Re: [PATCH] mm: shmem: don't set large-order range for internal anonymous shmem mapping

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

On 4/7/26 2:49 PM, Lance Yang wrote:
> 
> On Tue, Apr 07, 2026 at 02:07:27PM +0800, Baolin Wang wrote:
>> Anonymous shmem large order allocations are dynamically controlled via the
>> global THP sysfs knob (/sys/kernel/mm/transparent_hugepage/shmem_enabled)
>> and the per-size mTHP knobs (/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/shmem_enabled).
>>
>> Therefore, anonymous shmem uses shmem_allowable_huge_orders() to check
>> which large orders are allowed, rather than relying on mapping_max_folio_order().
>> Moreover, mapping_max_folio_order() is intended to control large order
>> allocations only for tmpfs mounts. Clarify this by not setting a large-order
>> range for internal anonymous shmem mappings, to avoid confusion, as discussed
>> in the previous thread[1].
>>
>> [1] https://lore.kernel.org/all/ec927492-4577-4192-8fad-85eb1bb43121@linux.alibaba.com/
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>> mm/shmem.c | 13 +++++++++++--
>> 1 file changed, 11 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 4ecefe02881d..a60fe067969c 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -3088,8 +3088,17 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
>> 	if (sbinfo->noswap)
>> 		mapping_set_unevictable(inode->i_mapping);
>>
>> -	/* Don't consider 'deny' for emergencies and 'force' for testing */
>> -	if (sbinfo->huge)
>> +	/*
>> +	 * Only set the large order range for tmpfs mounts. The large order
>> +	 * selection for the internal anonymous shmem mount is configured
>> +	 * dynamically via the 'shmem_enabled' interfaces, so there is no
>> +	 * need to set a large order range for the internal anonymous shmem
>> +	 * mapping.
>> +	 *
>> +	 * Note: Don't consider 'deny' for emergencies and 'force' for
>> +	 * testing.
>> +	 */
>> +	if (sbinfo->huge && !(sb->s_flags & SB_KERNMOUNT))
> 
> FWIW, SB_KERNMOUNT is broader than "internal anonymous shmem" and covers
> all shm_mnt users too.
> 
> So maybe "internal shmem mount" would be a better description of what
> this code is actually checking.

Right, good point. Will fix. Thanks.