Re: [PATCH] mm/mmap: leave adjust_next as virtual address instead of page frame number

[Wei Yang]

On Tue, Sep 08, 2020 at 05:31:22PM +0200, Vlastimil Babka wrote:
>On 8/28/20 10:10 AM, Wei Yang wrote:
>> Instead of convert adjust_next between virtual address and page frame
>> number, let's just store the virtual address into adjust_next.
>
>IMHO more precisely/less confusing it's "bytes" and "pages" instead of "virtual
>address" (which is absolute address, but this variable holds a difference) and
>"page frame number" (which is related to absolute physical address, but what we
>have is difference in pages in virtual address space).
>

Thanks for your comment.

To be honest, I am not sure which one is more precise. English is not my
mother tongue. If others think this is better, I am fine to adjust this.

>> Also, this patch fixes one typo in the comment of
>> vma_adjust_trans_huge().
>> 
>> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
>
>Other than that, seems like it leads to less shifting, so
>Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>
>> ---
>>  mm/huge_memory.c | 4 ++--
>>  mm/mmap.c        | 8 ++++----
>>  2 files changed, 6 insertions(+), 6 deletions(-)
>> 
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 78c84bee7e29..2c633ba14440 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2300,13 +2300,13 @@ void vma_adjust_trans_huge(struct vm_area_struct *vma,
>>  
>>  	/*
>>  	 * If we're also updating the vma->vm_next->vm_start, if the new
>> -	 * vm_next->vm_start isn't page aligned and it could previously
>> +	 * vm_next->vm_start isn't hpage aligned and it could previously
>>  	 * contain an hugepage: check if we need to split an huge pmd.
>>  	 */
>>  	if (adjust_next > 0) {
>>  		struct vm_area_struct *next = vma->vm_next;
>>  		unsigned long nstart = next->vm_start;
>> -		nstart += adjust_next << PAGE_SHIFT;
>> +		nstart += adjust_next;
>>  		if (nstart & ~HPAGE_PMD_MASK &&
>>  		    (nstart & HPAGE_PMD_MASK) >= next->vm_start &&
>>  		    (nstart & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE <= next->vm_end)
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 90b1298d4222..e4c9bbfd4103 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -758,7 +758,7 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>  			 * vma expands, overlapping part of the next:
>>  			 * mprotect case 5 shifting the boundary up.
>>  			 */
>> -			adjust_next = (end - next->vm_start) >> PAGE_SHIFT;
>> +			adjust_next = (end - next->vm_start);
>>  			exporter = next;
>>  			importer = vma;
>>  			VM_WARN_ON(expand != importer);
>> @@ -768,7 +768,7 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>  			 * split_vma inserting another: so it must be
>>  			 * mprotect case 4 shifting the boundary down.
>>  			 */
>> -			adjust_next = -((vma->vm_end - end) >> PAGE_SHIFT);
>> +			adjust_next = -(vma->vm_end - end);
>>  			exporter = vma;
>>  			importer = next;
>>  			VM_WARN_ON(expand != importer);
>> @@ -840,8 +840,8 @@ int __vma_adjust(struct vm_area_struct *vma, unsigned long start,
>>  	}
>>  	vma->vm_pgoff = pgoff;
>>  	if (adjust_next) {
>> -		next->vm_start += adjust_next << PAGE_SHIFT;
>> -		next->vm_pgoff += adjust_next;
>> +		next->vm_start += adjust_next;
>> +		next->vm_pgoff += adjust_next >> PAGE_SHIFT;
>>  	}
>>  
>>  	if (root) {
>> 

-- 
Wei Yang
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