Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap
From: Michal Hocko <mhocko@suse.com>
Date: 2021-10-22 08:03:38
Also in:
linux-mm, lkml
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@dhcp22.suse.cz That being said Acked-by: Michal Hocko <mhocko@suse.com> Thanks!
quoted hunk ↗ jump to hunk
Fixes: 884a7e5964e0 ("mm: introduce process_mrelease system call") Signed-off-by: Suren Baghdasaryan <surenb@google.com> --- 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