Re: [PATCH 2/2] mm: shmem: fix incorrect aligned index when checking conflicts

[David Hildenbrand]

On 31.07.24 07:46, Baolin Wang wrote:
> In the shmem_suitable_orders() function, xa_find() is used to check for
> conflicts in the pagecache to select suitable huge orders. However, when
> checking each huge order in every loop, the aligned index is calculated
> from the previous iteration, which may cause suitable huge orders to be
> missed.
>
> We should use the original index each time in the loop to calculate a
> new aligned index for checking conflicts to avoid this issue.
>
> Fixes: e7a2ab7b3bb5 ("mm: shmem: add mTHP support for anonymous shmem")
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> ---
>   mm/shmem.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index a4332a97558c..6e9836b1bd1d 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1686,6 +1686,7 @@ static unsigned long shmem_suitable_orders(struct inode *inode, struct vm_fault
>   					   unsigned long orders)
>   {
>   	struct vm_area_struct *vma = vmf->vma;
> +	pgoff_t aligned_index;
>   	unsigned long pages;
>   	int order;
>   
> @@ -1697,9 +1698,9 @@ static unsigned long shmem_suitable_orders(struct inode *inode, struct vm_fault
>   	order = highest_order(orders);
>   	while (orders) {
>   		pages = 1UL << order;
> -		index = round_down(index, pages);
> -		if (!xa_find(&mapping->i_pages, &index,
> -			     index + pages - 1, XA_PRESENT))
> +		aligned_index = round_down(index, pages);
> +		if (!xa_find(&mapping->i_pages, &aligned_index,
> +			     aligned_index + pages - 1, XA_PRESENT))
>   			break;
>   		order = next_order(&orders, order);
>   	}

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb