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

[Wei Yang <richard.weiyang@gmail.com>]

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.

>>
>> 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?

-- 
Wei Yang
Help you, Help me