Re: [PATCH 1/8] mm/rmap: improve anon_vma_clone(), unlink_anon_vmas() comments, add asserts

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Lorenzo Stoakes <lorenzo.stoakes@oracle.com> [251217 07:27]:
> Add kdoc comments, describe exactly what these functinos are used for in
> detail, pointing out importantly that the anon_vma_clone() !dst->anon_vma
> && src->anon_vma dance is ONLY for fork.
> 
> Both are confusing functions that will be refactored in a subsequent patch
> but the first stage is establishing documentation and some invariatns.
> 
> Add some basic CONFIG_DEBUG_VM asserts that help document expected state,
> specifically:
> 
> anon_vma_clone()
> - mmap write lock held.
> - We do nothing if src VMA is not faulted.
> - The destination VMA has no anon_vma_chain yet.
> - We are always operating on the same active VMA (i.e. vma->anon-vma).
> - If not forking, must operate on the same mm_struct.
> 
> unlink_anon_vmas()
> - mmap lock held (read on unmap downgraded).
> - That unfaulted VMAs are no-ops.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
>  mm/rmap.c | 82 +++++++++++++++++++++++++++++++++++++++++++------------
>  1 file changed, 64 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index d6799afe1114..0e34c0a69fbc 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -257,30 +257,60 @@ static inline void unlock_anon_vma_root(struct anon_vma *root)
>  		up_write(&root->rwsem);
>  }
>  
> -/*
> - * Attach the anon_vmas from src to dst.
> - * Returns 0 on success, -ENOMEM on failure.
> - *
> - * anon_vma_clone() is called by vma_expand(), vma_merge(), __split_vma(),
> - * copy_vma() and anon_vma_fork(). The first four want an exact copy of src,
> - * while the last one, anon_vma_fork(), may try to reuse an existing anon_vma to
> - * prevent endless growth of anon_vma. Since dst->anon_vma is set to NULL before
> - * call, we can identify this case by checking (!dst->anon_vma &&
> - * src->anon_vma).
> - *
> - * If (!dst->anon_vma && src->anon_vma) is true, this function tries to find
> - * and reuse existing anon_vma which has no vmas and only one child anon_vma.
> - * This prevents degradation of anon_vma hierarchy to endless linear chain in
> - * case of constantly forking task. On the other hand, an anon_vma with more
> - * than one child isn't reused even if there was no alive vma, thus rmap
> - * walker has a good chance of avoiding scanning the whole hierarchy when it
> - * searches where page is mapped.
> +static void check_anon_vma_clone(struct vm_area_struct *dst,
> +				 struct vm_area_struct *src)
> +{
> +	/* The write lock must be held. */
> +	mmap_assert_write_locked(src->vm_mm);
> +	/* If not a fork (implied by dst->anon_vma) then must be on same mm. */
> +	VM_WARN_ON_ONCE(dst->anon_vma && dst->vm_mm != src->vm_mm);
> +
> +	/* No source anon_vma is a no-op. */
> +	VM_WARN_ON_ONCE(!src->anon_vma && !list_empty(&src->anon_vma_chain));
> +	VM_WARN_ON_ONCE(!src->anon_vma && dst->anon_vma);
> +	/* We are establishing a new anon_vma_chain. */
> +	VM_WARN_ON_ONCE(!list_empty(&dst->anon_vma_chain));
> +	/*
> +	 * On fork, dst->anon_vma is set NULL (temporarily). Otherwise, anon_vma
> +	 * must be the same across dst and src.
> +	 */
> +	VM_WARN_ON_ONCE(dst->anon_vma && dst->anon_vma != src->anon_vma);
> +}
> +
> +/**
> + * anon_vma_clone - Establishes new anon_vma_chain objects in @dst linking to
> + * all of the anon_vma objects contained within @src anon_vma_chain's.
> + * @dst: The destination VMA with an empty anon_vma_chain.
> + * @src: The source VMA we wish to duplicate.
> + *
> + * This is the heart of the VMA side of the anon_vma implementation - we invoke
> + * this function whenever we need to set up a new VMA's anon_vma state.
> + *
> + * This is invoked for:
> + *
> + * - VMA Merge, but only when @dst is unfaulted and @src is faulted - meaning we
> + *   clone @src into @dst.
> + * - VMA split.
> + * - VMA (m)remap.
> + * - Fork of faulted VMA.
> + *
> + * In all cases other than fork this is simply a duplication. Fork additionally
> + * adds a new active anon_vma.
> + *
> + * ONLY in the case of fork do we try to 'reuse' existing anon_vma's in an
> + * anon_vma hierarchy, reusing anon_vma's which have no VMA associated with them
> + * but do have a single child. This is to avoid waste of memory when repeatedly
> + * forking.
> + *
> + * Returns: 0 on success, -ENOMEM on failure.
>   */
>  int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>  {
>  	struct anon_vma_chain *avc, *pavc;
>  	struct anon_vma *root = NULL;
>  
> +	check_anon_vma_clone(dst, src);
> +
>  	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
>  		struct anon_vma *anon_vma;
>  
> @@ -392,11 +422,27 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
>  	return -ENOMEM;
>  }
>  
> +/**
> + * unlink_anon_vmas() - remove all links between a VMA and anon_vma's, freeing
> + * anon_vma_chain objects.
> + * @vma: The VMA whose links to anon_vma objects is to be severed.
> + *
> + * As part of the process anon_vma_chain's are freed,
> + * anon_vma->num_children,num_active_vmas is updated as required and, if the
> + * relevant anon_vma references no further VMAs, its reference count is
> + * decremented.
> + */
>  void unlink_anon_vmas(struct vm_area_struct *vma)
>  {
>  	struct anon_vma_chain *avc, *next;
>  	struct anon_vma *root = NULL;
>  
> +	/* Always hold mmap lock, read-lock on unmap possibly. */
> +	mmap_assert_locked(vma->vm_mm);
> +
> +	/* Unfaulted is a no-op. */
> +	VM_WARN_ON_ONCE(!vma->anon_vma && !list_empty(&vma->anon_vma_chain));
> +
>  	/*
>  	 * Unlink each anon_vma chained to the VMA.  This list is ordered
>  	 * from newest to oldest, ensuring the root anon_vma gets freed last.
> -- 
> 2.52.0
>