Re: [PATCH v1 02/10] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

[Zi Yan]

On 27 Mar 2026, at 12:22, Lance Yang wrote:

> On Fri, Mar 27, 2026 at 11:00:26AM -0400, Zi Yan wrote:
>> On 27 Mar 2026, at 10:31, Lorenzo Stoakes (Oracle) wrote:
>>
>>> On Fri, Mar 27, 2026 at 10:26:53PM +0800, Baolin Wang wrote:
>>>>
>>>>
>>>> On 3/27/26 10:12 PM, Lorenzo Stoakes (Oracle) wrote:
>>>>> On Fri, Mar 27, 2026 at 09:45:03PM +0800, Baolin Wang wrote:
>>>>>>
>>>>>>
>>>>>> On 3/27/26 8:02 PM, Lorenzo Stoakes (Oracle) wrote:
>>>>>>> On Fri, Mar 27, 2026 at 05:44:49PM +0800, Baolin Wang wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On 3/27/26 9:42 AM, 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. shmem with
>>>>>>>>> huge option turned on also sets large folio order on mapping, so the check
>>>>>>>>> also applies to shmem.
>>>>>>>>>
>>>>>>>>> While at it, replace VM_BUG_ON with returning failure values.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
>>>>>>>>> ---
>>>>>>>>>     mm/khugepaged.c | 7 +++++--
>>>>>>>>>     1 file changed, 5 insertions(+), 2 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>>>>>>>>> index d06d84219e1b..45b12ffb1550 100644
>>>>>>>>> --- a/mm/khugepaged.c
>>>>>>>>> +++ b/mm/khugepaged.c
>>>>>>>>> @@ -1899,8 +1899,11 @@ 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));
>>>>>>>>> +	/* "huge" shmem sets mapping folio order and passes the check below */
>>>>>>>>> +	if (mapping_max_folio_order(mapping) < PMD_ORDER)
>>>>>>>>> +		return SCAN_FAIL;
>>>>>>>>
>>>>>>>> This is not true for anonymous shmem, since its large order allocation logic
>>>>>>>> is similar to anonymous memory. That means it will not call
>>>>>>>> mapping_set_large_folios() for anonymous shmem.
>>>>>>>>
>>>>>>>> So I think the check should be:
>>>>>>>>
>>>>>>>> if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER)
>>>>>>>>        return SCAN_FAIL;
>>>>>>>
>>>>>>> Hmm but in shmem_init() we have:
>>>>>>>
>>>>>>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>>>>> 	if (has_transparent_hugepage() && shmem_huge > SHMEM_HUGE_DENY)
>>>>>>> 		SHMEM_SB(shm_mnt->mnt_sb)->huge = shmem_huge;
>>>>>>> 	else
>>>>>>> 		shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */
>>>>>>>
>>>>>>> 	/*
>>>>>>> 	 * Default to setting PMD-sized THP to inherit the global setting and
>>>>>>> 	 * disable all other multi-size THPs.
>>>>>>> 	 */
>>>>>>> 	if (!shmem_orders_configured)
>>>>>>> 		huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER);
>>>>>>> #endif
>>>>>>>
>>>>>>> And shm_mnt->mnt_sb is the superblock used for anon shmem. Also
>>>>>>> shmem_enabled_store() updates that if necessary.
>>>>>>>
>>>>>>> So we're still fine right?
>>>>>>>
>>>>>>> __shmem_file_setup() (used for anon shmem) calls shmem_get_inode() ->
>>>>>>> __shmem_get_inode() which has:
>>>>>>>
>>>>>>> 	if (sbinfo->huge)
>>>>>>> 		mapping_set_large_folios(inode->i_mapping);
>>>>>>>
>>>>>>> Shared for both anon shmem and tmpfs-style shmem.
>>>>>>>
>>>>>>> So I think it's fine as-is.
>>>>>>
>>>>>> I'm afraid not. Sorry, I should have been clearer.
>>>>>>
>>>>>> First, anonymous shmem large order allocation is dynamically controlled via
>>>>>> the global interface (/sys/kernel/mm/transparent_hugepage/shmem_enabled) and
>>>>>> the mTHP interfaces
>>>>>> (/sys/kernel/mm/transparent_hugepage/hugepages-*kB/shmem_enabled).
>>>>>>
>>>>>> This means that during anonymous shmem initialization, these interfaces
>>>>>> might be set to 'never'. so it will not call mapping_set_large_folios()
>>>>>> because sbinfo->huge is 'SHMEM_HUGE_NEVER'.
>>>>>>
>>>>>> Even if shmem large order allocation is subsequently enabled via the
>>>>>> interfaces, __shmem_file_setup -> mapping_set_large_folios() is not called
>>>>>> again.
>>>>>
>>>>> I see your point, oh this is all a bit of a mess...
>>>>>
>>>>> It feels like entirely the wrong abstraction anyway, since at best you're
>>>>> getting a global 'is enabled'.
>>>>>
>>>>> I guess what happened before was we'd never call into this with ! r/o thp for fs
>>>>> && ! is_shmem.
>>>>
>>>> Right.
>>>>
>>>>> But now we are allowing it, but should STILL be gating on !is_shmem so yeah your
>>>>> suggestion is correct I think actualyl.
>>>>>
>>>>> I do hate:
>>>>>
>>>>> 	if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER)
>>>>>
>>>>> As a bit of code though. It's horrible.
>>>>
>>>> Indeed.
>>>>
>>>>> Let's abstract that...
>>>>>
>>>>> It'd be nice if we could find a way to clean things up in the lead up to changes
>>>>> in series like this instead of sticking with the mess, but I guess since it
>>>>> mostly removes stuff that's ok for now.
>>>>
>>>> I think this check can be removed from this patch.
>>>>
>>>> During the khugepaged's scan, it will call thp_vma_allowable_order() to
>>>> check if the VMA is allowed to collapse into a PMD.
>>>>
>>>> Specifically, within the call chain thp_vma_allowable_order() ->
>>>> __thp_vma_allowable_orders(), shmem is checked via
>>>> shmem_allowable_huge_orders(), while other FSes are checked via
>>>> file_thp_enabled().
>>
>> But for madvise(MADV_COLLAPSE) case, IIRC, it ignores shmem huge config
>> and can perform collapse anyway. This means without !is_shmem the check
>> will break madvise(MADV_COLLAPSE). Let me know if I get it wrong, since
>
> Right. That will break MADV_COLLAPSE, IIUC.
>
> For MADV_COLLAPSE on anonymous shmem, eligibility is determined by the
> TVA_FORCED_COLLAPSE path via shmem_allowable_huge_orders(), not by
> whether the inode mapping got mapping_set_large_folios() at creation
> time.
>
> Using mmap(MAP_SHARED | MAP_ANONYMOUS):
> - create time: shmem_enabled=never, hugepages-2048kB/shmem_enabled=never
> - collapse time: shmem_enabled=never, hugepages-2048kB/shmem_enabled=always
>
> With the !is_shmem guard, collapse succeeds. Without it, the same setup
> fails with -EINVAL.

Thank you for the confirmation. I will fix it.

>
> Thanks,
> Lance
>
>> I was in that TVA_FORCED_COLLAPSE email thread but does not remember
>> everything there.
>>
>>
>>>
>>> It sucks not to have an assert. Maybe in that case make it a
>>> VM_WARN_ON_ONCE().
>>
>> Will do that as I replied to David already.
>>
>>>
>>> I hate that you're left tracing things back like that...
>>>
>>>>
>>>> For those other filesystems, Patch 5 has already added the following check,
>>>> which I think is sufficient to filter out those FSes that do not support
>>>> large folios:
>>>>
>>>> if (mapping_max_folio_order(inode->i_mapping) < PMD_ORDER)
>>>> 	return false;
>>>
>>> 2 < 5, we won't tolerate bisection hazards.
>>>
>>>>
>>>>
>>>>>> Anonymous shmem behaves similarly to anonymous pages: it is controlled by
>>>>>> the 'shmem_enabled' interfaces and uses shmem_allowable_huge_orders() to
>>>>>> check for allowed large orders, rather than relying on
>>>>>> mapping_max_folio_order().
>>>>>>
>>>>>> The mapping_max_folio_order() is intended to control large page allocation
>>>>>> only for tmpfs mounts. Therefore, I find the current code confusing and
>>>>>> think it needs to be fixed:
>>>>>>
>>>>>> /* Don't consider 'deny' for emergencies and 'force' for testing */
>>>>>> if (sb != shm_mnt->mnt_sb && sbinfo->huge)
>>>>>>         mapping_set_large_folios(inode->i_mapping);
>>>>>
>>
>> Hi Baolin,
>>
>> Do you want to send a fix for this?
>>
>> Also I wonder how I can distinguish between anonymous shmem code and tmpfs code.
>> I thought they are the same thing except that they have different user interface,
>> but it seems that I was wrong.
>>
>>
>> Best Regards,
>> Yan, Zi
>>
>>


Best Regards,
Yan, Zi