linux-kernel - Re: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork

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Date:   Wed, 8 Apr 2020 16:10:44 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, Andrea Arcangeli <aarcange@...hat.com>,
        Zi Yan <ziy@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across
 fork

On Mon, Apr 06, 2020 at 01:50:56PM -0700, Yang Shi wrote:
> 
> 
> On 4/3/20 4:29 AM, Kirill A. Shutemov wrote:
> > The page can be included into collapse as long as it doesn't have extra
> > pins (from GUP or otherwise).
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >   mm/khugepaged.c | 25 ++++++++++++++-----------
> >   1 file changed, 14 insertions(+), 11 deletions(-)
> > 
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 57ff287caf6b..1e7e6543ebca 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -581,11 +581,18 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> >   		}
> >   		/*
> > -		 * cannot use mapcount: can't collapse if there's a gup pin.
> > -		 * The page must only be referenced by the scanned process
> > -		 * and page swap cache.
> > +		 * Check if the page has any GUP (or other external) pins.
> > +		 *
> > +		 * The page table that maps the page has been already unlinked
> > +		 * from the page table tree and this process cannot get
> > +		 * additinal pin on the page.
> > +		 *
> > +		 * New pins can come later if the page is shared across fork,
> > +		 * but not for the this process. It is fine. The other process
> > +		 * cannot write to the page, only trigger CoW.
> >   		 */
> > -		if (page_count(page) != 1 + PageSwapCache(page)) {
> > +		if (total_mapcount(page) + PageSwapCache(page) !=
> > +				page_count(page)) {
> 
> This check looks fine for base page, but what if the page is PTE-mapped THP?
> The following patch made this possible.
> 
> If it is PTE-mapped THP and the page is in swap cache, the refcount would be
> 512 + the number of PTE-mapped pages.
> 
> Shall we do the below change in the following patch?
> 
> extra_pins = PageSwapCache(page) ? nr_ccompound(page) - 1 : 0;
> if (total_mapcount(page) + PageSwapCache(page) != page_count(page) -
> extra_pins) {
> ...

Looks like you're right.

It would be nice to have a test case to demonstrate the issue.

Is there any way to trigger moving the page to swap cache? I don't see it
immediately.

-- 
 Kirill A. Shutemov