Re: [PATCH hotfix 6.12 8/8] mm: do not attempt second merge for file-backed VMAs

[Vlastimil Babka <vbabka@suse.cz>]

On 10/22/24 22:40, Lorenzo Stoakes wrote:
> Previously, we'd always try to merge a file-backed VMA if its flags were
> changed by the driver.
> 
> This however is rarely meaningful as typically the flags would be changed
> to VM_PFNMAP or other VM_SPECIAL flags which are inherently unmergable.
> 
> In cases where it is meaningful (for instance DAX) it is doubtful that this

Hm if that's true, I'm imagining many piecemeal mmap()s of DAX that used to
merge but now will create tons of VMA's, which doesn't sound great. Then it
has also potentially breaking effects on mremap() which doesn't work accross
multiple VMA's.

> optimisation is worth the effort and maintenance risk of having to unwind
> state and perform a merge.

What if we simply created a new vma but only then checked if the flags
changed and we can merge it with neighbours (i.e. like the mprotect()
merging case). Less efficient, but less tricky and with the same result
hopefully?

> Since we've observed bugs and resource leaks due to complexity in this
> area, it is simply not acceptable to have a 'nice to have' optimisation
> like this complicating an already very complicated code path, so let's
> simply eliminate it.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>  mm/vma.c | 39 ++-------------------------------------
>  1 file changed, 2 insertions(+), 37 deletions(-)
> 
> diff --git a/mm/vma.c b/mm/vma.c
> index a271e2b406ab..fe1fe5099e78 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -2260,8 +2260,7 @@ static int __mmap_prepare(struct mmap_state *map)
>  	return 0;
>  }
> 
> -static int __mmap_new_file_vma(struct mmap_state *map, struct vm_area_struct *vma,
> -			       struct vm_area_struct **mergep)
> +static int __mmap_new_file_vma(struct mmap_state *map, struct vm_area_struct *vma)
>  {
>  	struct vma_iterator *vmi = map->vmi;
>  	struct vma_merge_struct *vmg = map->vmg;
> @@ -2291,34 +2290,6 @@ static int __mmap_new_file_vma(struct mmap_state *map, struct vm_area_struct *vm
>  			(vma->vm_flags & VM_MAYWRITE));
> 
>  	vma_iter_config(vmi, vmg->start, vmg->end);
> -	/*
> -	 * If flags changed after mmap_file(), we should try merge
> -	 * vma again as we may succeed this time.
> -	 */
> -	if (unlikely(map->flags != vma->vm_flags && vmg->prev)) {
> -		struct vm_area_struct *merge;
> -
> -		vmg->flags = vma->vm_flags;
> -		/* If this fails, state is reset ready for a reattempt. */
> -		merge = vma_merge_new_range(vmg);
> -
> -		if (merge) {
> -			/*
> -			 * ->mmap() can change vma->vm_file and fput
> -			 * the original file. So fput the vma->vm_file
> -			 * here or we would add an extra fput for file
> -			 * and cause general protection fault
> -			 * ultimately.
> -			 */
> -			fput(vma->vm_file);
> -			vm_area_free(vma);
> -			vma_iter_free(vmi);
> -			*mergep = merge;
> -		} else {
> -			vma_iter_config(vmi, vmg->start, vmg->end);
> -		}
> -	}
> -
>  	map->flags = vma->vm_flags;
>  	return 0;
>  }
> @@ -2341,7 +2312,6 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
>  {
>  	struct vma_iterator *vmi = map->vmi;
>  	struct vma_merge_struct *vmg = map->vmg;
> -	struct vm_area_struct *merge = NULL;
>  	int error = 0;
>  	struct vm_area_struct *vma;
> 
> @@ -2365,7 +2335,7 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
>  	}
> 
>  	if (vmg->file)
> -		error = __mmap_new_file_vma(map, vma, &merge);
> +		error = __mmap_new_file_vma(map, vma);
>  	else if (map->flags & VM_SHARED)
>  		error = shmem_zero_setup(vma);
>  	else
> @@ -2374,9 +2344,6 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
>  	if (error)
>  		goto free_iter_vma;
> 
> -	if (merge)
> -		goto file_expanded;
> -
>  #ifdef CONFIG_SPARC64
>  	/* TODO: Fix SPARC ADI! */
>  	WARN_ON_ONCE(!arch_validate_flags(map->flags));
> @@ -2393,8 +2360,6 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap)
>  	 * call covers the non-merge case.
>  	 */
>  	khugepaged_enter_vma(vma, map->flags);
> -
> -file_expanded:
>  	ksm_add_vma(vma);
> 
>  	*vmap = vma;
> --
> 2.47.0