Re: [RFC 1/1] mm: introduce mmap_lock_speculation_{start|end}

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Aug 8, 2024 at 2:02 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> On Thu, Aug 8, 2024 at 8:19 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Wed, Aug 7, 2024 at 11:23 AM Suren Baghdasaryan <surenb@google.com> wrote:
> > >
> > > Add helper functions to speculatively perform operations without
> > > read-locking mmap_lock, expecting that mmap_lock will not be
> > > write-locked and mm is not modified from under us.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > Suggested-by: Peter Zijlstra <peterz@infradead.org>
> > > Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > > ---
> >
> > This change makes sense and makes mm's seq a bit more useful and
> > meaningful. I've also tested it locally with uprobe stress-test, and
> > it seems to work great, I haven't run into any problems with a
> > multi-hour stress test run so far. Thanks!
>
> Thanks for testing and feel free to include this patch into your set.

Will do!

>
> I've been thinking about this some more and there is a very unlikely
> corner case if between mmap_lock_speculation_start() and
> mmap_lock_speculation_end() mmap_lock is write-locked/unlocked so many
> times that mm->mm_lock_seq (int) overflows and just happen to reach
> the same value as we recorded in mmap_lock_speculation_start(). This
> would generate a false positive, which would show up as if the
> mmap_lock was never touched. Such overflows are possible for vm_lock
> as well (see: https://elixir.bootlin.com/linux/v6.10.3/source/include/linux/mm_types.h#L688)
> but they are not critical because a false result would simply lead to
> a retry under mmap_lock. However for your case this would be a
> critical issue. This is an extremely low probability scenario but
> should we still try to handle it?
>

No, I think it's fine. Similar problems could happen with refcount_t,
for instance (it has a logic to have a sticky "has overflown" state,
which I believe relies on the fact that we'll never be able to
increment refcount 2bln+ times in between some resetting logic).
Anyways, I think it's utterly unrealistic and should be considered
impossible.



> I'm CC'ing several mm folks and Jann Horn to chime in.
>
> >
> > Acked-by: Andrii Nakryiko <andrii@kernel.org>
> >
> > > Discussion [1] follow-up. If proves to be useful can be included in that
> > > patchset. Based on mm-unstable.
> > >
> > > [1] https://lore.kernel.org/all/20240730134605.GO33588@noisy.programming.kicks-ass.net/
> > >
> > >  include/linux/mm_types.h  |  3 +++
> > >  include/linux/mmap_lock.h | 53 +++++++++++++++++++++++++++++++--------
> > >  kernel/fork.c             |  3 ---
> > >  3 files changed, 46 insertions(+), 13 deletions(-)
> > >
> > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > > index 003619fab20e..a426e6ced604 100644
> > > --- a/include/linux/mm_types.h
> > > +++ b/include/linux/mm_types.h
> > > @@ -887,6 +887,9 @@ struct mm_struct {
> > >                  * Roughly speaking, incrementing the sequence number is
> > >                  * equivalent to releasing locks on VMAs; reading the sequence
> > >                  * number can be part of taking a read lock on a VMA.
> > > +                * Incremented every time mmap_lock is write-locked/unlocked.
> > > +                * Initialized to 0, therefore odd values indicate mmap_lock
> > > +                * is write-locked and even values that it's released.
> > >                  *
> > >                  * Can be modified under write mmap_lock using RELEASE
> > >                  * semantics.
> > > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> > > index de9dc20b01ba..5410ce741d75 100644
> > > --- a/include/linux/mmap_lock.h
> > > +++ b/include/linux/mmap_lock.h
> > > @@ -71,15 +71,12 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> > >  }
> > >
> > >  #ifdef CONFIG_PER_VMA_LOCK
> > > -/*
> > > - * Drop all currently-held per-VMA locks.
> > > - * This is called from the mmap_lock implementation directly before releasing
> > > - * a write-locked mmap_lock (or downgrading it to read-locked).
> > > - * This should normally NOT be called manually from other places.
> > > - * If you want to call this manually anyway, keep in mind that this will release
> > > - * *all* VMA write locks, including ones from further up the stack.
> > > - */
> > > -static inline void vma_end_write_all(struct mm_struct *mm)
> > > +static inline void init_mm_lock_seq(struct mm_struct *mm)
> > > +{
> > > +       mm->mm_lock_seq = 0;
> > > +}
> > > +
> > > +static inline void inc_mm_lock_seq(struct mm_struct *mm)
> > >  {
> > >         mmap_assert_write_locked(mm);
> > >         /*
> > > @@ -91,19 +88,52 @@ static inline void vma_end_write_all(struct mm_struct *mm)
> > >          */
> > >         smp_store_release(&mm->mm_lock_seq, mm->mm_lock_seq + 1);
> > >  }
> > > +
> > > +static inline bool mmap_lock_speculation_start(struct mm_struct *mm, int *seq)
> > > +{
> > > +       /* Pairs with RELEASE semantics in inc_mm_lock_seq(). */
> > > +       *seq = smp_load_acquire(&mm->mm_lock_seq);
> > > +       /* Allow speculation if mmap_lock is not write-locked */
> > > +       return (*seq & 1) == 0;
> > > +}
> > > +
> > > +static inline bool mmap_lock_speculation_end(struct mm_struct *mm, int seq)
> > > +{
> > > +       /* Pairs with RELEASE semantics in inc_mm_lock_seq(). */
> > > +       return seq == smp_load_acquire(&mm->mm_lock_seq);
> > > +}
> > > +
> > >  #else
> > > -static inline void vma_end_write_all(struct mm_struct *mm) {}
> > > +static inline void init_mm_lock_seq(struct mm_struct *mm) {}
> > > +static inline void inc_mm_lock_seq(struct mm_struct *mm) {}
> > > +static inline bool mmap_lock_speculation_start(struct mm_struct *mm, int *seq) { return false; }
> > > +static inline bool mmap_lock_speculation_end(struct mm_struct *mm, int seq) { return false; }
> > >  #endif
> > >
> > > +/*
> > > + * Drop all currently-held per-VMA locks.
> > > + * This is called from the mmap_lock implementation directly before releasing
> > > + * a write-locked mmap_lock (or downgrading it to read-locked).
> > > + * This should normally NOT be called manually from other places.
> > > + * If you want to call this manually anyway, keep in mind that this will release
> > > + * *all* VMA write locks, including ones from further up the stack.
> > > + */
> > > +static inline void vma_end_write_all(struct mm_struct *mm)
> > > +{
> > > +       inc_mm_lock_seq(mm);
> > > +}
> > > +
> > >  static inline void mmap_init_lock(struct mm_struct *mm)
> > >  {
> > >         init_rwsem(&mm->mmap_lock);
> > > +       init_mm_lock_seq(mm);
> > >  }
> > >
> > >  static inline void mmap_write_lock(struct mm_struct *mm)
> > >  {
> > >         __mmap_lock_trace_start_locking(mm, true);
> > >         down_write(&mm->mmap_lock);
> > > +       inc_mm_lock_seq(mm);
> > >         __mmap_lock_trace_acquire_returned(mm, true, true);
> > >  }
> > >
> > > @@ -111,6 +141,7 @@ static inline void mmap_write_lock_nested(struct mm_struct *mm, int subclass)
> > >  {
> > >         __mmap_lock_trace_start_locking(mm, true);
> > >         down_write_nested(&mm->mmap_lock, subclass);
> > > +       inc_mm_lock_seq(mm);
> > >         __mmap_lock_trace_acquire_returned(mm, true, true);
> > >  }
> > >
> > > @@ -120,6 +151,8 @@ static inline int mmap_write_lock_killable(struct mm_struct *mm)
> > >
> > >         __mmap_lock_trace_start_locking(mm, true);
> > >         ret = down_write_killable(&mm->mmap_lock);
> > > +       if (!ret)
> > > +               inc_mm_lock_seq(mm);
> > >         __mmap_lock_trace_acquire_returned(mm, true, ret == 0);
> > >         return ret;
> > >  }
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > index 3d590e51ce84..73e37af8a24d 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -1259,9 +1259,6 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
> > >         seqcount_init(&mm->write_protect_seq);
> > >         mmap_init_lock(mm);
> > >         INIT_LIST_HEAD(&mm->mmlist);
> > > -#ifdef CONFIG_PER_VMA_LOCK
> > > -       mm->mm_lock_seq = 0;
> > > -#endif
> > >         mm_pgtables_bytes_init(mm);
> > >         mm->map_count = 0;
> > >         mm->locked_vm = 0;
> > >
> > > base-commit: 98808d08fc0f78ee638e0c0a88020fbbaf581ec6
> > > --
> > > 2.46.0.rc2.264.g509ed76dc8-goog
> > >