linux-kernel - Re: [PATCH v4] hugetlbfs: check for shareable lock before calling huge_pmd

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Message-ID: <adf3cb95-916b-4513-b763-48aa8fbfb700@redhat.com>
Date: Mon, 13 Oct 2025 10:27:28 +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 v4] hugetlbfs: check for shareable lock before calling
 huge_pmd_unshare()

On 08.10.25 07:27, 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.
> 
> Instead of skipping VMAs or adding new flags, check __vma_shareable_lock()
> directly in __unmap_hugepage_range() right before calling huge_pmd_unshare().
> This ensures PMD unsharing only happens when the VMA has a shareable lock
> structure, while still allowing page unmapping and freeing to proceed for
> all VMAs.
> 
> Reported-by: syzbot+f26d7c75c26ec19790e7@...kaller.appspotmail.com
> Tested-by: syzbot+f26d7c75c26ec19790e7@...kaller.appspotmail.com
> Reported-by: Mark Brown <broonie@...nel.org>
> Fixes: dd83609b8898 ("hugetlbfs: skip VMAs without shareable locks in hugetlb_vmdelete_list")
> Suggested-by: Oscar Salvador <osalvador@...e.de>
> Suggested-by: David Hildenbrand <david@...hat.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]
> Signed-off-by: Deepanshu Kartikey <kartikey406@...il.com>
> ---
> Changes in v4:
> - Simplified approach per Oscar's suggestion: check __vma_shareable_lock()
>    directly in __unmap_hugepage_range() before calling huge_pmd_unshare()
> - Removed ZAP_FLAG_NO_UNSHARE flag per David's feedback to avoid polluting
>    generic mm.h header
> - Reverted hugetlb_vmdelete_list() to not skip VMAs
> 
> Changes in v3:
> - Added ZAP_FLAG_NO_UNSHARE to skip only PMD unsharing, not entire VMA
> 
> Changes in v2:
> - Skip entire VMAs without shareable locks in hugetlb_vmdelete_list()
>    (caused PUNCH_HOLE regression)
> 
> Changes in v1:
> - Initial fix attempt
> ---
>   fs/hugetlbfs/inode.c | 10 +---------
>   mm/hugetlb.c         |  2 +-
>   2 files changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index 9c94ed8c3ab0..1e040db18b20 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -478,14 +478,6 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
>   		if (!hugetlb_vma_trylock_write(vma))
>   			continue;
>   
> -		/*
> -		 * Skip VMAs without shareable locks. Per the design in commit
> -		 * 40549ba8f8e0, these will be handled by remove_inode_hugepages()
> -		 * called after this function with proper locking.
> -		 */
> -		if (!__vma_shareable_lock(vma))
> -			goto skip;
> -
>   		v_start = vma_offset_start(vma, start);
>   		v_end = vma_offset_end(vma, end);
>   
> @@ -496,7 +488,7 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
>   		 * vmas.  Therefore, lock is not held when calling
>   		 * unmap_hugepage_range for private vmas.
>   		 */
> -skip:
> +
>   		hugetlb_vma_unlock_write(vma);
>   	}
>   }
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6cac826cb61f..9ed85ab8420e 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 (huge_pmd_unshare(mm, vma, address, ptep)) {
> +		if (__vma_shareable_lock(vma) && huge_pmd_unshare(mm, vma, address, ptep)) {
>   			spin_unlock(ptl);
>   			tlb_flush_pmd_range(tlb, address & PUD_MASK, PUD_SIZE);
>   			force_flush = true;

Wondering, couldn't we handle that in huge_pmd_unshare()?

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index eed59cfb5d218..f167cec4a5acc 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -7598,13 +7598,14 @@ 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_pts_count(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

-- 
Cheers

David / dhildenb