Re: [PATCH] mm/vma: the pgoff is correct if can_merge_right

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Oct 24, 2024 at 09:10:12AM +0000, Wei Yang wrote:
> On Thu, Oct 24, 2024 at 10:03:34AM +0100, Lorenzo Stoakes wrote:
> >On Thu, Oct 24, 2024 at 08:42:22AM +0000, Wei Yang wrote:
> >> can_merge_right implies can_vma_merge_right() has checked the pgoff.
> >>
> >> Don't need to assign it again.
> >
> >Would prefer a bigger commit message something like:
> >
> >By this point can_vma_merge_right() must have returned true, which implies
> >can_vma_merge_before() also returned true, which already asserts that the
> >pgoff is as expected for a merge with the following VMA, thus this
> >assignment is redundant.
> >
>
> Will change to this in next version.

Thanks. The actual change itself looks good!

>
> >>
> >> Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
> >
> >Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> >
> >> CC: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> >> ---
> >>  mm/vma.c | 2 --
> >>  1 file changed, 2 deletions(-)
> >>
> >> diff --git a/mm/vma.c b/mm/vma.c
> >> index 4737afcb064c..fb4f1863f88e 100644
> >> --- a/mm/vma.c
> >> +++ b/mm/vma.c
> >> @@ -915,7 +915,6 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
> >>  	unsigned long start = vmg->start;
> >>  	unsigned long end = vmg->end;
> >>  	pgoff_t pgoff = vmg->pgoff;
> >> -	pgoff_t pglen = PHYS_PFN(end - start);
> >>  	bool can_merge_left, can_merge_right;
> >>
> >>  	mmap_assert_write_locked(vmg->mm);
> >> @@ -936,7 +935,6 @@ struct vm_area_struct *vma_merge_new_range(struct vma_merge_struct *vmg)
> >>  	if (can_merge_right) {
> >>  		vmg->end = next->vm_end;
> >>  		vmg->vma = next;
> >> -		vmg->pgoff = next->vm_pgoff - pglen;
> >>  	}
> >>
> >>  	/* If we can merge with the previous VMA, adjust vmg accordingly. */
> >> --
> >> 2.34.1
> >>
> >>
> >
> >Thanks, nice spot!
> >
> >For the purposes of explaining it on-list this is because:
> >
> >static bool can_vma_merge_right(struct vma_merge_struct *vmg,
> >				bool can_merge_left)
> >{
> >	if (!vmg->next || vmg->end != vmg->next->vm_start ||
> >	    !can_vma_merge_before(vmg))
> >		return false;
> >	...
> >}
> >
> >And:
> >
> >static bool can_vma_merge_before(struct vma_merge_struct *vmg)
> >{
> >	pgoff_t pglen = PHYS_PFN(vmg->end - vmg->start);
> >...
> >		if (vmg->next->vm_pgoff == vmg->pgoff + pglen)
> >			return true;
> >...
> >}
> >
> >Which implies vmg->pgoff == vmg->next->vm_pgoff - pglen.
> >
> >None of these values are changed between the check and prior assignment, so
> >this was an entirely redundant assignment.
>
> Do you suggest me to add this in change log?

Sure, I am a big believer in putting as much detail as possible in commit
messages, we definitely need to explain why we're doing this to future
observers (including me... ;)

>
> --
> Wei Yang
> Help you, Help me
>