Re: [PATCH 7.2 v2 01/12] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

[David Hildenbrand (Arm)]

On 4/13/26 21:20, Zi Yan wrote:
> collapse_file() requires FSes supporting large folio with at least
> PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that.
> MADV_COLLAPSE ignores shmem huge config, so exclude the check for shmem.
> 
> While at it, replace VM_BUG_ON with VM_WARN_ON_ONCE.
> 
> In collapse_scan_file(), add FS eligibility check to avoid redundant scans.
> 
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
>  mm/khugepaged.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b8452dbdb043..d2f0acd2dac2 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1892,8 +1892,9 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
>  	int nr_none = 0;
>  	bool is_shmem = shmem_file(file);
>  
> -	VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
> -	VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
> +	/* MADV_COLLAPSE ignores shmem huge config, so do not check shmem */
> +	VM_WARN_ON_ONCE(!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER);
> +	VM_WARN_ON_ONCE(start & (HPAGE_PMD_NR - 1));
>  
>  	result = alloc_charge_folio(&new_folio, mm, cc);
>  	if (result != SCAN_SUCCEED)
> @@ -2321,6 +2322,13 @@ static enum scan_result collapse_scan_file(struct mm_struct *mm,
>  	int node = NUMA_NO_NODE;
>  	enum scan_result result = SCAN_SUCCEED;
>  
> +	/*
> +	 * skip files without PMD-order folio support
> +	 * do not check shmem, since MADV_COLLAPSE ignores shmem huge config
> +	 */

How is the !collapse path handled? Through thp_vma_allowable_order() in
collapse_scan_mm_slot()?

Wouldn't it be better to have that check exactly there?

-- 
Cheers,

David