linux-kernel - Re: [PATCH v6] hugetlbfs: move lock assertions after early returns in huge_pmd

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Message-ID: <e6bb05f7-8f05-409f-9d87-2d25f66942a9@redhat.com>
Date: Tue, 14 Oct 2025 11:35:46 +0200
From: David Hildenbrand <david@...hat.com>
To: Deepanshu Kartikey <kartikey406@...il.com>, muchun.song@...ux.dev,
 osalvador@...e.de, akpm@...ux-foundation.org, broonie@...nel.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 syzbot+f26d7c75c26ec19790e7@...kaller.appspotmail.com
Subject: Re: [PATCH v6] hugetlbfs: move lock assertions after early returns in
 huge_pmd_unshare()

On 14.10.25 07:06, Deepanshu Kartikey wrote:
> When hugetlb_vmdelete_list() processes VMAs during truncate operations,
> it may encounter VMAs where huge_pmd_unshare() is called without the
> required shareable lock. This triggers an assertion failure in
> hugetlb_vma_assert_locked().
> 
> The previous fix in commit dd83609b8898 ("hugetlbfs: skip VMAs without
> shareable locks in hugetlb_vmdelete_list") skipped entire VMAs without
> shareable locks to avoid the assertion. However, this prevented pages
> from being unmapped and freed, causing a regression in fallocate(PUNCH_HOLE)
> operations where pages were not freed immediately, as reported by Mark Brown.
> 
> A subsequent fix in commit 06e8ca1b3dca ("hugetlbfs: check for shareable
> lock before calling huge_pmd_unshare()") addressed this by checking
> __vma_shareable_lock() in the caller before calling huge_pmd_unshare().
> However, a cleaner approach is to move the lock assertions in
> huge_pmd_unshare() itself to after the early return checks. The assertions
> are only needed when actual PMD unsharing work will be performed. If the
> function returns early because sz != PMD_SIZE or the PMD is not shared,
> no locks are required.
> 
> This patch removes the check added in commit 06e8ca1b3dca ("hugetlbfs:
> check for shareable lock before calling huge_pmd_unshare()") and instead
> moves the assertions inside huge_pmd_unshare(), keeping all the logic
> within the function itself.
> 
> Reported-by: syzbot+f26d7c75c26ec19790e7@...kaller.appspotmail.com
> Reported-by: Mark Brown <broonie@...nel.org>
> Closes: https://syzkaller.appspot.com/bug?extid=f26d7c75c26ec19790e7
> Fixes: dd83609b8898 ("hugetlbfs: skip VMAs without shareable locks in hugetlb_vmdelete_list")
> Suggested-by: David Hildenbrand <david@...hat.com>
> Suggested-by: Oscar Salvador <osalvador@...e.de>
> Tested-by: syzbot+f26d7c75c26ec19790e7@...kaller.appspotmail.com
> Link: https://lore.kernel.org/mm-commits/20250925203504.7BE02C4CEF7@smtp.kernel.org/ [v1]
> Link: https://lore.kernel.org/mm-commits/20250928185232.BEDB6C4CEF0@smtp.kernel.org/ [v2]
> Link: https://lore.kernel.org/linux-mm/20251003174553.3078839-1-kartikey406@gmail.com/ [v3]
> Link: https://lore.kernel.org/linux-mm/20251008052759.469714-1-kartikey406@gmail.com/ [v4]
> Link: https://lore.kernel.org/linux-mm/CADhLXY72yEVDjXWfxBUXfXhNfb8MWqwJmcb1daEHmDeFW+DRGw@mail.gmail.com/ [v5]
> Signed-off-by: Deepanshu Kartikey <kartikey406@...il.com>
> ---
> Changes in v6:
> - Remove __vma_shareable_lock() check from __unmap_hugepage_range()
>    that was added in v4 (commit 06e8ca1b3dca)
> - Move lock assertions after early returns in huge_pmd_unshare()
> - Complete implementation of David's cleaner approach
> 
> Changes in v5:
> - Incomplete: only moved assertions, forgot to remove v4 check
> 
> Changes in v4:
> - Check __vma_shareable_lock() in __unmap_hugepage_range() before calling
>    huge_pmd_unshare() per Oscar's suggestion
> 
> Changes in v3:
> - Add ZAP_FLAG_NO_UNSHARE to skip only PMD unsharing
> 
> Changes in v2:
> - Skip entire VMAs without shareable locks (caused PUNCH_HOLE regression)
> 
> Changes in v1:
> - Initial fix attempt
> ---
>   mm/hugetlb.c | 7 +++----
>   1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 85b2dac79d25..0455119716ec 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5885,7 +5885,7 @@ void __unmap_hugepage_range(struct mmu_gather *tlb, struct vm_area_struct *vma,
>   		}
>   
>   		ptl = huge_pte_lock(h, mm, ptep);
> -		if (__vma_shareable_lock(vma) && huge_pmd_unshare(mm, vma, address, ptep)) {
> +		if (huge_pmd_unshare(mm, vma, address, ptep)) {
>   			spin_unlock(ptl);
>   			tlb_flush_pmd_range(tlb, address & PUD_MASK, PUD_SIZE);
>   			force_flush = true;
> @@ -7614,13 +7614,12 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
>   	p4d_t *p4d = p4d_offset(pgd, addr);
>   	pud_t *pud = pud_offset(p4d, addr);
>   
> -	i_mmap_assert_write_locked(vma->vm_file->f_mapping);
> -	hugetlb_vma_assert_locked(vma);
>   	if (sz != PMD_SIZE)
>   		return 0;
>   	if (!ptdesc_pmd_is_shared(virt_to_ptdesc(ptep)))
>   		return 0;
> -
> +	i_mmap_assert_write_locked(vma->vm_file->f_mapping);
> +	hugetlb_vma_assert_locked(vma);
>   	pud_clear(pud);
>   	/*
>   	 * Once our caller drops the rmap lock, some other process might be

Can you resend based on mm/mm-stable please? Looks like this one is 
based on v5.

-- 
Cheers

David / dhildenb