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@xxxxxxxxxxxxxxxxx>
>>> ---
>>> 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
