linux-kernel - Re: [PATCHv2] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on SCAN_PMD

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <c00d14a8-c34a-4f0d-8c7d-b1b1ec329c0b@arm.com>
Date: Mon, 15 Sep 2025 20:38:06 +0530
From: Dev Jain <dev.jain@....com>
To: Kiryl Shutsemau <kirill@...temov.name>,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Barry Song <baohua@...nel.org>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, nd@....com
Subject: Re: [PATCHv2] mm/khugepaged: Do not fail collapse_pte_mapped_thp() on
 SCAN_PMD_NULL


On 15/09/25 7:22 pm, Kiryl Shutsemau wrote:
> From: Kiryl Shutsemau <kas@...nel.org>
>
> MADV_COLLAPSE on a file mapping behaves inconsistently depending on if
> PMD page table is installed or not.
>
> Consider following example:
>
> 	p = mmap(NULL, 2UL << 20, PROT_READ | PROT_WRITE,
> 		 MAP_SHARED, fd, 0);
> 	err = madvise(p, 2UL << 20, MADV_COLLAPSE);
>
> fd is a populated tmpfs file.
>
> The result depends on the address that the kernel returns on mmap().
> If it is located in an existing PMD table, the madvise() will succeed.
> However, if the table does not exist, it will fail with -EINVAL.
>
> This occurs because find_pmd_or_thp_or_none() returns SCAN_PMD_NULL when
> a page table is missing, which causes collapse_pte_mapped_thp() to fail.
>
> SCAN_PMD_NULL and SCAN_PMD_NONE should be treated the same in
> collapse_pte_mapped_thp(): install the PMD leaf entry and allocate page
> tables as needed.
>
> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
> ---
>
> v2:
>   - Modify set_huge_pmd() instead of introducing install_huge_pmd();
>
> ---
>   mm/khugepaged.c | 20 +++++++++++++++++++-
>   1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b486c1d19b2d..986718599355 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1472,15 +1472,32 @@ static void collect_mm_slot(struct khugepaged_mm_slot *mm_slot)
>   static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr,
>   			pmd_t *pmdp, struct folio *folio, struct page *page)
>   {
> +	struct mm_struct *mm = vma->vm_mm;
>   	struct vm_fault vmf = {
>   		.vma = vma,
>   		.address = addr,
>   		.flags = 0,
> -		.pmd = pmdp,
>   	};
> +	pgd_t *pgdp;
> +	p4d_t *p4dp;
> +	pud_t *pudp;
>   
>   	mmap_assert_locked(vma->vm_mm);

I was going to reply to v1 - you could replace vma->vm_mm
with mm here.

Reviewed-by: Dev Jain <dev.jain@....com>