On Mon 09-01-23 12:53:07, Suren Baghdasaryan wrote:
> Introduce a per-VMA rw_semaphore to be used during page fault handling
> instead of mmap_lock. Because there are cases when multiple VMAs need
> to be exclusively locked during VMA tree modifications, instead of the
> usual lock/unlock patter we mark a VMA as locked by taking per-VMA lock
> exclusively and setting vma->lock_seq to the current mm->lock_seq. When
> mmap_write_lock holder is done with all modifications and drops mmap_lock,
> it will increment mm->lock_seq, effectively unlocking all VMAs marked as
> locked.
I have to say I was struggling a bit with the above and only understood
what you mean by reading the patch several times. I would phrase it like
this (feel free to use if you consider this to be an improvement).
Introduce a per-VMA rw_semaphore. The lock implementation relies on a
per-vma and per-mm sequence counters to note exclusive locking:
- read lock - (implemented by vma_read_trylock) requires the the
vma (vm_lock_seq) and mm (mm_lock_seq) sequence counters to
differ. If they match then there must be a vma exclusive lock
held somewhere.
- read unlock - (implemented by vma_read_unlock) is a trivial
vma->lock unlock.
- write lock - (vma_write_lock) requires the mmap_lock to be
held exclusively and the current mm counter is noted to the vma
side. This will allow multiple vmas to be locked under a single
mmap_lock write lock (e.g. during vma merging). The vma counter
is modified under exclusive vma lock.
- write unlock - (vma_write_unlock_mm) is a batch release of all
vma locks held. It doesn't pair with a specific
vma_write_lock! It is done before exclusive mmap_lock is
released by incrementing mm sequence counter (mm_lock_seq).
- write downgrade - if the mmap_lock is downgraded to the read
lock all vma write locks are released as well (effectivelly
same as write unlock).
> VMA lock is placed on the cache line boundary so that its 'count' field
> falls into the first cache line while the rest of the fields fall into
> the second cache line. This lets the 'count' field to be cached with
> other frequently accessed fields and used quickly in uncontended case
> while 'owner' and other fields used in the contended case will not
> invalidate the first cache line while waiting on the lock.
>
> Signed-off-by: Suren Baghdasaryan <sur...@google.com>
> ---
> include/linux/mm.h | 80 +++++++++++++++++++++++++++++++++++++++
> include/linux/mm_types.h | 8 ++++
> include/linux/mmap_lock.h | 13 +++++++
> kernel/fork.c | 4 ++
> mm/init-mm.c | 3 ++
> 5 files changed, 108 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f3f196e4d66d..ec2c4c227d51 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -612,6 +612,85 @@ struct vm_operations_struct {
> unsigned long addr);
> };
>
> +#ifdef CONFIG_PER_VMA_LOCK
> +static inline void vma_init_lock(struct vm_area_struct *vma)
> +{
> + init_rwsem(&vma->lock);
> + vma->vm_lock_seq = -1;
> +}
> +
> +static inline void vma_write_lock(struct vm_area_struct *vma)
> +{
> + int mm_lock_seq;
> +
> + mmap_assert_write_locked(vma->vm_mm);
> +
> + /*
> + * current task is holding mmap_write_lock, both vma->vm_lock_seq and
> + * mm->mm_lock_seq can't be concurrently modified.
> + */
> + mm_lock_seq = READ_ONCE(vma->vm_mm->mm_lock_seq);
> + if (vma->vm_lock_seq == mm_lock_seq)
> + return;
> +
> + down_write(&vma->lock);
> + vma->vm_lock_seq = mm_lock_seq;
> + up_write(&vma->lock);
> +}
> +
> +/*
> + * Try to read-lock a vma. The function is allowed to occasionally yield
> false
> + * locked result to avoid performance overhead, in which case we fall back to
> + * using mmap_lock. The function should never yield false unlocked result.
> + */
> +static inline bool vma_read_trylock(struct vm_area_struct *vma)
> +{
> + /* Check before locking. A race might cause false locked result. */
> + if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
> + return false;
> +
> + if (unlikely(down_read_trylock(&vma->lock) == 0))
> + return false;
> +
> + /*
> + * Overflow might produce false locked result.
> + * False unlocked result is impossible because we modify and check
> + * vma->vm_lock_seq under vma->lock protection and mm->mm_lock_seq
> + * modification invalidates all existing locks.
> + */
> + if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) {
> + up_read(&vma->lock);
> + return false;
> + }
> + return true;
> +}
> +
> +static inline void vma_read_unlock(struct vm_area_struct *vma)
> +{
> + up_read(&vma->lock);
> +}
> +
> +static inline void vma_assert_write_locked(struct vm_area_struct *vma)
> +{
> + mmap_assert_write_locked(vma->vm_mm);
> + /*
> + * current task is holding mmap_write_lock, both vma->vm_lock_seq and
> + * mm->mm_lock_seq can't be concurrently modified.
> + */
> + VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
> vma);
> +}
> +
> +#else /* CONFIG_PER_VMA_LOCK */
> +
> +static inline void vma_init_lock(struct vm_area_struct *vma) {}
> +static inline void vma_write_lock(struct vm_area_struct *vma) {}
> +static inline bool vma_read_trylock(struct vm_area_struct *vma)
> + { return false; }
> +static inline void vma_read_unlock(struct vm_area_struct *vma) {}
> +static inline void vma_assert_write_locked(struct vm_area_struct *vma) {}
> +
> +#endif /* CONFIG_PER_VMA_LOCK */
> +
> static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm)
> {
> static const struct vm_operations_struct dummy_vm_ops = {};
> @@ -620,6 +699,7 @@ static inline void vma_init(struct vm_area_struct *vma,
> struct mm_struct *mm)
> vma->vm_mm = mm;
> vma->vm_ops = &dummy_vm_ops;
> INIT_LIST_HEAD(&vma->anon_vma_chain);
> + vma_init_lock(vma);
> }
>
> static inline void vma_set_anonymous(struct vm_area_struct *vma)
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index d5cdec1314fe..5f7c5ca89931 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -555,6 +555,11 @@ struct vm_area_struct {
> pgprot_t vm_page_prot;
> unsigned long vm_flags; /* Flags, see mm.h. */
>
> +#ifdef CONFIG_PER_VMA_LOCK
> + int vm_lock_seq;
> + struct rw_semaphore lock;
> +#endif
> +
> /*
> * For areas with an address space and backing store,
> * linkage into the address_space->i_mmap interval tree.
> @@ -680,6 +685,9 @@ struct mm_struct {
> * init_mm.mmlist, and are protected
> * by mmlist_lock
> */
> +#ifdef CONFIG_PER_VMA_LOCK
> + int mm_lock_seq;
> +#endif
>
>
> unsigned long hiwater_rss; /* High-watermark of RSS usage */
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index e49ba91bb1f0..40facd4c398b 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -72,6 +72,17 @@ static inline void mmap_assert_write_locked(struct
> mm_struct *mm)
> VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
> }
>
> +#ifdef CONFIG_PER_VMA_LOCK
> +static inline void vma_write_unlock_mm(struct mm_struct *mm)
> +{
> + mmap_assert_write_locked(mm);
> + /* No races during update due to exclusive mmap_lock being held */
> + WRITE_ONCE(mm->mm_lock_seq, mm->mm_lock_seq + 1);
> +}
> +#else
> +static inline void vma_write_unlock_mm(struct mm_struct *mm) {}
> +#endif
> +
> static inline void mmap_init_lock(struct mm_struct *mm)
> {
> init_rwsem(&mm->mmap_lock);
> @@ -114,12 +125,14 @@ static inline bool mmap_write_trylock(struct mm_struct
> *mm)
> static inline void mmap_write_unlock(struct mm_struct *mm)
> {
> __mmap_lock_trace_released(mm, true);
> + vma_write_unlock_mm(mm);
> up_write(&mm->mmap_lock);
> }
>
> static inline void mmap_write_downgrade(struct mm_struct *mm)
> {
> __mmap_lock_trace_acquire_returned(mm, false, true);
> + vma_write_unlock_mm(mm);
> downgrade_write(&mm->mmap_lock);
> }
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 5986817f393c..c026d75108b3 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -474,6 +474,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct
> *orig)
> */
> *new = data_race(*orig);
> INIT_LIST_HEAD(&new->anon_vma_chain);
> + vma_init_lock(new);
> dup_anon_vma_name(orig, new);
> }
> return new;
> @@ -1145,6 +1146,9 @@ 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
> + WRITE_ONCE(mm->mm_lock_seq, 0);
> +#endif
> mm_pgtables_bytes_init(mm);
> mm->map_count = 0;
> mm->locked_vm = 0;
> diff --git a/mm/init-mm.c b/mm/init-mm.c
> index c9327abb771c..33269314e060 100644
> --- a/mm/init-mm.c
> +++ b/mm/init-mm.c
> @@ -37,6 +37,9 @@ struct mm_struct init_mm = {
> .page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
> .arg_lock = __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
> .mmlist = LIST_HEAD_INIT(init_mm.mmlist),
> +#ifdef CONFIG_PER_VMA_LOCK
> + .mm_lock_seq = 0,
> +#endif
> .user_ns = &init_user_ns,
> .cpu_bitmap = CPU_BITS_NONE,
> #ifdef CONFIG_IOMMU_SVA
> --
> 2.39.0
--
Michal Hocko
SUSE Labs
