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

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Message-ID: <D61B9FA2-4EAC-4F0E-AF56-236D37A766BE@nvidia.com>
Date: Thu, 29 May 2025 22:17:15 -0400
From: Zi Yan <ziy@...dia.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: shmem: disallow hugepages if the system-wide
 shmem THP sysfs settings are disabled

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@...ux.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?

Best Regards,
Yan, Zi