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

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Date:   Tue, 8 Sep 2020 17:31:22 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Wei Yang <richard.weiyang@...ux.alibaba.com>,
        akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/mmap: leave adjust_next as virtual address instead of
 page frame number

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).

> Also, this patch fixes one typo in the comment of
> vma_adjust_trans_huge().
> 
> Signed-off-by: Wei Yang <richard.weiyang@...ux.alibaba.com>

Other than that, seems like it leads to less shifting, so
Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  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) {
>