[1] https://lore.kernel.org/r/Y31s%2FK8T85jh05wH@google.com/

Link: https://lkml.kernel.org/r/20221230215252.2628425-1-yuzhao@google.com
Signed-off-by: Yu Zhao <yuzhao@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Righi <andrea.righi@canonical.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michael Larabel <Michael@MichaelLarabel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions