Re: [PATCH] mm/memcg: reorder retry checks for clarity in try_charge_memcg

[Dipendra Khadka <kdipendra88@gmail.com>]

Hi Johannes,

Thank you for the feedback. Let me clarify the scenario this patch addresses.

The task_is_dying() check in try_charge_memcg() identifies when the
CURRENT task (the caller) is the OOM victim - not when some other 
process was killed.

Two scenarios:

1. Normal allocator triggers OOM:
  - Process A allocates → triggers OOM
  - Process B is killed (victim)
  - Process A continues with reset retries - task_is_dying() = false for A
  → Unchanged by my patch

2. Victim tries to allocate:
 - Process B (victim, TIF_MEMDIE set) tries to allocate
  - task_is_dying() = true
  - Current code: wastes retries on hopeless reclaims
  - My patch: exits immediately
  → Optimization for this case

The victim has three safety mechanisms that make the retries unnecessary:
1. oom_reaper proactively frees its memory
2. __alloc_pages_slowpath() grants reserves via oom_reserves_allowed()
3. Critical allocations with __GFP_NOFAIL still reach force: label

The retry loop for a dying victim is futile because:
- Reclaim won't help (victim is being killed to free memory!)
- Victim will exit regardless
- Just wastes CPU cycles

Would you like me to provide evidence showing the unnecessary retries,
or run specific tests to verify the safety mechanisms are sufficient?

Best Regards,
Dipendra