Re: [PATCH v2 08/14] mm: copy_pte_range() use pte_offset_map_rw_nolock()

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2024/8/29 23:36, David Hildenbrand wrote:
> On 22.08.24 09:13, Qi Zheng wrote:
>> In copy_pte_range(), we may modify the src_pte entry after holding the
>> src_ptl, so convert it to using pte_offset_map_rw_nolock(). But since we
>> already hold the write lock of mmap_lock, there is no need to get pmdval
>> to do pmd_same() check, just pass a dummy variable to it.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>>   mm/memory.c | 11 ++++++++++-
>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 7b6071a0e21e2..30d98025b2a40 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1083,6 +1083,7 @@ copy_pte_range(struct vm_area_struct *dst_vma, 
>> struct vm_area_struct *src_vma,
>>       struct mm_struct *src_mm = src_vma->vm_mm;
>>       pte_t *orig_src_pte, *orig_dst_pte;
>>       pte_t *src_pte, *dst_pte;
>> +    pmd_t dummy_pmdval;
>>       pte_t ptent;
>>       spinlock_t *src_ptl, *dst_ptl;
>>       int progress, max_nr, ret = 0;
>> @@ -1108,7 +1109,15 @@ copy_pte_range(struct vm_area_struct *dst_vma, 
>> struct vm_area_struct *src_vma,
>>           ret = -ENOMEM;
>>           goto out;
>>       }
>> -    src_pte = pte_offset_map_nolock(src_mm, src_pmd, addr, &src_ptl);
>> +
>> +    /*
>> +     * Use the maywrite version to indicate that dst_pte will be 
>> modified,
>> +     * but since we already hold the write lock of mmap_lock, there 
>> is no
>> +     * need to get pmdval to do pmd_same() check, just pass a dummy 
>> variable
>> +     * to it.
> 
> As we hold the mmap lock write lock, I assume it will prevent any page 
> table removal, because they need *at least* the mmap lock in read mode, 
> right?

Except for retract_page_tables(), all others hold the read lock of
mmap_lock.

> 
> We should probably document the rules for removing a page table -- which 
> locks must be held in which mode (if not already done).

Agree, I will document it in the v3.

>