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

Zi Yan
2026-03-27T15:00:47Z

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@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.
>>>
>>> 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
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