Re: [patch v2] mm, oom: fix concurrent munlock and oom reaper unmap

Tetsuo Handa
2018-04-18T11:49:17Z

Michal Hocko wrote:
> On Tue 17-04-18 19:52:41, David Rientjes wrote:
> > Since exit_mmap() is done without the protection of mm->mmap_sem, it is
> > possible for the oom reaper to concurrently operate on an mm until
> > MMF_OOM_SKIP is set.
> >
> > This allows munlock_vma_pages_all() to concurrently run while the oom
> > reaper is operating on a vma. Since munlock_vma_pages_range() depends on
> > clearing VM_LOCKED from vm_flags before actually doing the munlock to
> > determine if any other vmas are locking the same memory, the check for
> > VM_LOCKED in the oom reaper is racy.
> >
> > This is especially noticeable on architectures such as powerpc where
> > clearing a huge pmd requires serialize_against_pte_lookup(). If the pmd
> > is zapped by the oom reaper during follow_page_mask() after the check for
> > pmd_none() is bypassed, this ends up deferencing a NULL ptl.
> >
> > Fix this by reusing MMF_UNSTABLE to specify that an mm should not be
> > reaped. This prevents the concurrent munlock_vma_pages_range() and
> > unmap_page_range(). The oom reaper will simply not operate on an mm that
> > has the bit set and leave the unmapping to exit_mmap().
>
> This will further complicate the protocol and actually theoretically
> restores the oom lockup issues because the oom reaper doesn't set
> MMF_OOM_SKIP when racing with exit_mmap so we fully rely that nothing
> blocks there... So the resulting code is more fragile and tricky.
>
> Can we try a simpler way and get back to what I was suggesting before
> [1] and simply not play tricks with
> down_write(&mm->mmap_sem);
> up_write(&mm->mmap_sem);
>
> and use the write lock in exit_mmap for oom_victims?

You mean something like this?
Then, I'm tempted to call __oom_reap_task_mm() before holding mmap_sem for write.
It would be OK to call __oom_reap_task_mm() at the beginning of __mmput()...

diff --git a/mm/mmap.c b/mm/mmap.c
index 188f195..ba7083b 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3011,17 +3011,22 @@ void exit_mmap(struct mm_struct *mm)
struct mmu_gather tlb;
struct vm_area_struct *vma;
unsigned long nr_accounted = 0;
+ const bool is_oom_mm = mm_is_oom_victim(mm);

/* mm's last user has gone, and its about to be pulled down */
mmu_notifier_release(mm);

if (mm->locked_vm) {
+ if (is_oom_mm)
+ down_write(&mm->mmap_sem);
vma = mm->mmap;
while (vma) {
if (vma->vm_flags & VM_LOCKED)
munlock_vma_pages_all(vma);
vma = vma->vm_next;
}
+ if (is_oom_mm)
+ up_write(&mm->mmap_sem);
}

arch_exit_mmap(mm);
@@ -3037,7 +3042,7 @@ void exit_mmap(struct mm_struct *mm)
/* Use -1 here to ensure all VMAs in the mm are unmapped */
unmap_vmas(&tlb, vma, 0, -1);

- if (unlikely(mm_is_oom_victim(mm))) {
+ if (unlikely(is_oom_mm)) {
/*
* Wait for oom_reap_task() to stop working on this
* mm. Because MMF_OOM_SKIP is already set before