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Message-ID: <6132583a-1754-4eb1-9b84-19b55cac176c@linux.alibaba.com>
Date: Fri, 30 May 2025 10:32:24 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Zi Yan <ziy@...dia.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 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@...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?
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.
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