Re: [PATCH v1 3/3] mm: process_mrelease: introduce PROCESS_MRELEASE_REAP_KILL flag

[Michal Hocko <mhocko@suse.com>]

On Tue 21-04-26 16:02:39, Minchan Kim wrote:
> Currently, process_mrelease() requires userspace to send a SIGKILL signal
> prior to the call. This separation introduces a scheduling race window
> where the victim task may receive the signal and enter the exit path
> before the reaper can invoke process_mrelease().
> 
> When the victim enters the exit path (do_exit -> exit_mm), it clears its
> task->mm immediately. This causes process_mrelease() to fail with -ESRCH,
> leaving the actual address space teardown (exit_mmap) to be deferred until
> the mm's reference count drops to zero. In Android, arbitrary reference counts
> (e.g., async I/O, reading /proc/<pid>/cmdline, or various other remote
> VM accesses) frequently delay this teardown indefinitely, defeating the
> purpose of expedited reclamation.
> 
> This delay keeps memory pressure high, forcing the system to unnecessarily
> kill additional innocent background apps before the memory from the first
> victim is recovered.

Thanks, this makes the motivation much more clear and usecase very
sound.

> This patch introduces the PROCESS_MRELEASE_REAP_KILL UAPI flag to support
> an integrated auto-kill mode. When specified, process_mrelease() directly
> injects a SIGKILL into the target task.
> 
> To solve the race condition deterministically, we grab the mm reference
> via mmget() and set the MMF_UNSTABLE flag *before* sending the SIGKILL.
> Using mmget() instead of mmgrab() keeps mm_users > 0, preventing the
> victim from calling exit_mmap() in its own exit path.

Why is this needed? Address space tear down is an operation that can run
from several execution contexts.

> This ensures that
> the memory is reclaimed synchronously and deterministically by the reaper
> in the context of process_mrelease(), avoiding delays caused by
> non-deterministic scheduling of the victim task.

The memory is still reclaimed synchronously from the mrelease context.
This is really confusing.

Please also explain why do you need to do all that ugly
task_will_free_mem hoops. Why cannot you simply kill the task if
task_will_free_mem fails (if PROCESS_MRELEASE_REAP_KILL is used).
-- 
Michal Hocko
SUSE Labs