Re: [PATCH v1 05/10] mm/huge_memory: remove READ_ONLY_THP_FOR_FS from file_thp_enabled()

[Zi Yan]

On 27 Mar 2026, at 12:08, Lorenzo Stoakes (Oracle) wrote:

> On Fri, Mar 27, 2026 at 11:43:57AM -0400, Zi Yan wrote:
>> On 27 Mar 2026, at 11:29, Lorenzo Stoakes (Oracle) wrote:
>>
>>> On Fri, Mar 27, 2026 at 11:12:46AM -0400, Zi Yan wrote:
>>>> On 27 Mar 2026, at 8:42, Lorenzo Stoakes (Oracle) wrote:
>>>>
>>>>> On Thu, Mar 26, 2026 at 09:42:50PM -0400, Zi Yan wrote:
>>>>>> Replace it with a check on the max folio order of the file's address space
>>>>>> mapping, making sure PMD_ORDER is supported.
>>>>>>
>>>>>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>>>>>> ---
>>>>>>  mm/huge_memory.c | 6 +++---
>>>>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>>>> index c7873dbdc470..1da1467328a3 100644
>>>>>> --- a/mm/huge_memory.c
>>>>>> +++ b/mm/huge_memory.c
>>>>>> @@ -89,9 +89,6 @@ 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;
>>>>>>
>>>>>> @@ -100,6 +97,9 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
>>>>>>  	if (IS_ANON_FILE(inode))
>>>>>>  		return false;
>>>>>>
>>>>>> +	if (mapping_max_folio_order(inode->i_mapping) < PMD_ORDER)
>>>>>> +		return false;
>>>>>> +
>>>>>
>>>>> At this point I think this should be a separate function quite honestly and
>>>>> share it with 2/10's use, and then you can put the comment in here re: anon
>>>>> shmem etc.
>>>>>
>>>>> Though that won't apply here of course as shmem_allowable_huge_orders() would
>>>>> have been invoked :)
>>>>>
>>>>> But no harm in refactoring it anyway, and the repetitive < PMD_ORDER stuff is
>>>>> unfortunate.
>>>>>
>>>>> Buuut having said that is this right actually?
>>>>>
>>>>> Because we have:
>>>>>
>>>>> 		if (((in_pf || smaps)) && vma->vm_ops->huge_fault)
>>>>> 			return orders;
>>>>>
>>>>> Above it, and now you're enabling huge folio file systems to do non-page fault
>>>>> THP and that's err... isn't that quite a big change?
>>>>
>>>> That is what READ_ONLY_THP_FOR_FS does, creating THPs after page faults, right?
>>>> This patchset changes the condition from all FSes to FSes with large folio
>>>> support.
>>>
>>> No, READ_ONLY_THP_FOR_FS operates differently.
>>>
>>> It explicitly _only_ is allowed for MADV_COLLAPSE and only if the file is
>>> mounted read-only.
>>>
>>> So due to:
>>>
>>> 		if (((in_pf || smaps)) && vma->vm_ops->huge_fault)
>>> 			return orders;
>>>
>>> 		if (((!in_pf || smaps)) && file_thp_enabled(vma))
>>> 			return orders;
>>>
>>>                       |    PF     | MADV_COLLAPSE | khugepaged |
>>> 		      |-----------|---------------|------------|
>>> large folio fs        |     ✓     |       x       |      x     |
>>> READ_ONLY_THP_FOR_FS  |     x     |       ✓       |      ✓     |
>>>
>>> After this change:
>>>
>>>                       |    PF     | MADV_COLLAPSE | khugepaged |
>>> 		      |-----------|---------------|------------|
>>> large folio fs        |     ✓     |       ✓       |      ?     |
>>>
>>> (I hope we're not enabling khugepaged for large folio fs - which shouldn't
>>> be necessary anyway as we try to give them folios on page fault and they
>>> use thp-friendly get_unused_area etc. :)
>>>
>>> We shouldn't be doing this.
>>>
>>> It should remain:
>>>
>>>                       |    PF     | MADV_COLLAPSE | khugepaged |
>>> 		      |-----------|---------------|------------|
>>> large folio fs        |     ✓     |       x       |      x     |
>>>
>>> If we're going to remove it, we should first _just remove it_, not
>>> simultaneously increase the scope of what all the MADV_COLLAPSE code is
>>> doing without any confidence in any of it working properly.
>>>
>>> And it makes the whole series misleading - you're actually _enabling_ a
>>> feature not (only) _removing_ one.
>>
>> That is what my RFC patch does, but David and willy told me to do this. :)
>> IIUC, with READ_ONLY_THP_FOR_FS, FSes with large folio support will
>> get THP via MADV_COLLAPSE or khugepaged. So removing the code like I
>> did in RFC would cause regressions.
>
> OK I think we're dealing with a union of the two states here.
>
> READ_ONLY_THP_FOR_FS is separate from large folio support, as checked by
> file_thp_enabled():
>
> 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;
>
> 	inode = file_inode(vma->vm_file);
>
> 	if (IS_ANON_FILE(inode))
> 		return false;
>
> 	return !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);
> }
>
> So actually:
>
>                        |    PF     | MADV_COLLAPSE | khugepaged |
> 		       |-----------|---------------|------------|
>  large folio fs        |     ✓     |       x       |      x     |
>  READ_ONLY_THP_FOR_FS  |     x     |       ✓       |      ✓     |
>  both!                 |     ✓     |       ✓       |      ✓     |
>
> (Where it's impllied it's a read-only mapping obviously for the later two
> cases.)
>
> Now without READ_ONLY_THP_FOR_FS you're going to:
>
>                        |    PF     | MADV_COLLAPSE | khugepaged |
> 		       |-----------|---------------|------------|
>  large folio fs        |     ✓     |       x       |      x     |
>  large folio + r/o     |     ✓     |       ✓       |      ✓     |
>
> And intentionally leaving behind the 'not large folio fs, r/o' case because
> those file systems need to implement large folio support.
>
> I guess we'll regress those users but we don't care?

Yes. This also motivates FSes without large folio support to add large folio
support instead of relying on READ_ONLY_THP_FOR_FS hack.

>
> I do think all this needs to be spelled out in the commit message though as it's
> subtle.
>
> Turns out this PitA config option is going to kick and scream a bit first before
> it goes...

Sure. I will shameless steal your tables. Thank you for the contribution. ;)

>
>>
>> I guess I need to rename the series to avoid confusion. How about?
>>
>> Remove read-only THP support for FSes without large folio support.
>
> Yup that'd be better :)
>
> Cheers, Lorenzo
>
>>
>> [1] https://lore.kernel.org/all/7382046f-7c58-4a3e-ab34-b2704355b7d5@xxxxxxxxxx/
>>
>>>
>>> So let's focus as David suggested on one thing at a time, incrementally.
>>>
>>> And let's please try and sort some of this confusing mess out in the code
>>> if at all possible...
>>>
>>>>
>>>> Will add a helper, mapping_support_pmd_folio(), for
>>>> mapping_max_folio_order(inode->i_mapping) < PMD_ORDER.
>>>>
>>>>>
>>>>> So yeah probably no to this patch as is :) we should just drop
>>>>> file_thp_enabled()?
>>>>
>>>>
>>>>
>>>>>
>>>>>>  	return !inode_is_open_for_write(inode) && S_ISREG(inode->i_mode);
>>>>>>  }
>>>>>>
>>>>>> --
>>>>>> 2.43.0
>>>>>>
>>>>
>>>>
>>>> Best Regards,
>>>> Yan, Zi
>>>
>>> Cheers, Lorenzo
>>
>>
>> Best Regards,
>> Yan, Zi


Best Regards,
Yan, Zi