Re: [PATCH v4 20/21] mm/mmap: Move may_expand_vm() check in mmap_region()

[Lorenzo Stoakes]

On Wed, Jul 10, 2024 at 03:22:49PM GMT, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
>
> The MAP_FIXED page count is available after the vms_gather_munmap_vmas()
> call, so use it instead of looping over the vmas twice.

This commit message doesn't explain the 'Move may_expand_vm()' part of this
change which is the patch's subject. Should add a little blurb about that.

>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> ---
>  mm/mmap.c | 36 ++++--------------------------------
>  1 file changed, 4 insertions(+), 32 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 18c269bf8703..7a440e7da55a 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -401,27 +401,6 @@ anon_vma_interval_tree_post_update_vma(struct vm_area_struct *vma)
>  		anon_vma_interval_tree_insert(avc, &avc->anon_vma->rb_root);
>  }
>
> -static unsigned long count_vma_pages_range(struct mm_struct *mm,
> -		unsigned long addr, unsigned long end,
> -		unsigned long *nr_accounted)
> -{
> -	VMA_ITERATOR(vmi, mm, addr);
> -	struct vm_area_struct *vma;
> -	unsigned long nr_pages = 0;
> -
> -	*nr_accounted = 0;
> -	for_each_vma_range(vmi, vma, end) {
> -		unsigned long vm_start = max(addr, vma->vm_start);
> -		unsigned long vm_end = min(end, vma->vm_end);
> -
> -		nr_pages += PHYS_PFN(vm_end - vm_start);
> -		if (vma->vm_flags & VM_ACCOUNT)
> -			*nr_accounted += PHYS_PFN(vm_end - vm_start);
> -	}
> -
> -	return nr_pages;
> -}
> -
>  static void __vma_link_file(struct vm_area_struct *vma,
>  			    struct address_space *mapping)
>  {
> @@ -2949,17 +2928,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	pgoff_t vm_pgoff;
>  	int error = -ENOMEM;
>  	VMA_ITERATOR(vmi, mm, addr);
> -	unsigned long nr_pages, nr_accounted;
> -
> -	nr_pages = count_vma_pages_range(mm, addr, end, &nr_accounted);
> -
> -	/*
> -	 * Check against address space limit.
> -	 * MAP_FIXED may remove pages of mappings that intersects with requested
> -	 * mapping. Account for the pages it would unmap.
> -	 */
> -	if (!may_expand_vm(mm, vm_flags, pglen - nr_pages))
> -		return -ENOMEM;
>
>
>  	/* Find the first overlapping VMA */
> @@ -2987,6 +2955,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  			vma_iter_next_range(&vmi);
>  	}
>
> +	/* Check against address space limit. */
> +	if (!may_expand_vm(mm, vm_flags, pglen - vms.nr_pages))
> +		goto abort_munmap;
> +
>  	/*
>  	 * Private writable mapping: check memory availability
>  	 */
> --
> 2.43.0
>

This is fine though as discussed previously, though obviously need to think
about the arch_unmap() bit :)

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>