Re: [PATCH v10 14/69] kernel/fork: use maple tree for dup_mmap() during forking

[David Hildenbrand]

On 21.06.22 22:46, Liam Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> 
> The maple tree was already tracking VMAs in this function by an earlier
> commit, but the rbtree iterator was being used to iterate the list.
> Change the iterator to use a maple tree native iterator and switch to the
> maple tree advanced API to avoid multiple walks of the tree during insert
> operations.  Unexport the now-unused vma_store() function.
> 
> For performance reasons we bulk allocate the maple tree nodes.  The node
> calculations are done internally to the tree and use the VMA count and
> assume the worst-case node requirements.  The VM_DONT_COPY flag does not
> allow for the most efficient copy method of the tree and so a bulk loading
> algorithm is used.

You could track the #VM_DONT_COPY per mm and optimize? Do you have that
planned or maybe it already is part of this patch set? :)

> 
> Link: https://lkml.kernel.org/r/20220504010716.661115-16-Liam.Howlett@xxxxxxxxxx
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: SeongJae Park <sj@xxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/mm.h |  2 --
>  kernel/fork.c      | 12 +++++++++---
>  2 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index f22c6f71a18c..27fa36a7c093 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2612,8 +2612,6 @@ extern bool arch_has_descending_max_zone_pfns(void);
>  /* nommu.c */
>  extern atomic_long_t mmap_pages_allocated;
>  extern int nommu_shrink_inode_mappings(struct inode *, size_t, size_t);
> -/* mmap.c */
> -void vma_mas_store(struct vm_area_struct *vma, struct ma_state *mas);
>  
>  /* interval_tree.c */
>  void vma_interval_tree_insert(struct vm_area_struct *node,
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 1840da0732f6..4ecca79556eb 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -583,9 +583,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>  	struct vm_area_struct *mpnt, *tmp, *prev, **pprev;
>  	struct rb_node **rb_link, *rb_parent;
>  	int retval;
> -	unsigned long charge;
> -	LIST_HEAD(uf);
> +	unsigned long charge = 0;

^ why that change?

> +	MA_STATE(old_mas, &oldmm->mm_mt, 0, 0);
>  	MA_STATE(mas, &mm->mm_mt, 0, 0);
> +	LIST_HEAD(uf);

^ why that reshuffling?

>  
>  	uprobe_start_dup_mmap();
>  	if (mmap_write_lock_killable(oldmm)) {
> @@ -620,7 +621,12 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>  		goto out;
>  
>  	prev = NULL;
> -	for (mpnt = oldmm->mmap; mpnt; mpnt = mpnt->vm_next) {
> +
> +	retval = mas_expected_entries(&mas, oldmm->map_count);
> +	if (retval)
> +		goto out;

mas_expected_entries is a fancy function :)



> +
> +	mas_for_each(&old_mas, mpnt, ULONG_MAX) {
>  		struct file *file;
>  
>  		if (mpnt->vm_flags & VM_DONTCOPY) {


-- 
Thanks,

David / dhildenb