Re: [PATCH 1/2] mm: shmem: fix incorrect index alignment for within_size policy

[David Hildenbrand <david@redhat.com>]

On 19.12.24 08:30, Baolin Wang wrote:
> With enabling the shmem per-size within_size policy, using an incorrect
> 'order' size to round_up() the index can lead to incorrect i_size checks,
> resulting in an inappropriate large orders being returned.
> 
> Changing to use '1 << order' to round_up() the index to fix this issue.
> Additionally, adding an 'aligned_index' variable to avoid affecting the
> index checks.
> 
> Fixes: e7a2ab7b3bb5 ("mm: shmem: add mTHP support for anonymous shmem")
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
> Hi Andrew,
> 
> These two bugfix patches are based on the mm-hotfixes-unstable branch,
> and this patch has a slight conflict with my previous patch set:
> "Support large folios for tmpfs". However, I think the conflicts are
> easy to resolve. If you need me to rebase and resend the
> "Support large folios for tmpfs" patch set, please let me know.
> Sorry for the troubles :)
> ---
>   mm/shmem.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index f6fb053ac50d..dec659e84562 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1689,6 +1689,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>   	unsigned long mask = READ_ONCE(huge_shmem_orders_always);
>   	unsigned long within_size_orders = READ_ONCE(huge_shmem_orders_within_size);
>   	unsigned long vm_flags = vma ? vma->vm_flags : 0;
> +	pgoff_t aligned_index;
>   	bool global_huge;
>   	loff_t i_size;
>   	int order;
> @@ -1723,9 +1724,9 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>   	/* Allow mTHP that will be fully within i_size. */
>   	order = highest_order(within_size_orders);
>   	while (within_size_orders) {
> -		index = round_up(index + 1, order);
> +		aligned_index = round_up(index + 1, 1 << order);
>   		i_size = round_up(i_size_read(inode), PAGE_SIZE);
> -		if (i_size >> PAGE_SHIFT >= index) {
> +		if (i_size >> PAGE_SHIFT >= aligned_index) {
>   			mask |= within_size_orders;
>   			break;
>   		}


Yes, that matches the logic in shmem_huge_global_enabled().

Acked-by: David Hildenbrand <david@redhat.com>


Was wondering if one can factor that out into a helper where one could 
pass an optional write_end ...

-- 
Cheers,

David / dhildenb