Re: [PATCH v2 1/2] khugepaged: enable collapse pmd for pte-mapped THP

[Song Liu <songliubraving@fb.com>]

> On Aug 2, 2019, at 3:31 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> 
> On 08/01, Song Liu wrote:
>> 
>> 
>>> On Aug 1, 2019, at 7:50 AM, Oleg Nesterov <oleg@redhat.com> wrote:
>>> 
>>> On 07/31, Song Liu wrote:
>>>> 
>>>> +static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
>>>> +					 unsigned long addr)
>>>> +{
>>>> +	struct mm_slot *mm_slot;
>>>> +	int ret = 0;
>>>> +
>>>> +	/* hold mmap_sem for khugepaged_test_exit() */
>>>> +	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
>>>> +	VM_BUG_ON(addr & ~HPAGE_PMD_MASK);
>>>> +
>>>> +	if (unlikely(khugepaged_test_exit(mm)))
>>>> +		return 0;
>>>> +
>>>> +	if (!test_bit(MMF_VM_HUGEPAGE, &mm->flags) &&
>>>> +	    !test_bit(MMF_DISABLE_THP, &mm->flags)) {
>>>> +		ret = __khugepaged_enter(mm);
>>>> +		if (ret)
>>>> +			return ret;
>>>> +	}
>>> 
>>> could you explain why do we need mm->mmap_sem, khugepaged_test_exit() check
>>> and __khugepaged_enter() ?
>> 
>> If the mm doesn't have a mm_slot, we would like to create one here (by
>> calling __khugepaged_enter()).
> 
> I can be easily wrong, I never read this code before, but this doesn't
> look correct.
> 
> Firstly, mm->mmap_sem cam ONLY help if a) the task already has mm_slot
> and b) this mm_slot is khugepaged_scan.mm_slot. Otherwise khugepaged_exit()
> won't take mmap_sem for writing and thus we can't rely on test_exit().
> 
> and this means that down_read(mmap_sem) before khugepaged_add_pte_mapped_thp()
> is pointless and can't help; this mm was found by vma_interval_tree_foreach().
> 
> so __khugepaged_enter() can race with khugepaged_exit() and this is wrong
> in any case.
> 
>> This happens when the THP is created by another mm, or by tmpfs with
>> "huge=always"; and then page table of this mm got split by split_huge_pmd().
>> With current kernel, this happens when we attach/detach uprobe to a file
>> in tmpfs with huge=always.
> 
> Well. In this particular case khugepaged_enter() was likely already called
> by shmem_mmap() or khugepaged_enter_vma_merge(), or madvise.
> 
> (in fact I think do_set_pmd() or shmem_fault() should call _enter() too,
> like do_huge_pmd_anonymous_page() does, but this is another story).

Hmm.. I didn't notice the one in shmem_mmap(). And yes, you are right, we 
don't really need __khugepaged_enter() in khugepaged_add_pte_mapped_thp(),
especially when uprobes.c doesn't call it. 

This should simplify this patch. 

> 
> 
> And I forgot to mention... I don't understand why
> khugepaged_collapse_pte_mapped_thps() has to be called with khugepaged_mm_lock.

You are right that khugepaged_collapse_pte_mapped_thps() doesn't need 
khugepaged_mm_lock in this version. For v1, when uprobes.c calls 
khugepaged_add_pte_mapped_thp(), this is necessary. 

Let me clean this up. 

Thanks,
Song