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

[Zi Yan <ziy@nvidia.com>]

On 29 May 2025, at 22:32, Baolin Wang wrote:

> On 2025/5/30 10:17, Zi Yan wrote:
>> On 29 May 2025, at 21:58, Baolin Wang wrote:
>>
>>> 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.
>>
>> Got it. If we want to enforce sysfs, why not just get rid of TVA_ENFORCE_SYSFS
>> and make everyone follow sysfs?
>
> Now MADV_COLLAPSE will ignore the VM_HUGEPAGE, while the others will check the VM_HUGEPAGE flag before using 'huge_shmem_orders_madvise' with the TVA_ENFORCE_SYSFS flag set.
>
> That is to follow the rule: “allowing for collapsing in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".

Can you add this rule in your commit message? It clarifies things.

>
> So I think we should still keep the TVA_ENFORCE_SYSFS flag.

Got it. Thank you for the explanation.

Acked-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi