Re: [PATCH mm-unstable v4 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd()

[Lance Yang <lance.yang@linux.dev>]

On 2026/3/31 22:01, Lorenzo Stoakes (Oracle) wrote:
> OK we need a fairly urgent fix for this as this has triggered a syzbot. See [0]
> for an analysis.
> 
> I show inline where the issue is, and attach a fix-patch for the bug.
> 
> [0]: https://lore.kernel.org/all/e1cb33b8-c1f7-4972-8628-3a2169077d6e@lucifer.local/
> 
> See below for details.
> 
> Cheers, Lorenzo
> 
[...]
> 
> Fix patch follows:
> 
> ----8<----
>  From a4dfc7718a15035449f344a0bc7f58e449366405 Mon Sep 17 00:00:00 2001
> From: "Lorenzo Stoakes (Oracle)" <ljs@kernel.org>
> Date: Tue, 31 Mar 2026 13:11:18 +0100
> Subject: [PATCH] mm/khugepaged: fix issue with tracking lock
> 
> We are incorrectly treating lock_dropped to track both whether the lock is
> currently held and whether or not the lock was ever dropped.

Good catch!

Right, lock_dropped is not supposed to mean "is the mmap lock currently
unlocked?", it should mean "was the mmap lock dropped at any point
during MADV_COLLAPSE?"

> 
> Update this change to account for this.
> 
> Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> ---

Thanks for the fix!
Reviewed-by: Lance Yang <lance.yang@linux.dev>

>   mm/khugepaged.c | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index d21348b85a59..b8452dbdb043 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2828,6 +2828,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>   	unsigned long hstart, hend, addr;
>   	enum scan_result last_fail = SCAN_FAIL;
>   	int thps = 0;
> +	bool mmap_unlocked = false;
> 
>   	BUG_ON(vma->vm_start > start);
>   	BUG_ON(vma->vm_end < end);
> @@ -2850,10 +2851,11 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>   	for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
>   		enum scan_result result = SCAN_FAIL;
> 
> -		if (*lock_dropped) {
> +		if (mmap_unlocked) {
>   			cond_resched();
>   			mmap_read_lock(mm);
> -			*lock_dropped = false;
> +			mmap_unlocked = false;
> +			*lock_dropped = true;
>   			result = hugepage_vma_revalidate(mm, addr, false, &vma,
>   							cc);
>   			if (result  != SCAN_SUCCEED) {
> @@ -2864,7 +2866,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
>   			hend = min(hend, vma->vm_end & HPAGE_PMD_MASK);
>   		}
> 
> -		result = collapse_single_pmd(addr, vma, lock_dropped, cc);
> +		result = collapse_single_pmd(addr, vma, &mmap_unlocked, cc);
> 
>   		switch (result) {
>   		case SCAN_SUCCEED:
> @@ -2893,8 +2895,10 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
> 
>   out_maybelock:
>   	/* Caller expects us to hold mmap_lock on return */
> -	if (*lock_dropped)
> +	if (mmap_unlocked) {
> +		*lock_dropped = true;
>   		mmap_read_lock(mm);
> +	}
>   out_nolock:
>   	mmap_assert_locked(mm);
>   	mmdrop(mm);
> --
> 2.53.0