The OOM reaper makes it possible to immediately release anonymous memory
from a dying process in order to free up memory faster. This provides
immediate relief under heavy memory pressure instead of waiting for victim
processes to naturally release their memory.
Utilize the OOM reaper by creating another kthread in Simple LMK to perform
victim reaping. Similar to the OOM reaper kthread (which is unused with
Simple LMK), this new kthread allows reaping to race with exit_mmap() in
order to preclude the need to take a reference to an mm's address space and
thus potentially mmput() an mm's last reference. Doing so would stall the
reaper kthread, preventing it from being able to quickly reap new victims.
Reaping is done on victims one at a time by descending order of anonymous
pages, so that the most promising victims with the most anonymous pages
are reaped first. Victims are also marked for reaping via MMF_OOM_VICTIM so
that they reap themselves first in exit_mmap(). Even if a victim isn't
reaped by the reaper thread, it'll free its anonymous memory first thing in
exit_mmap() as a small win towards making memory available sooner.
By relieving memory pressure faster via reaping, Simple LMK not only
doesn't need to kill as many processes, but also improves system
responsiveness when memory is low since memory pressure is relieved sooner.
Although not strictly required, Simple LMK should be the only one utilizing
the OOM reaper. Any other code that may utilize the OOM reaper, such as
patches that invoke the OOM reaper for all SIGKILLs, should be disabled.
Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
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