linux-kernel - Re: [PATCH v3 16/16] mm/mmap: Move may_expand_vm() check in mmap

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Message-ID: <8fbb424d-a781-4e61-af7a-904e281eba8c@lucifer.local>
Date: Mon, 8 Jul 2024 13:52:59 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lstoakes@...il.com>,
        Matthew Wilcox <willy@...radead.org>, sidhartha.kumar@...cle.com,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Bert Karwatzki <spasswolf@....de>, Jiri Olsa <olsajiri@...il.com>,
        linux-kernel@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v3 16/16] mm/mmap: Move may_expand_vm() check in
 mmap_region()

On Thu, Jul 04, 2024 at 02:27:18PM GMT, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
>
> The MAP_FIXED page count is available after the vms_gather_munmap_vmas()
> call, so use it instead of looping over the vmas twice.

Predictably indeed you removed the thing I commented on in the last patch
;) but at least this time I predicted it! ;)

>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  mm/mmap.c | 36 ++++--------------------------------
>  1 file changed, 4 insertions(+), 32 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index b2de26683903..62edaabf3987 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -400,27 +400,6 @@ 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)
>  {
> @@ -2946,17 +2925,6 @@ 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;
>
>  	if (unlikely(!can_modify_mm(mm, addr, end)))
>  		return -EPERM;
> @@ -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;
> +

I know you can literally only do this after the vms_gather_munmap_vmas(),
but this does change where we check this, so for instance we do
arch_unmap() without having checked may_expand_vm().

However I assume this is fine?

>  	/*
>  	 * Private writable mapping: check memory availability
>  	 */
> --
> 2.43.0
>

Looks good to me generally,

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