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)? > 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 > >
