On Fri, Oct 22, 2021 at 1:03 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > 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. I agree, that would probably be the cleanest way. > There was a push back against > that though so arguments would have to be re-evaluated. I'll review that discussion to better understand the reasons for the push back. Thanks for the link. > > [1] http://lkml.kernel.org/r/YQzZqFwDP7eUxwcn@xxxxxxxxxxxxxx > > That being said > Acked-by: Michal Hocko <mhocko@xxxxxxxx> Thanks! > > 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
