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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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@nvidia.com>
>>> ---
>>>    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.

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);