Re: [PATCH v2 1/1] mm: introduce mmap_lock_speculation_{start|end}
From: Suren Baghdasaryan
Date: Thu Sep 12 2024 - 18:25:12 EST
On Thu, Sep 12, 2024 at 3:20 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Sep 12, 2024 at 2:04 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >
> > On Thu, Sep 12, 2024 at 2:02 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> 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.
> >
> > Here you go. I hope I got the ordering right this time around, but I
> > would feel much better if Jann reviewed it before it's included in
> > your next patchset :)
>
> Thanks a lot! And yes, I'll give it a bit of time for reviews before
> sending a new revision.
>
> Did you by any chance get any new ideas for possible benchmarks
> (beyond what I did with will-it-scale)?
Hmm. You could use Mel Gorman's mmtests suite I guess.
>
>
> > Thanks,
> > Suren.
> >
> > >
> > > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > > ---
> > > Changes since v1 [1]:
> > > - Made memory barriers in inc_mm_lock_seq and mmap_lock_speculation_end
> > > more strict, per Jann Horn
> > >
> > > [1] https://lore.kernel.org/all/20240906051205.530219-2-andrii@xxxxxxxxxx/
> > >
> > > include/linux/mm_types.h | 3 ++
> > > include/linux/mmap_lock.h | 74 ++++++++++++++++++++++++++++++++-------
> > > kernel/fork.c | 3 --
> > > 3 files changed, 65 insertions(+), 15 deletions(-)
> > >
> > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > > index 6e3bdf8e38bc..5d8cdebd42bc 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..a281519d0c12 100644
> > > --- a/include/linux/mmap_lock.h
> > > +++ b/include/linux/mmap_lock.h
> > > @@ -71,39 +71,86 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> > > }
> > >
> > > #ifdef CONFIG_PER_VMA_LOCK
> > > +static inline void init_mm_lock_seq(struct mm_struct *mm)
> > > +{
> > > + mm->mm_lock_seq = 0;
> > > +}
> > > +
> > > /*
> > > - * 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.
> > > + * Increment mm->mm_lock_seq when mmap_lock is write-locked (ACQUIRE semantics)
> > > + * or write-unlocked (RELEASE semantics).
> > > */
> > > -static inline void vma_end_write_all(struct mm_struct *mm)
> > > +static inline void inc_mm_lock_seq(struct mm_struct *mm, bool acquire)
> > > {
> > > mmap_assert_write_locked(mm);
> > > /*
> > > * Nobody can concurrently modify mm->mm_lock_seq due to exclusive
> > > * mmap_lock being held.
> > > - * We need RELEASE semantics here to ensure that preceding stores into
> > > - * the VMA take effect before we unlock it with this store.
> > > - * Pairs with ACQUIRE semantics in vma_start_read().
> > > */
> > > - smp_store_release(&mm->mm_lock_seq, mm->mm_lock_seq + 1);
> > > +
> > > + if (acquire) {
> > > + WRITE_ONCE(mm->mm_lock_seq, mm->mm_lock_seq + 1);
> > > + /*
> > > + * For ACQUIRE semantics we should ensure no following stores are
> > > + * reordered to appear before the mm->mm_lock_seq modification.
> > > + */
> > > + smp_wmb();
> > > + } else {
> > > + /*
> > > + * We need RELEASE semantics here to ensure that preceding stores
> > > + * into the VMA take effect before we unlock it with this store.
> > > + * Pairs with ACQUIRE semantics in vma_start_read().
> > > + */
> > > + 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 ACQUIRE semantics in inc_mm_lock_seq(). */
> > > + smp_rmb();
> > > + return seq == READ_ONCE(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, bool acquire) {}
> > > +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, false);
> > > +}
> > > +
> > > 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, true);
> > > __mmap_lock_trace_acquire_returned(mm, true, true);
> > > }
> > >
> > > @@ -111,6 +158,7 @@ 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, true);
> > > __mmap_lock_trace_acquire_returned(mm, true, true);
> > > }
> > >
> > > @@ -120,6 +168,8 @@ 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, true);
> > > __mmap_lock_trace_acquire_returned(mm, true, ret == 0);
> > > return ret;
> > > }
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > index 61070248a7d3..c86e87ed172b 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: 015bdfcb183759674ba1bd732c3393014e35708b
> > > --
> > > 2.46.0.662.g92d0881bb0-goog
> > >