Re: [PATCH v7 5/6] mm,thp: add read-only THP support for (non-shmem) FS

[Song Liu <songliubraving@fb.com>]

> On Jun 24, 2019, at 8:15 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> 
> On Mon, Jun 24, 2019 at 03:04:21PM +0000, Song Liu wrote:
>> 
>> 
>>> On Jun 24, 2019, at 7:54 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>>> 
>>> On Mon, Jun 24, 2019 at 02:42:13PM +0000, Song Liu wrote:
>>>> 
>>>> 
>>>>> On Jun 24, 2019, at 7:27 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
>>>>> 
>>>>> On Mon, Jun 24, 2019 at 02:01:05PM +0000, Song Liu wrote:
>>>>>>>> @@ -1392,6 +1403,23 @@ static void collapse_file(struct mm_struct *mm,
>>>>>>>> 				result = SCAN_FAIL;
>>>>>>>> 				goto xa_unlocked;
>>>>>>>> 			}
>>>>>>>> +		} else if (!page || xa_is_value(page)) {
>>>>>>>> +			xas_unlock_irq(&xas);
>>>>>>>> +			page_cache_sync_readahead(mapping, &file->f_ra, file,
>>>>>>>> +						  index, PAGE_SIZE);
>>>>>>>> +			lru_add_drain();
>>>>>>> 
>>>>>>> Why?
>>>>>> 
>>>>>> isolate_lru_page() is likely to fail if we don't drain the pagevecs. 
>>>>> 
>>>>> Please add a comment.
>>>> 
>>>> Will do. 
>>>> 
>>>>> 
>>>>>>>> +			page = find_lock_page(mapping, index);
>>>>>>>> +			if (unlikely(page == NULL)) {
>>>>>>>> +				result = SCAN_FAIL;
>>>>>>>> +				goto xa_unlocked;
>>>>>>>> +			}
>>>>>>>> +		} else if (!PageUptodate(page)) {
>>>>>>> 
>>>>>>> Maybe we should try wait_on_page_locked() here before give up?
>>>>>> 
>>>>>> Are you referring to the "if (!PageUptodate(page))" case? 
>>>>> 
>>>>> Yes.
>>>> 
>>>> I think this case happens when another thread is reading the page in. 
>>>> I could not think of a way to trigger this condition for testing. 
>>>> 
>>>> On the other hand, with current logic, we will retry the page on the 
>>>> next scan, so I guess this is OK. 
>>> 
>>> What I meant that calling wait_on_page_locked() on !PageUptodate() page
>>> will likely make it up-to-date and we don't need to SCAN_FAIL the attempt.
>>> 
>> 
>> Yeah, I got the point. My only concern is that I don't know how to 
>> reliably trigger this case for testing. I can try to trigger it. But I 
>> don't know whether it will happen easily. 
> 
> Atrifically slowing down IO should do the trick.
> 

Let me try that. 

Thanks,
Song