Re: [PATCH v2 6/6] mm: move vma locking out of vma_prepare

[Liam R. Howlett]

* Suren Baghdasaryan <surenb@xxxxxxxxxx> [230801 18:07]:
> vma_prepare() is currently the central place where vmas are being locked
> before vma_complete() applies changes to them. While this is convenient,
> it also obscures vma locking and makes it hard to follow the locking rules.
> Move vma locking out of vma_prepare() and take vma locks explicitly at the
> locations where vmas are being modified.
> 
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> ---
>  mm/mmap.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 850a39dee075..e59d83cb1d7a 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -476,16 +476,6 @@ static inline void init_vma_prep(struct vma_prepare *vp,
>   */
>  static inline void vma_prepare(struct vma_prepare *vp)
>  {
> -	vma_start_write(vp->vma);
> -	if (vp->adj_next)
> -		vma_start_write(vp->adj_next);
> -	if (vp->insert)
> -		vma_start_write(vp->insert);
> -	if (vp->remove)
> -		vma_start_write(vp->remove);
> -	if (vp->remove2)
> -		vma_start_write(vp->remove2);
> -
>  	if (vp->file) {
>  		uprobe_munmap(vp->vma, vp->vma->vm_start, vp->vma->vm_end);
>  
> @@ -650,6 +640,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	bool remove_next = false;
>  	struct vma_prepare vp;
>  
> +	vma_start_write(vma);

This lock made me think about the lock in dup_anon_vma().. the only
reason we need to dup the anon vma is because the VMA _will_ be
modified..  So if we are to remove next the dup_anon_vma() call may lock
vma again.  This happens multiple times through this patch set.

If we lock both dst and src VMAs before calling dup_anon_vma, then I
think it will become more clear in the code below which VMAs are locked.
We could remove the lock in the dup_anon_vma() (and replace with an
assert?), and drop the vma locking of "vma".

I think this would address the confusion of the locking below that I
experienced and avoid calling the vma_start_write() multiple times for
the same vma.  I mean, having the vma_start_write(vma) obscures which
VMA is locked as well.


>  	if (next && (vma != next) && (end == next->vm_end)) {
>  		int ret;
>  
> @@ -657,6 +648,7 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		ret = dup_anon_vma(vma, next);
>  		if (ret)
>  			return ret;
> +		vma_start_write(next);
>  	}
>  
>  	init_multi_vma_prep(&vp, vma, NULL, remove_next ? next : NULL, NULL);
> @@ -708,6 +700,8 @@ int vma_shrink(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	if (vma_iter_prealloc(vmi))
>  		return -ENOMEM;
>  
> +	vma_start_write(vma);
> +
>  	init_vma_prep(&vp, vma);
>  	vma_prepare(&vp);
>  	vma_adjust_trans_huge(vma, start, end, 0);
> @@ -946,10 +940,12 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	/* Can we merge both the predecessor and the successor? */
>  	if (merge_prev && merge_next &&
>  	    is_mergeable_anon_vma(prev->anon_vma, next->anon_vma, NULL)) {
> +		vma_start_write(next);
>  		remove = next;				/* case 1 */
>  		vma_end = next->vm_end;
>  		err = dup_anon_vma(prev, next);
>  		if (curr) {				/* case 6 */
> +			vma_start_write(curr);
>  			remove = curr;
>  			remove2 = next;
>  			if (!next->anon_vma)
> @@ -958,6 +954,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	} else if (merge_prev) {			/* case 2 */
>  		if (curr) {
>  			err = dup_anon_vma(prev, curr);
> +			vma_start_write(curr);
>  			if (end == curr->vm_end) {	/* case 7 */
>  				remove = curr;
>  			} else {			/* case 5 */
> @@ -969,6 +966,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  		res = next;
>  		if (prev && addr < prev->vm_end) {	/* case 4 */
>  			vma_end = addr;
> +			vma_start_write(next);
>  			adjust = next;
>  			adj_start = -(prev->vm_end - addr);
>  			err = dup_anon_vma(next, prev);
> @@ -983,6 +981,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  			vma_pgoff = next->vm_pgoff - pglen;
>  			if (curr) {			/* case 8 */
>  				vma_pgoff = curr->vm_pgoff;
> +				vma_start_write(curr);
>  				remove = curr;
>  				err = dup_anon_vma(next, curr);
>  			}
> @@ -996,6 +995,8 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
>  	if (vma_iter_prealloc(vmi))
>  		return NULL;
>  
> +	vma_start_write(vma);
> +
>  	init_multi_vma_prep(&vp, vma, adjust, remove, remove2);
>  	VM_WARN_ON(vp.anon_vma && adjust && adjust->anon_vma &&
>  		   vp.anon_vma != adjust->anon_vma);
> @@ -2373,6 +2374,9 @@ int __split_vma(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  	if (new->vm_ops && new->vm_ops->open)
>  		new->vm_ops->open(new);
>  
> +	vma_start_write(vma);
> +	vma_start_write(new);
> +
>  	init_vma_prep(&vp, vma);
>  	vp.insert = new;
>  	vma_prepare(&vp);
> @@ -3078,6 +3082,8 @@ static int do_brk_flags(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		if (vma_iter_prealloc(vmi))
>  			goto unacct_fail;
>  
> +		vma_start_write(vma);
> +
>  		init_vma_prep(&vp, vma);
>  		vma_prepare(&vp);
>  		vma_adjust_trans_huge(vma, vma->vm_start, addr + len, 0);
> -- 
> 2.41.0.585.gd2178a4bd4-goog
>