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 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. 

Thanks,
Song