Re: [PATCH v3 4/4] mm/mmap/vma_merge: init cleanup, be explicit about the non-mergeable case

[Liam R. Howlett]

* Vlastimil Babka <vbabka@xxxxxxx> [230322 05:28]:
> On 3/22/23 08:13, Lorenzo Stoakes wrote:
> > Rather than setting err = -1 and only resetting if we hit merge cases,
> > explicitly check the non-mergeable case to make it abundantly clear that we
> > only proceed with the rest if something is mergeable, default err to 0 and
> > only update if an error might occur.
> > 
> > Move the merge_prev, merge_next cases closer to the logic determining curr,
> > next and reorder initial variables so they are more logically grouped.
> > 
> > This has no functional impact.
> > 
> > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> 
> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
> 
> Some nits:
> 
> > ---
> >  mm/mmap.c | 57 ++++++++++++++++++++++++++++---------------------------
> >  1 file changed, 29 insertions(+), 28 deletions(-)
> > 
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 2a4f63716231..642f3d063be1 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -909,18 +909,18 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >  			struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
> >  			struct anon_vma_name *anon_name)
> >  {
> > -	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
> > -	pgoff_t vma_pgoff;
> >  	struct vm_area_struct *curr, *next, *res;
> >  	struct vm_area_struct *vma, *adjust, *remove, *remove2;
> > -	int err = -1;
> > +	struct vma_prepare vp;
> > +	pgoff_t vma_pgoff;
> > +	int err = 0;
> >  	bool merge_prev = false;
> >  	bool merge_next = false;
> >  	bool vma_expanded = false;
> > -	struct vma_prepare vp;
> > +	unsigned long vma_start = addr;
> >  	unsigned long vma_end = end;
> > +	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
> >  	long adj_start = 0;
> > -	unsigned long vma_start = addr;
> >  
> >  	validate_mm(mm);
> >  	/*
> > @@ -939,36 +939,38 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >  	else
> >  		next = NULL;		/* case 5 */
> >  
> > -	/*
> > -	 * By default, we return prev. Cases 3, 4, 8 will instead return next
> > -	 * and cases 3, 8 will also update vma to point at next.
> > -	 */
> > -	res = vma = prev;
> > -
> > -	/* Verify some invariant that must be enforced by the caller. */
> > -	VM_WARN_ON(prev && addr <= prev->vm_start);
> > -	VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> > -	VM_WARN_ON(addr >= end);
> > -
> >  	if (prev) {
> >  		vma_start = prev->vm_start;
> >  		vma_pgoff = prev->vm_pgoff;
> > +
> >  		/* Can we merge the predecessor? */
> > -		if (prev->vm_end == addr && mpol_equal(vma_policy(prev), policy)
> > +		if (addr == prev->vm_end && mpol_equal(vma_policy(prev), policy)
> >  		    && can_vma_merge_after(prev, vm_flags, anon_vma, file,
> > -				   pgoff, vm_userfaultfd_ctx, anon_name)) {
> > +					   pgoff, vm_userfaultfd_ctx, anon_name)) {
> >  			merge_prev = true;
> >  			vma_prev(vmi);
> >  		}
> >  	}
> >  
> >  	/* Can we merge the successor? */
> > -	if (next && mpol_equal(policy, vma_policy(next)) &&
> > -	    can_vma_merge_before(next, vm_flags,
> > -				 anon_vma, file, pgoff+pglen,
> > -				 vm_userfaultfd_ctx, anon_name)) {
> > -		merge_next = true;
> > -	}
> > +	merge_next = next && mpol_equal(policy, vma_policy(next)) &&
> > +		can_vma_merge_before(next, vm_flags,
> > +				     anon_vma, file, pgoff+pglen,
> > +				     vm_userfaultfd_ctx, anon_name);
> 
> Not a great fan of this, I think the if() is more readable, but if you and
> Liam agree, I won't mind much. Either way could consolidate the parameters
> on less lines maybe.

I think it's more readable with the if() as well, fwiw.  If you revert
this to an if(), then you should keep the braces as it looks very
awkward without them.

> 
> > +
> > +	if (!merge_prev && !merge_next)
> > +		return NULL; /* Not mergeable. */
> > +
> > +	/*
> > +	 * By default, we return prev. Cases 3, 4, 8 will instead return next
> > +	 * and cases 3, 8 will also update vma to point at next.
> > +	 */
> > +	res = vma = prev;
> > +
> > +	/* Verify some invariant that must be enforced by the caller. */
> > +	VM_WARN_ON(prev && addr <= prev->vm_start);
> > +	VM_WARN_ON(curr && (addr != curr->vm_start || end > curr->vm_end));
> > +	VM_WARN_ON(addr >= end);
> >  
> >  	remove = remove2 = adjust = NULL;
> >  	/* Can we merge both the predecessor and the successor? */
> > @@ -984,7 +986,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >  				err = dup_anon_vma(prev, curr);
> >  		}
> >  	} else if (merge_prev) {
> > -		err = 0;				/* case 2 */
> > +							/* case 2 */
> 
> Move the comment from this now weirdly empty line to the "else if" one above?

Yeah, this also makes sense to me, and brings it in line with case 8/4,
etc.

> 
> >  		if (curr) {
> >  			err = dup_anon_vma(prev, curr);
> >  			if (end == curr->vm_end) {	/* case 7 */
> > @@ -994,7 +996,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >  				adj_start = (end - curr->vm_start);
> >  			}
> >  		}
> > -	} else if (merge_next) {
> > +	} else { /* merge_next */
> >  		res = next;
> >  		if (prev && addr < prev->vm_end) {	/* case 4 */
> >  			vma_end = addr;
> > @@ -1010,7 +1012,6 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >  			vma_start = addr;
> >  			vma_end = next->vm_end;
> >  			vma_pgoff = next->vm_pgoff;
> > -			err = 0;
> >  			if (curr) {			/* case 8 */
> >  				vma_pgoff = curr->vm_pgoff;
> >  				remove = curr;
> > @@ -1019,7 +1020,7 @@ struct vm_area_struct *vma_merge(struct vma_iterator *vmi, struct mm_struct *mm,
> >  		}
> >  	}
> >  
> > -	/* Cannot merge or error in anon_vma clone */
> > +	/* Error in anon_vma clone. */
> >  	if (err)
> >  		return NULL;
> >  
> 
>