linux-kernel - Re: [PATCH v2 4/5] mm: abstract merge for new VMAs into vma_merge_new

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Message-ID: <20231011015140.arngzv47bdyyzfie@revolver>
Date:   Tue, 10 Oct 2023 21:51:40 -0400
From:   "Liam R. Howlett" <Liam.Howlett@...cle.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 4/5] mm: abstract merge for new VMAs into
 vma_merge_new_vma()

* Lorenzo Stoakes <lstoakes@...il.com> [231009 16:53]:
> Only in mmap_region() and copy_vma() do we attempt to merge VMAs which
> occupy entirely new regions of virtual memory.
> 
> We can abstract this logic and make the intent of this invocations of it
> completely explicit, rather than invoking vma_merge() with an inscrutable
> wall of parameters.
> 
> This also paves the way for a simplification of the core vma_merge()
> implementation, as we seek to make it entirely an implementation detail.
> 
> Note that on mmap_region(), VMA fields are initialised to zero, so we can
> simply reference these rather than explicitly specifying NULL.

I don't think that's accurate.. mmap_region() sets the start, end,
offset, flags.  It also passes this vma into a driver, so I'm not sure
we can rely on them being anything after that?  The whole reason
vma_merge() is attempted in this case is because the driver may have
changed vma->vm_flags on us.  Your way may actually be better since the
driver may set something we assume is NULL today.

> 
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Lorenzo Stoakes <lstoakes@...il.com>
> ---
>  mm/mmap.c | 27 ++++++++++++++++++++-------
>  1 file changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 17c0dcfb1527..33aafd23823b 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2482,6 +2482,22 @@ struct vm_area_struct *vma_modify(struct vma_iterator *vmi,
>  	return NULL;
>  }
>  
> +/*
> + * Attempt to merge a newly mapped VMA with those adjacent to it. The caller
> + * must ensure that [start, end) does not overlap any existing VMA.
> + */
> +static struct vm_area_struct *vma_merge_new_vma(struct vma_iterator *vmi,
> +						struct vm_area_struct *prev,
> +						struct vm_area_struct *vma,
> +						unsigned long start,
> +						unsigned long end,
> +						pgoff_t pgoff)

It's not a coding style, but if you used two tabs here, it may make this
more condensed.

> +{
> +	return vma_merge(vmi, vma->vm_mm, prev, start, end, vma->vm_flags,
> +			 vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
> +			 vma->vm_userfaultfd_ctx, anon_vma_name(vma));
> +}
> +
>  /*
>   * do_vmi_align_munmap() - munmap the aligned region from @start to @end.
>   * @vmi: The vma iterator
> @@ -2837,10 +2853,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  		 * vma again as we may succeed this time.
>  		 */
>  		if (unlikely(vm_flags != vma->vm_flags && prev)) {
> -			merge = vma_merge(&vmi, mm, prev, vma->vm_start,
> -				    vma->vm_end, vma->vm_flags, NULL,
> -				    vma->vm_file, vma->vm_pgoff, NULL,
> -				    NULL_VM_UFFD_CTX, NULL);
> +			merge = vma_merge_new_vma(&vmi, prev, vma,
> +						  vma->vm_start, vma->vm_end,
> +						  pgoff);
                                                   └ vma->vm_pgoff
>  			if (merge) {
>  				/*
>  				 * ->mmap() can change vma->vm_file and fput
> @@ -3382,9 +3397,7 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
>  	if (new_vma && new_vma->vm_start < addr + len)
>  		return NULL;	/* should never get here */
>  
> -	new_vma = vma_merge(&vmi, mm, prev, addr, addr + len, vma->vm_flags,
> -			    vma->anon_vma, vma->vm_file, pgoff, vma_policy(vma),
> -			    vma->vm_userfaultfd_ctx, anon_vma_name(vma));
> +	new_vma = vma_merge_new_vma(&vmi, prev, vma, addr, addr + len, pgoff);
>  	if (new_vma) {
>  		/*
>  		 * Source vma may have been merged into new_vma
> -- 
> 2.42.0
>