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 2025/5/30 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".

So I think we should still keep the TVA_ENFORCE_SYSFS flag.