linux-kernel - Re: [PATCH v14 67/70] mm/vmscan: Use vma iterator instead of vm

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Date:   Mon, 12 Sep 2022 00:55:08 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v14 67/70] mm/vmscan: Use vma iterator instead of vm_next

On Tue, Sep 06, 2022 at 07:49:05PM +0000, Liam Howlett wrote:
> Use the vma iterator in in get_next_vma() instead of the linked list.
> 
> Suggested-by: Yu Zhao <yuzhao@...gle.com>

Apologies for the bad suggestion.

> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3776,23 +3776,14 @@ static bool get_next_vma(unsigned long mask, unsigned long size, struct mm_walk
>  {
>  	unsigned long start = round_up(*vm_end, size);
>  	unsigned long end = (start | ~mask) + 1;
> +	VMA_ITERATOR(vmi, args->mm, start);
>  
>  	VM_WARN_ON_ONCE(mask & size);
>  	VM_WARN_ON_ONCE((start & mask) != (*vm_start & mask));
>  
> -	while (args->vma) {
> -		if (start >= args->vma->vm_end) {
> -			args->vma = args->vma->vm_next;
> +	for_each_vma_range(vmi, args->vma, end) {
> +		if (should_skip_vma(args->vma->vm_start, args->vma->vm_end, args))
>  			continue;
> -		}
> -
> -		if (end && end <= args->vma->vm_start)
> -			return false;

Here the original code leaves args->vma pointing the first vma out of
the range [start, end). This allows the caller (page table walker) to
resume at that vma, if it chooses to.

With for_each_vma_range(), under the same condition, args->vma is set to
NULL. And the page table walker may terminate prematurely. Apparently I
overlooked until I was told MGLRU in mm-unstable is slower than itself
on 6.0-rc4 yesterday.

> -
> -		if (should_skip_vma(args->vma->vm_start, args->vma->vm_end, args)) {
> -			args->vma = args->vma->vm_next;
> -			continue;
> -		}
>  
>  		*vm_start = max(start, args->vma->vm_start);
>  		*vm_end = min(end - 1, args->vma->vm_end - 1) + 1;

The following should work properly. Please take a look. Thanks!

---
 mm/vmscan.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 11a86d47e85e..b22d3efe3031 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3776,23 +3776,17 @@ static bool get_next_vma(unsigned long mask, unsigned long size, struct mm_walk
 {
 	unsigned long start = round_up(*vm_end, size);
 	unsigned long end = (start | ~mask) + 1;
+	VMA_ITERATOR(vmi, args->mm, start);
 
 	VM_WARN_ON_ONCE(mask & size);
 	VM_WARN_ON_ONCE((start & mask) != (*vm_start & mask));
 
-	while (args->vma) {
-		if (start >= args->vma->vm_end) {
-			args->vma = args->vma->vm_next;
-			continue;
-		}
-
+	for_each_vma(vmi, args->vma) {
 		if (end && end <= args->vma->vm_start)
 			return false;
 
-		if (should_skip_vma(args->vma->vm_start, args->vma->vm_end, args)) {
-			args->vma = args->vma->vm_next;
+		if (should_skip_vma(args->vma->vm_start, args->vma->vm_end, args))
 			continue;
-		}
 
 		*vm_start = max(start, args->vma->vm_start);
 		*vm_end = min(end - 1, args->vma->vm_end - 1) + 1;
-- 
2.37.2.789.g6183377224-goog