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

[Kirill A. Shutemov]

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@xxxxxxxxxxxxx> 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.

-- 
 Kirill A. Shutemov