Re: [PATCH v3] mm/hugetlb: fix overbroad MMU notifiers for unshared PMDs

["Li Zhe" <lizhe.67@bytedance.com>]

On 9/24/26 5:51 PM, Muchun Song wrote:
>
>
> On 2026/9/24 16:20, Li Zhe wrote:
>> Hugetlb currently expands MMU notifier ranges to PUD boundaries whenever
>> PMD sharing is possible. That is only needed when huge_pmd_unshare()
>> actually detaches a shared PMD page table, because clearing the PUD
>> invalidates the whole PUD-sized virtual address range.
>>
>> For hugetlbfs hole punch, and similarly for other hugetlb unmap paths,
>> a shared mapping can pass the "PMD sharing is possible" range test in
>> adjust_range_if_pmd_sharing_possible() even when the hugetlbfs file does
>> not currently have any shared PMD page tables. KVM then receives a 1G
>> invalidation for a 2M operation and zaps unrelated secondary mappings,
>> so the guest has to fault them back in.
>>
>> Avoid this by tracking active PMD-sharing attachments per hugetlbfs
>> inode. The count is incremented only after huge_pmd_share() successfully
>> installs a shared PMD table, and decremented when __huge_pmd_unshare()
>> actually detaches one. Since huge_pmd_share() can run concurrently under
>> i_mmap_lock_read(), use atomic operations for the count. A zero count is
>> used to skip the conservative notifier range expansion only after
>> excluding concurrent PMD sharing with the mapping write lock.
>>
>> On a Redis-in-VM workload that punches cold 2M hugetlb pages, this patch
>> improves P99 QPS stability while punching pages, reducing the QPS
>> degradation ratio from 7.09% to 1.45%.
>>
>> Reported-by: aiqi.i7 <aiqi.i7@bytedance.com>
>> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
>> ---
>> v2: 
>> https://lore.kernel.org/all/20260922090749.24905-1-lizhe.67@bytedance.com/
>> v1: 
>> https://lore.kernel.org/all/20260831091023.66581-1-lizhe.67@bytedance.com/
>>
>> ChangeLogs:
>> v2->v3:
>> - Rework the sticky state based on Andrew's feedback: use a per-inode
>>    counter of active PMD-sharing attachments, so files can return to the
>>    no-active-sharing state after PMD sharing ends.
>>
>> v1->v2:
>> - Rework the implementation based on David's suggestion: remember
>>    whether PMD sharing ever happened for a hugetlbfs file, and skip the
>>    conservative notifier range expansion while it has not. This avoids
>>    the per-unmap page-table walk.
>>
>>   fs/hugetlbfs/inode.c    |  1 +
>>   include/linux/hugetlb.h | 43 +++++++++++++++++++++++++++++++++++++++++
>>   mm/hugetlb.c            | 13 ++++++++++---
>>   3 files changed, 54 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
>> index 7611a8470ea26..78e27ce0a6f63 100644
>> --- a/fs/hugetlbfs/inode.c
>> +++ b/fs/hugetlbfs/inode.c
>> @@ -921,6 +921,7 @@ static struct inode *hugetlbfs_get_inode(struct 
>> super_block *sb,
>>           simple_inode_init_ts(inode);
>>           info->resv_map = resv_map;
>>           info->seals = F_SEAL_SEAL;
>> +        hugetlbfs_pmd_sharing_init(inode);
>>           switch (mode & S_IFMT) {
>>           default:
>>               init_special_inode(inode, mode, dev);
>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>> index 16c4c4caa126c..6b4f92b7f7ae4 100644
>> --- a/include/linux/hugetlb.h
>> +++ b/include/linux/hugetlb.h
>> @@ -12,6 +12,7 @@
>>   #include <linux/page_ref.h>
>>   #include <linux/list.h>
>>   #include <linux/kref.h>
>> +#include <linux/atomic.h>
>>   #include <linux/pgtable.h>
>>   #include <linux/gfp.h>
>>   #include <linux/userfaultfd_k.h>
>> @@ -509,6 +510,9 @@ struct hugetlbfs_inode_info {
>>       struct inode vfs_inode;
>>       struct resv_map *resv_map;
>>       unsigned int seals;
>> +#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
>> +    atomic_t pmd_sharing_count;
>> +#endif
>>   };
>>     static inline struct hugetlbfs_inode_info *HUGETLBFS_I(struct 
>> inode *inode)
>> @@ -516,6 +520,45 @@ static inline struct hugetlbfs_inode_info 
>> *HUGETLBFS_I(struct inode *inode)
>>       return container_of(inode, struct hugetlbfs_inode_info, 
>> vfs_inode);
>>   }
>>   +#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
>> +static inline void hugetlbfs_pmd_sharing_init(struct inode *inode)
>> +{
>> +    atomic_set(&HUGETLBFS_I(inode)->pmd_sharing_count, 0);
>> +}
>> +
>> +static inline void hugetlbfs_pmd_sharing_inc(struct inode *inode)
>> +{
>> +    atomic_inc(&HUGETLBFS_I(inode)->pmd_sharing_count);
>> +}
>> +
>> +static inline void hugetlbfs_pmd_sharing_dec(struct inode *inode)
>> +{
>> +    atomic_dec(&HUGETLBFS_I(inode)->pmd_sharing_count);
>> +}
>> +
>> +/*
>> + * A 32-bit counter can theoretically wrap, but doing so would require
>> + * billions of active PMD-sharing attachments to the same inode and 
>> is not
>> + * expected in practice. Treat any non-zero value as active so a 
>> wrapped
>> + * negative value still takes the conservative notifier range.
>> + */
>> +static inline bool hugetlbfs_pmd_sharing_active(struct inode *inode)
>> +{
>> +    return atomic_read(&HUGETLBFS_I(inode)->pmd_sharing_count) != 0;
>
> Testing for non zero covers the negative half of the cycle,
> but the 2^32nd increment changes the value back to zero.
>
> The inode-wide total is not bounded by PID_MAX_LIMIT because
> one mm can have an attachment in every PUD-sized part of a
> large mapping.  For example, a 4-level x86 mm has about
> 131,000 user PUD slots.  Roughly 32,769 child mms can therefore
> create more than 2^32 attachments by faulting one address in
> each PUD of a single large inherited MAP_SHARED hugetlb VMA.
> This also does not require one backing huge page per
> attachment: huge_pte_alloc() installs or shares the PMD table
> before hugetlb_no_page() attempts to obtain the huge page.
>
> At the zero value, an unmap can take i_mmap_rwsem for write,
> observe no active sharing, and issue only the original narrow
> notifier.  Its subsequent __huge_pmd_unshare() can still
> clear a PUD and decrement the counter from zero to -1, leaving
> the rest of the PUD-sized invalidation unreported.
>
> Could this use a 64-bit or saturating counter so an active
> count can never be mistaken for zero?
>
> Therefore, I recommend using atomic64_t.
>
> Thanks,
> Muchun


Thanks for the detailed explanation.

I did consider that such an extreme case could theoretically cause a
problem, but I expected it to be practically unreachable. But since we
can cover this corner case simply by using atomic64_t, that is definitely
better.

I will switch the counter to atomic64_t in v4 and drop the 32-bit wrap
comment.

Thanks,
Zhe

>
>> +}
>> +#else
>> +static inline void hugetlbfs_pmd_sharing_init(struct inode *inode) {}
>> +
>> +static inline void hugetlbfs_pmd_sharing_inc(struct inode *inode) {}
>> +
>> +static inline void hugetlbfs_pmd_sharing_dec(struct inode *inode) {}
>> +
>> +static inline bool hugetlbfs_pmd_sharing_active(struct inode *inode)
>> +{
>> +    return false;
>> +}
>> +#endif
>> +
>>   extern const struct vm_operations_struct hugetlb_vm_ops;
>>   struct file *hugetlb_file_setup(const char *name, size_t size, 
>> vma_flags_t acct,
>>                   int creat_flags, int page_size_log);
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 4f6f58bf3db6c..cb27c06d1f7a7 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -5361,10 +5361,12 @@ void __hugetlb_zap_begin(struct 
>> vm_area_struct *vma,
>>       if (!vma->vm_file)    /* hugetlbfs_file_mmap error */
>>           return;
>>   -    adjust_range_if_pmd_sharing_possible(vma, start, end);
>>       hugetlb_vma_lock_write(vma);
>> -    if (vma->vm_file)
>> +    if (vma->vm_file) {
>>           i_mmap_lock_write(vma->vm_file->f_mapping);
>> +        if (hugetlbfs_pmd_sharing_active(file_inode(vma->vm_file)))
>> +            adjust_range_if_pmd_sharing_possible(vma, start, end);
>> +    }
>>   }
>>     void __hugetlb_zap_end(struct vm_area_struct *vma,
>> @@ -5403,7 +5405,10 @@ void unmap_hugepage_range(struct 
>> vm_area_struct *vma, unsigned long start,
>>         mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm,
>>                   start, end);
>> -    adjust_range_if_pmd_sharing_possible(vma, &range.start, 
>> &range.end);
>> +    i_mmap_assert_write_locked(vma->vm_file->f_mapping);
>> +    if (hugetlbfs_pmd_sharing_active(file_inode(vma->vm_file)))
>> +        adjust_range_if_pmd_sharing_possible(vma, &range.start,
>> +                             &range.end);
>>       mmu_notifier_invalidate_range_start(&range);
>>       tlb_gather_mmu(&tlb, vma->vm_mm);
>>   @@ -7006,6 +7011,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, 
>> struct vm_area_struct *vma,
>>       if (pud_none(*pud)) {
>>           pud_populate(mm, pud,
>>                   (pmd_t *)((unsigned long)spte & PAGE_MASK));
>> +        hugetlbfs_pmd_sharing_inc(file_inode(vma->vm_file));
>>           mm_inc_nr_pmds(mm);
>>       } else {
>>           ptdesc_pmd_pts_dec(virt_to_ptdesc(spte));
>> @@ -7037,6 +7043,7 @@ static int __huge_pmd_unshare(struct mmu_gather 
>> *tlb,
>>       pud_clear(pud);
>>         tlb_unshare_pmd_ptdesc(tlb, virt_to_ptdesc(ptep), addr);
>> +    hugetlbfs_pmd_sharing_dec(file_inode(vma->vm_file));
>>         mm_dec_nr_pmds(mm);
>>       return 1;
>