Re: [PATCH 1/4] mm: pagewalk: assert write mmap lock only for walking the user page tables

[Muchun Song <muchun.song@linux.dev>]

> On Dec 1, 2023, at 19:09, Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
> 
> 
> 
> On 2023/11/27 16:46, Muchun Song wrote:
>> The 8782fb61cc848 ("mm: pagewalk: Fix race between unmap and page walker")
>> introduces an assertion to walk_page_range_novma() to make all the users
>> of page table walker is safe. However, the race only exists for walking the
>> user page tables. And it is ridiculous to hold a particular user mmap write
>> lock against the changes of the kernel page tables. So only assert at least
>> mmap read lock when walking the kernel page tables. And some users matching
>> this case could downgrade to a mmap read lock to relief the contention of
>> mmap lock of init_mm, it will be nicer in hugetlb (only holding mmap read
>> lock) in the next patch.
>> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>> ---
>>  mm/pagewalk.c | 29 ++++++++++++++++++++++++++++-
>>  1 file changed, 28 insertions(+), 1 deletion(-)
>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c
>> index b7d7e4fcfad7a..f46c80b18ce4f 100644
>> --- a/mm/pagewalk.c
>> +++ b/mm/pagewalk.c
>> @@ -539,6 +539,11 @@ int walk_page_range(struct mm_struct *mm, unsigned long start,
>>   * not backed by VMAs. Because 'unusual' entries may be walked this function
>>   * will also not lock the PTEs for the pte_entry() callback. This is useful for
>>   * walking the kernel pages tables or page tables for firmware.
>> + *
>> + * Note: Be careful to walk the kernel pages tables, the caller may be need to
>> + * take other effective approache (mmap lock may be insufficient) to prevent
>> + * the intermediate kernel page tables belonging to the specified address range
>> + * from being freed (e.g. memory hot-remove).
>>   */
>>  int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
>>    unsigned long end, const struct mm_walk_ops *ops,
>> @@ -556,7 +561,29 @@ int walk_page_range_novma(struct mm_struct *mm, unsigned long start,
>>   if (start >= end || !walk.mm)
>>   return -EINVAL;
>>  - mmap_assert_write_locked(walk.mm);
>> + /*
>> + * 1) For walking the user virtual address space:
>> + *
>> + * The mmap lock protects the page walker from changes to the page
>> + * tables during the walk.  However a read lock is insufficient to
>> + * protect those areas which don't have a VMA as munmap() detaches
>> + * the VMAs before downgrading to a read lock and actually tearing
>> + * down PTEs/page tables. In which case, the mmap write lock should
>> + * be hold.
>> + *
>> + * 2) For walking the kernel virtual address space:
>> + *
>> + * The kernel intermediate page tables usually do not be freed, so
>> + * the mmap map read lock is sufficient. But there are some exceptions.
>> + * E.g. memory hot-remove. In which case, the mmap lock is insufficient
>> + * to prevent the intermediate kernel pages tables belonging to the
>> + * specified address range from being freed. The caller should take
>> + * other actions to prevent this race.
>> + */
>> + if (mm == &init_mm)
>> + 	mmap_assert_locked(walk.mm);
>> + else
>> + 	mmap_assert_write_locked(walk.mm);
> 
> Maybe just use process_mm_walk_lock() and set correct page_walk_lock in struct mm_walk_ops?

No. You also need to make sure the users do not pass the wrong
walk_lock, so you also need to add something like following:

if (mm == &init_mm)
	VM_BUG_ON(walk_lock != PGWALK_RDLOCK);
else
	VM_BUG_ON(walk_lock == PGWALK_RDLOCK);

I do not think the code will be simple.

> 
>>     return walk_pgd_range(start, end, &walk);
>>  }