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

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/25/26 14:00, Zi Yan wrote:
> 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@nvidia.com>
>> ---
>>  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.

Makes sense. The comment above the filemap_flush() is valuable :)

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

That would be better. Do we plan on handling races with concurrent writable-opening?

Or could we simply gate the filemap_flush() by a inode_is_open_for_write(), to
not have it turn into a kwritebackd()?

-- 
Cheers,

David