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

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Date:   Mon, 24 Jun 2019 18:15:28 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Song Liu <songliubraving@...com>
Cc:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        Kernel Team <Kernel-team@...com>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hdanton@...a.com" <hdanton@...a.com>
Subject: Re: [PATCH v7 5/6] mm,thp: add read-only THP support for (non-shmem)
 FS

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@...temov.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@...temov.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.

-- 
 Kirill A. Shutemov