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Date:   Tue, 17 Jan 2023 16:04:25 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, michel@lespinasse.org,
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Subject: Re: [PATCH 12/41] mm: add per-VMA lock and helper functions to
 control it
Message-ID: <Y8a4+bV1dYNAiUkD@dhcp22.suse.cz>
References: <20230109205336.3665937-1-surenb@google.com>
 <20230109205336.3665937-13-surenb@google.com>
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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 <surenb@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