Re: [PATCH v1 1/7] mm: khugepaged: retract_page_tables() use pte_offset_map_lock()

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

On 2024/10/18 02:00, Jann Horn wrote:
> On Thu, Oct 17, 2024 at 11:47 AM Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>> In retract_page_tables(), we may modify the pmd entry after acquiring the
>> pml and ptl, so we should also check whether the pmd entry is stable.
>> Using pte_offset_map_lock() to do it, and then we can also remove the
>> calling of the pte_lockptr().
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>> ---
>>   mm/khugepaged.c | 9 ++++++++-
>>   1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 94feb85ce996c..b4f49d323c8d9 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1721,6 +1721,7 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
>>                  spinlock_t *pml;
>>                  spinlock_t *ptl;
>>                  bool skipped_uffd = false;
>> +               pte_t *pte;
>>
>>                  /*
>>                   * Check vma->anon_vma to exclude MAP_PRIVATE mappings that
>> @@ -1757,9 +1758,15 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
>>                  mmu_notifier_invalidate_range_start(&range);
>>
>>                  pml = pmd_lock(mm, pmd);
>> -               ptl = pte_lockptr(mm, pmd);
>> +               pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
> 
> This takes the lock "ptl" on the success path...
> 
>> +               if (!pte) {
>> +                       spin_unlock(pml);
>> +                       mmu_notifier_invalidate_range_end(&range);
>> +                       continue;
>> +               }
>>                  if (ptl != pml)
>>                          spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
> 
> ... and this takes the same lock again, right? I think this will

Oh my god, my mistake, I used pte_offset_map_rw_nolock() at first, then
I changed it to pte_offset_map_lock() but forgot to delete this, and
because my test did not trigger retract_page_tables(), so I did not
find this error.

Will change in v2.

Thanks!

> deadlock on kernels with CONFIG_SPLIT_PTE_PTLOCKS=y. Did you test this
> on a machine with less than 4 CPU cores, or something like that? Or am
> I missing something?