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On 9/26/2025 2:00 PM, Barry Song wrote: >> 2) __oom_reap_task_mm() >> It holds the mmap read lock when setting the MMF_UNSTABLE as far as I can tell. >> So checking the MMF_UNSTABLE with another __oom_reap_task_mm() does >> not exclude each other. >> This is more of a question for oom reaping. >> Does MMF_UNSTABLE have the test vs set racing here? It seems this >> check does not protect against __oom_reap_task_mm(). I have no idea >> if this race is triggerable. Just want someone else to double check if >> my understanding is correct or not. > I haven’t actually run the code. > My guess is there’s a race when checking MMF_UNSTABLE against the > OOM reaper. I think it’s fine either way—whether we skip an OOM-reaped > mm upfront or take a middle path—since the OOM reaper will handle those > PTEs with the PTL just like unuse_pte() does and eventually free the mm > of the reaped process. It’s probably better to skip it early and avoid > unnecessary unuse_pte() calls. > I am sorry that I can't really see any stability issue b/n oom reaper and unuse_mm(), but yes that unnecessary unuse_pte() calls, as Barry mentioned, after reaping. >> I can see this patch does protect the intended race in dup_mm() vs >> unuse_mm(), it adds value. > This also seems to add values for OOM-reaped processes to avoid a > useless unuse(), in case we aren’t skipping this mm right now. I’m > not sure if we’ve been skipping OOM-reaped processes elsewhere. > I don't see any explicit flags that tells the process is already oom-reaped/under it. There is MMF_OOM_REAP_QUEUED, but this doesn't tell if it is already reaped. If the unnecessary calls to unuse_vma() is really of a concern, then check the MMF_UNSTABLE while traversing VMA may be a solution but this looks ugly. > Hi Charan, do you have any observations on this? If an additional value is > added, could we record it in the changelog? Otherwise, can we add some > description in the changelog to address Chris’ concern? I do see that Chris ask can go as completely different change as the mentioned problem exist even before this change, please CMIW. Thanks, Charan