Re: [PATCH v13 mm-new 07/16] khugepaged: introduce collapse_max_ptes_none helper function

[Nico Pache <npache@redhat.com>]

Hi Andrew,

The bot has reported a potential uninitialized use of a variable.

Can you please squash the following fixup to this commit.

Thank you,
Nico

----8<----

From 846f79d91a25ebad76cbab3690ae315cfe3cf278 Mon Sep 17 00:00:00 2001
From: Nico Pache <npache@redhat.com>
Date: Wed, 3 Dec 2025 13:42:18 -0700
Subject: [PATCH] khugepaged: fixup unintialized _pte variable

There is a potential use of an uninitialized variable after
`khugepaged: introduce collapse_max_ptes_none helper function`

Andrew can you please append this to Patch 7 of my series

as reported by the kernel test robot
>> mm/khugepaged.c:593:6: warning: variable '_pte' is used uninitialized
whenever 'if' condition is true [-Wsometimes-uninitialized]
593 | if (max_ptes_none == -EINVAL)
| ^~~~~~~~~~~~~~~~~~~~~~~~
mm/khugepaged.c:724:25: note: uninitialized use occurs here
724 | release_pte_pages(pte, _pte, compound_pagelist);
| ^~~~
mm/khugepaged.c:593:2: note: remove the 'if' if its condition is always false
593 | if (max_ptes_none == -EINVAL)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
594 | goto out;
| ~~~~~~~~
mm/khugepaged.c:588:13: note: initialize the variable '_pte' to silence this warning
588 | pte_t *_pte;
| ^
| = NULL
1 warning generated.

Signed-off-by: Nico Pache <npache@redhat.com>
---
mm/khugepaged.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index f425238d5d4f..7c7d04d6737e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -585,7 +585,7 @@ static int __collapse_huge_page_isolate(struct
vm_area_struct *vma,
struct page *page = NULL;
struct folio *folio = NULL;
unsigned long addr = start_addr;
- pte_t *_pte;
+ pte_t *_pte = pte;
int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
const unsigned long nr_pages = 1UL << order;
int max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged);
@@ -593,8 +593,7 @@ static int __collapse_huge_page_isolate(struct
vm_area_struct *vma,
if (max_ptes_none == -EINVAL)
goto out;
- for (_pte = pte; _pte < pte + nr_pages;
- _pte++, addr += PAGE_SIZE) {
+ for (; _pte < pte + nr_pages; _pte++, addr += PAGE_SIZE) {
pte_t pteval = ptep_get(_pte);
if (pte_none_or_zero(pteval)) {
++none_or_zero;

--
2.52.0

On 12/1/25 10:46 AM, Nico Pache wrote:
> The current mechanism for determining mTHP collapse scales the
> khugepaged_max_ptes_none value based on the target order. This
> introduces an undesirable feedback loop, or "creep", when max_ptes_none
> is set to a value greater than HPAGE_PMD_NR / 2.
> 
> With this configuration, a successful collapse to order N will populate
> enough pages to satisfy the collapse condition on order N+1 on the next
> scan. This leads to unnecessary work and memory churn.
> 
> To fix this issue introduce a helper function that will limit mTHP
> collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1.
> This effectively supports two modes:
> 
> - max_ptes_none=0: never introduce new none-pages for mTHP collapse.
> - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest
>   available mTHP order.
> 
> This removes the possiblilty of "creep", while not modifying any uAPI
> expectations. A warning will be emitted if any non-supported
> max_ptes_none value is configured with mTHP enabled.
> 
> The limits can be ignored by passing full_scan=true, this is useful for
> madvise_collapse (which ignores limits), or in the case of
> collapse_scan_pmd(), allows the full PMD to be scanned when mTHP
> collapse is available.
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  mm/khugepaged.c | 43 ++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 42 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 8dab49c53128..f425238d5d4f 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -463,6 +463,44 @@ void __khugepaged_enter(struct mm_struct *mm)
>  		wake_up_interruptible(&khugepaged_wait);
>  }
>  
> +/**
> + * collapse_max_ptes_none - Calculate maximum allowed empty PTEs for collapse
> + * @order: The folio order being collapsed to
> + * @full_scan: Whether this is a full scan (ignore limits)
> + *
> + * For madvise-triggered collapses (full_scan=true), all limits are bypassed
> + * and allow up to HPAGE_PMD_NR - 1 empty PTEs.
> + *
> + * For PMD-sized collapses (order == HPAGE_PMD_ORDER), use the configured
> + * khugepaged_max_ptes_none value.
> + *
> + * For mTHP collapses, we currently only support khugepaged_max_pte_none values
> + * of 0 or (HPAGE_PMD_NR - 1). Any other value will emit a warning and no mTHP
> + * collapse will be attempted
> + *
> + * Return: Maximum number of empty PTEs allowed for the collapse operation
> + */
> +static unsigned int collapse_max_ptes_none(unsigned int order, bool full_scan)
> +{
> +	/* ignore max_ptes_none limits */
> +	if (full_scan)
> +		return HPAGE_PMD_NR - 1;
> +
> +	if (!is_mthp_order(order))
> +		return khugepaged_max_ptes_none;
> +
> +	/* Zero/non-present collapse disabled. */
> +	if (!khugepaged_max_ptes_none)
> +		return 0;
> +
> +	if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1)
> +		return (1 << order) - 1;
> +
> +	pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %d\n",
> +		      HPAGE_PMD_NR - 1);
> +	return -EINVAL;
> +}
> +
>  void khugepaged_enter_vma(struct vm_area_struct *vma,
>  			  vm_flags_t vm_flags)
>  {
> @@ -550,7 +588,10 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  	pte_t *_pte;
>  	int none_or_zero = 0, shared = 0, result = SCAN_FAIL, referenced = 0;
>  	const unsigned long nr_pages = 1UL << order;
> -	int max_ptes_none = khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - order);
> +	int max_ptes_none = collapse_max_ptes_none(order, !cc->is_khugepaged);
> +
> +	if (max_ptes_none == -EINVAL)
> +		goto out;
>  
>  	for (_pte = pte; _pte < pte + nr_pages;
>  	     _pte++, addr += PAGE_SIZE) {