On Thu 21-10-21 18:46:58, Suren Baghdasaryan wrote: > Race between process_mrelease and exit_mmap, where free_pgtables is > called while __oom_reap_task_mm is in progress, leads to kernel crash > during pte_offset_map_lock call. oom-reaper avoids this race by setting > MMF_OOM_VICTIM flag and causing exit_mmap to take and release > mmap_write_lock, blocking it until oom-reaper releases mmap_read_lock. > Reusing MMF_OOM_VICTIM for process_mrelease would be the simplest way to > fix this race, however that would be considered a hack. Fix this race > by elevating mm->mm_users and preventing exit_mmap from executing until > process_mrelease is finished. Patch slightly refactors the code to adapt > for a possible mmget_not_zero failure. > This fix has considerable negative impact on process_mrelease performance > and will likely need later optimization. I am not sure there is any promise that process_mrelease will run in parallel with the exiting process. In fact the primary purpose of this syscall is to provide a reliable way to oom kill from user space. If you want to optimize process exit resp. its exit_mmap part then you should be using other means. So I would be careful calling this a regression. I do agree that taking the reference count is the right approach here. I was wrong previously [1] when saying that pinning the mm struct is sufficient. I have completely forgot about the subtle sync in exit_mmap. One way we can approach that would be to take exclusive mmap_sem throughout the exit_mmap unconditionally. There was a push back against that though so arguments would have to be re-evaluated. [1] http://lkml.kernel.org/r/YQzZqFwDP7eUxwcn@xxxxxxxxxxxxxx That being said Acked-by: Michal Hocko <mhocko@xxxxxxxx> Thanks! > Fixes: 884a7e5964e0 ("mm: introduce process_mrelease system call") > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > --- > mm/oom_kill.c | 23 ++++++++++++----------- > 1 file changed, 12 insertions(+), 11 deletions(-) > > diff --git a/mm/oom_kill.c b/mm/oom_kill.c > index 831340e7ad8b..989f35a2bbb1 100644 > --- a/mm/oom_kill.c > +++ b/mm/oom_kill.c > @@ -1150,7 +1150,7 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags) > struct task_struct *task; > struct task_struct *p; > unsigned int f_flags; > - bool reap = true; > + bool reap = false; > struct pid *pid; > long ret = 0; > > @@ -1177,15 +1177,15 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags) > goto put_task; > } > > - mm = p->mm; > - mmgrab(mm); > - > - /* If the work has been done already, just exit with success */ > - if (test_bit(MMF_OOM_SKIP, &mm->flags)) > - reap = false; > - else if (!task_will_free_mem(p)) { > - reap = false; > - ret = -EINVAL; > + if (mmget_not_zero(p->mm)) { > + mm = p->mm; > + if (task_will_free_mem(p)) > + reap = true; > + else { > + /* Error only if the work has not been done already */ > + if (!test_bit(MMF_OOM_SKIP, &mm->flags)) > + ret = -EINVAL; > + } > } > task_unlock(p); > > @@ -1201,7 +1201,8 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags) > mmap_read_unlock(mm); > > drop_mm: > - mmdrop(mm); > + if (mm) > + mmput(mm); > put_task: > put_task_struct(task); > put_pid: > -- > 2.33.0.1079.g6e70778dc9-goog -- Michal Hocko SUSE Labs
