Re: [PATCH 2/2] mm: shmem: disallow hugepages if the system-wide shmem THP sysfs settings are disabled

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

On 2025/5/29 23:21, Zi Yan wrote:
> On 29 May 2025, at 4:23, Baolin Wang wrote:
> 
>> The MADV_COLLAPSE will ignore the system-wide shmem THP sysfs settings, which
>> means that even though we have disabled the shmem THP configuration, MADV_COLLAPSE
>> will still attempt to collapse into a shmem THP. This violates the rule we have
>> agreed upon: never means never.
>>
>> Then the current strategy is:
>> For shmem, if none of always, madvise, within_size, and inherit have enabled
>> PMD-sized mTHP, then MADV_COLLAPSE will be prohibited from collapsing PMD-sized mTHP.
>>
>> For tmpfs, if the mount option is set with the 'huge=never' parameter, then
>> MADV_COLLAPSE will be prohibited from collapsing PMD-sized mTHP.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   mm/huge_memory.c |  2 +-
>>   mm/shmem.c       | 12 ++++++------
>>   2 files changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index d3e66136e41a..a8cfa37cae72 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -166,7 +166,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>>   	 * own flags.
>>   	 */
>>   	if (!in_pf && shmem_file(vma->vm_file))
>> -		return shmem_allowable_huge_orders(file_inode(vma->vm_file),
>> +		return orders & shmem_allowable_huge_orders(file_inode(vma->vm_file),
>>   						   vma, vma->vm_pgoff, 0,
>>   						   !enforce_sysfs);
> 
> OK, here orders is checked against allowed orders.
> 
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 4b42419ce6b2..4dbb28d85cd9 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -613,7 +613,7 @@ static unsigned int shmem_get_orders_within_size(struct inode *inode,
>>   }
>>
>>   static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index,
>> -					      loff_t write_end, bool shmem_huge_force,
>> +					      loff_t write_end,
>>   					      struct vm_area_struct *vma,
>>   					      unsigned long vm_flags)
>>   {
>> @@ -625,7 +625,7 @@ static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index
>>   		return 0;
>>   	if (shmem_huge == SHMEM_HUGE_DENY)
>>   		return 0;
>> -	if (shmem_huge_force || shmem_huge == SHMEM_HUGE_FORCE)
>> +	if (shmem_huge == SHMEM_HUGE_FORCE)
>>   		return maybe_pmd_order;
> 
> shmem_huge is set by sysfs?

Yes, through the '/sys/kernel/mm/transparent_hugepage/shmem_enabled' 
interface.

>>   	/*
>> @@ -860,7 +860,7 @@ static unsigned long shmem_unused_huge_shrink(struct shmem_sb_info *sbinfo,
>>   }
>>
>>   static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index,
>> -					      loff_t write_end, bool shmem_huge_force,
>> +					      loff_t write_end,
>>   					      struct vm_area_struct *vma,
>>   					      unsigned long vm_flags)
>>   {
>> @@ -1261,7 +1261,7 @@ static int shmem_getattr(struct mnt_idmap *idmap,
>>   			STATX_ATTR_NODUMP);
>>   	generic_fillattr(idmap, request_mask, inode, stat);
>>
>> -	if (shmem_huge_global_enabled(inode, 0, 0, false, NULL, 0))
>> +	if (shmem_huge_global_enabled(inode, 0, 0, NULL, 0))
>>   		stat->blksize = HPAGE_PMD_SIZE;
>>
>>   	if (request_mask & STATX_BTIME) {
>> @@ -1768,7 +1768,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>>   		return 0;
>>
>>   	global_orders = shmem_huge_global_enabled(inode, index, write_end,
>> -						  shmem_huge_force, vma, vm_flags);
>> +						  vma, vm_flags);
>>   	/* Tmpfs huge pages allocation */
>>   	if (!vma || !vma_is_anon_shmem(vma))
>>   		return global_orders;
>> @@ -1790,7 +1790,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
>>   	/* Allow mTHP that will be fully within i_size. */
>>   	mask |= shmem_get_orders_within_size(inode, within_size_orders, index, 0);
>>
>> -	if (vm_flags & VM_HUGEPAGE)
>> +	if (shmem_huge_force || (vm_flags & VM_HUGEPAGE))
>>   		mask |= READ_ONCE(huge_shmem_orders_madvise);
>>
>>   	if (global_orders > 0)
>> -- 
>> 2.43.5
> 
> shmem_huge_force comes from !enforce_sysfs in __thp_vma_allowable_orders().
> Do you know when sysfs is not enforced and why?

IIUC, shmem_huge_force will only be set during MADV_COLLAPSE. 
Originally, MADV_COLLAPSE was intended to ignore the system-wide THP 
sysfs settings. However, if all system-wide shmem THP settings are 
disabled, we should not allow MADV_COLLAPSE to collapse a THP. This is 
the issue this patchset aims to fix. Thanks for the review.