Re: [PATCH v5 4/6] mm: make vma cache SLAB_TYPESAFE_BY_RCU

[Vlastimil Babka <vbabka@suse.cz>]

On 12/6/24 23:52, Suren Baghdasaryan wrote:
> To enable SLAB_TYPESAFE_BY_RCU for vma cache we need to ensure that
> object reuse before RCU grace period is over will be detected inside
> lock_vma_under_rcu().
> lock_vma_under_rcu() enters RCU read section, finds the vma at the
> given address, locks the vma and checks if it got detached or remapped
> to cover a different address range. These last checks are there
> to ensure that the vma was not modified after we found it but before
> locking it.
> vma reuse introduces several new possibilities:
> 1. vma can be reused after it was found but before it is locked;
> 2. vma can be reused and reinitialized (including changing its vm_mm)
> while being locked in vma_start_read();
> 3. vma can be reused and reinitialized after it was found but before
> it is locked, then attached at a new address or to a new mm while
> read-locked;
> For case #1 current checks will help detecting cases when:
> - vma was reused but not yet added into the tree (detached check)
> - vma was reused at a different address range (address check);
> We are missing the check for vm_mm to ensure the reused vma was not
> attached to a different mm. This patch adds the missing check.
> For case #2, we pass mm to vma_start_read() to prevent access to
> unstable vma->vm_mm. This might lead to vma_start_read() returning
> a false locked result but that's not critical if it's rare because
> it will only lead to a retry under mmap_lock.
> For case #3, we ensure the order in which vma->detached flag and
> vm_start/vm_end/vm_mm are set and checked. vma gets attached after
> vm_start/vm_end/vm_mm were set and lock_vma_under_rcu() should check
> vma->detached before checking vm_start/vm_end/vm_mm. This is required
> because attaching vma happens without vma write-lock, as opposed to
> vma detaching, which requires vma write-lock. This patch adds memory
> barriers inside is_vma_detached() and vma_mark_attached() needed to
> order reads and writes to vma->detached vs vm_start/vm_end/vm_mm.
> After these provisions, SLAB_TYPESAFE_BY_RCU is added to vm_area_cachep.
> This will facilitate vm_area_struct reuse and will minimize the number
> of call_rcu() calls.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  include/linux/mm.h               |  36 +++++--
>  include/linux/mm_types.h         |  10 +-
>  include/linux/slab.h             |   6 --
>  kernel/fork.c                    | 157 +++++++++++++++++++++++++------
>  mm/memory.c                      |  15 ++-
>  mm/vma.c                         |   2 +-
>  tools/testing/vma/vma_internal.h |   7 +-
>  7 files changed, 179 insertions(+), 54 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2bf38c1e9cca..3568bcbc7c81 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -257,7 +257,7 @@ struct vm_area_struct *vm_area_alloc(struct mm_struct *);
>  struct vm_area_struct *vm_area_dup(struct vm_area_struct *);
>  void vm_area_free(struct vm_area_struct *);
>  /* Use only if VMA has no other users */
> -void __vm_area_free(struct vm_area_struct *vma);
> +void vm_area_free_unreachable(struct vm_area_struct *vma);
>  
>  #ifndef CONFIG_MMU
>  extern struct rb_root nommu_region_tree;
> @@ -706,8 +706,10 @@ static inline void vma_lock_init(struct vm_area_struct *vma)
>   * 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.
> + * False locked result is possible if mm_lock_seq overflows or if vma gets
> + * reused and attached to a different mm before we lock it.
>   */
> -static inline bool vma_start_read(struct vm_area_struct *vma)
> +static inline bool vma_start_read(struct mm_struct *mm, struct vm_area_struct *vma)
>  {
>  	/*
>  	 * Check before locking. A race might cause false locked result.
> @@ -716,7 +718,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
>  	 * we don't rely on for anything - the mm_lock_seq read against which we
>  	 * need ordering is below.
>  	 */
> -	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence))
> +	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(mm->mm_lock_seq.sequence))
>  		return false;
>  
>  	if (unlikely(down_read_trylock(&vma->vm_lock.lock) == 0))
> @@ -733,7 +735,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
>  	 * after it has been unlocked.
>  	 * This pairs with RELEASE semantics in vma_end_write_all().
>  	 */
> -	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
> +	if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&mm->mm_lock_seq))) {
>  		up_read(&vma->vm_lock.lock);
>  		return false;
>  	}

This could have been perhaps another preparatory patch to make this one smaller?

>  
> +static void vm_area_ctor(void *data)
> +{
> +	struct vm_area_struct *vma = (struct vm_area_struct *)data;
> +
> +#ifdef CONFIG_PER_VMA_LOCK
> +	/* vma is not locked, can't use vma_mark_detached() */
> +	vma->detached = true;
> +#endif
> +	INIT_LIST_HEAD(&vma->anon_vma_chain);
> +	vma_lock_init(vma);
> +}
> +
> +#ifdef CONFIG_PER_VMA_LOCK
> +
> +static void vma_clear(struct vm_area_struct *vma, struct mm_struct *mm)
> +{
> +	vma->vm_mm = mm;
> +	vma->vm_ops = &vma_dummy_vm_ops;
> +	vma->vm_start = 0;
> +	vma->vm_end = 0;
> +	vma->anon_vma = NULL;
> +	vma->vm_pgoff = 0;
> +	vma->vm_file = NULL;
> +	vma->vm_private_data = NULL;
> +	vm_flags_init(vma, 0);
> +	memset(&vma->vm_page_prot, 0, sizeof(vma->vm_page_prot));
> +	memset(&vma->shared, 0, sizeof(vma->shared));
> +	memset(&vma->vm_userfaultfd_ctx, 0, sizeof(vma->vm_userfaultfd_ctx));
> +	vma_numab_state_init(vma);
> +#ifdef CONFIG_ANON_VMA_NAME
> +	vma->anon_name = NULL;
> +#endif
> +#ifdef CONFIG_SWAP
> +	memset(&vma->swap_readahead_info, 0, sizeof(vma->swap_readahead_info));
> +#endif
> +#ifndef CONFIG_MMU
> +	vma->vm_region = NULL;
> +#endif
> +#ifdef CONFIG_NUMA
> +	vma->vm_policy = NULL;
> +#endif

This isn't the ideal pattern I think, now that we have a ctor. Ideally the
ctor would do all this (except setting the vm_mm), and then we need to make
sure it's also done when freeing the vma, to make sure the freed object is
in the same state as a new object after the constructor.

On freeing, things like numab_state and anon_name could be NULL'd (by the
respective destructors) only when they are non-NULL and thus freeing the
objects pointed to. vm_policy and vm_file could perhaps be handled same way
after some refactoring (see remove_vma()), vma_dummy_vm_ops are possibly
already reset by vma_close(), etc.

> +}
> +
> +static void vma_copy(const struct vm_area_struct *src, struct vm_area_struct *dest)
> +{
> +	dest->vm_mm = src->vm_mm;
> +	dest->vm_ops = src->vm_ops;
> +	dest->vm_start = src->vm_start;
> +	dest->vm_end = src->vm_end;
> +	dest->anon_vma = src->anon_vma;
> +	dest->vm_pgoff = src->vm_pgoff;
> +	dest->vm_file = src->vm_file;
> +	dest->vm_private_data = src->vm_private_data;
> +	vm_flags_init(dest, src->vm_flags);
> +	memcpy(&dest->vm_page_prot, &src->vm_page_prot,
> +	       sizeof(dest->vm_page_prot));
> +	memcpy(&dest->shared, &src->shared, sizeof(dest->shared));
> +	memcpy(&dest->vm_userfaultfd_ctx, &src->vm_userfaultfd_ctx,
> +	       sizeof(dest->vm_userfaultfd_ctx));
> +#ifdef CONFIG_ANON_VMA_NAME
> +	dest->anon_name = src->anon_name;
> +#endif
> +#ifdef CONFIG_SWAP
> +	memcpy(&dest->swap_readahead_info, &src->swap_readahead_info,
> +	       sizeof(dest->swap_readahead_info));
> +#endif
> +#ifndef CONFIG_MMU
> +	dest->vm_region = src->vm_region;
> +#endif
> +#ifdef CONFIG_NUMA
> +	dest->vm_policy = src->vm_policy;
> +#endif
> +}
> +
> +#else /* CONFIG_PER_VMA_LOCK */