Re: [PATCH 7.2 v3 03/12] mm/huge_memory: remove READ_ONLY_THP_FOR_FS from file_thp_enabled()

[Zi Yan]

On 17 Apr 2026, at 22:44, Zi Yan wrote:

> Replace it with a check on the max folio order of the file's address space
> mapping, making sure PMD THP is supported. Also remove the read-only fd
> check, since collapse_file() now makes sure all to-be-collapsed folios are
> clean and the created PMD file THP can be handled by FSes properly.
>
> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 970e077019b7..7e9cf8c0985f 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -86,9 +86,6 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
>  {
>  	struct inode *inode;
>
> -	if (!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS))
> -		return false;
> -
>  	if (!vma->vm_file)
>  		return false;
>
> @@ -97,7 +94,10 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
>  	if (IS_ANON_FILE(inode))
>  		return false;
>
> -	return !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);

Hi Matthew, David, and Lorenzo,

After some discussions on irc, I feel that we probably should not allow
read-write fd for PMD THP collapse at the moment. Combining with the
filemap_flush() under folio_dirty() check from collapse_file(), khugepaged
would become a kwritebackd that scans pagecache folios and writes them back.
If we limit it to read-only fds, at least khugepaged would only write
back once for the pagecache folios from these fds.

I am planning to restore inode_is_open_for_write() check in the next version.
Let me know your thoughts.

> +	if (!mapping_pmd_thp_support(inode->i_mapping))
> +		return false;
> +
> +	return S_ISREG(inode->i_mode);
>  }
>
>  /* If returns true, we are unable to access the VMA's folios. */
> -- 
> 2.43.0


--
Best Regards,
Yan, Zi