Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   "Zi Yan" <zi.yan@sent.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     akpm@linux-foundation.org,
        "Andrea Arcangeli" <aarcange@redhat.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH 4/7] khugepaged: Allow to callapse a page shared across
 fork
Date:   Fri, 27 Mar 2020 14:19:50 -0400
Message-ID: <AD80A72A-8FB7-4F89-A2C9-CDD5C616A479@sent.com>
In-Reply-To: <20200327170601.18563-5-kirill.shutemov@linux.intel.com>
References: <20200327170601.18563-1-kirill.shutemov@linux.intel.com>
 <20200327170601.18563-5-kirill.shutemov@linux.intel.com>
MIME-Version: 1.0
Content-Type: multipart/signed;
 boundary="=_MailMate_80997A0A-BCA7-4724-8B72-2A1A1D67B3D7_=";
 micalg=pgp-sha512; protocol="application/pgp-signature"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

This is an OpenPGP/MIME signed message (RFC 3156 and 4880).

--=_MailMate_80997A0A-BCA7-4724-8B72-2A1A1D67B3D7_=
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

On 27 Mar 2020, at 13:05, 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@linux.intel.com>
> ---
>  mm/khugepaged.c | 28 ++++++++++++++++------------
>  1 file changed, 16 insertions(+), 12 deletions(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 39e0994abeb8..b47edfe57f7b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -581,18 +581,26 @@ 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) !=3D 1 + PageSwapCache(page)) {
> +		if (total_mapcount(page) + PageSwapCache(page) !=3D
> +				page_count(page)) {

Do you think having a function for this check would be better? Since the =
check is used three times.

>  			/*
>  			 * Drain pagevec and retry just in case we can get rid
>  			 * of the extra pin, like in swapin case.
>  			 */
>  			lru_add_drain();
>  		}
> -		if (page_count(page) !=3D 1 + PageSwapCache(page)) {
> +		if (total_mapcount(page) + PageSwapCache(page) !=3D
> +				page_count(page)) {
>  			unlock_page(page);
>  			result =3D SCAN_PAGE_COUNT;
>  			goto out;
> @@ -680,7 +688,6 @@ static void __collapse_huge_page_copy(pte_t *pte, s=
truct page *page,
>  		} else {
>  			src_page =3D pte_page(pteval);
>  			copy_user_highpage(page, src_page, address, vma);
> -			VM_BUG_ON_PAGE(page_mapcount(src_page) !=3D 1, src_page);

Maybe replace it with this?

VM_BUG_ON_PAGE(page_mapcount(src_page) + PageSwapCache(src_page) !=3D pag=
e_count(src_page), src_page);


>  			release_pte_page(src_page);
>  			/*
>  			 * ptl mostly unnecessary, but preempt has to
> @@ -1209,12 +1216,9 @@ static int khugepaged_scan_pmd(struct mm_struct =
*mm,
>  			goto out_unmap;
>  		}
>
> -		/*
> -		 * 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.
> -		 */
> -		if (page_count(page) !=3D 1 + PageSwapCache(page)) {
> +		/* Check if the page has any GUP (or other external) pins */
> +		if (total_mapcount(page) + PageSwapCache(page) !=3D
> +				page_count(page)) {
>  			result =3D SCAN_PAGE_COUNT;
>  			goto out_unmap;
>  		}
> -- =

> 2.26.0

Thanks.

=E2=80=94
Best Regards,
Yan Zi

--=_MailMate_80997A0A-BCA7-4724-8B72-2A1A1D67B3D7_=
Content-Description: OpenPGP digital signature
Content-Disposition: attachment; filename=signature.asc
Content-Type: application/pgp-signature; name=signature.asc

-----BEGIN PGP SIGNATURE-----

iQFEBAEBCgAuFiEEOXBxLIohamfZUwd5QYsvEZxOpswFAl5+Q8YQHHppLnlhbkBz
ZW50LmNvbQAKCRBBiy8RnE6mzMUiCACSQ0+eDkMnjyy+ajzLsbTzYN30M5/XuDS4
2iflrQbLK3OZDH5A/L6/3nW5drgfu0AV6opHNrsksfrw+ClQVrI2q19kz0uZJKj5
KqAnN49evZTDdMZd64u4QR320fqNRLPN7tWRQ9Kzj/AwDVLG1GDQgCKfI7q/0CP0
vIG1R6fkmOUz/6dQsqa+RYJJ6Wlg7oFWrSZNHCvHJ7d3N/goS41BQtyiW9n+fcj3
VLjlgy2ljDUc6zOV2zUcR63CgKWxkhSVeIhmhfjJtnX2vKfrOgp6MPP6Pc49MeXx
LQSp0oNZ6q7bPdkn6uJAI0e4qZDyJPQIeQO6vDABx1q1CiAJ1ZBB
=/MsL
-----END PGP SIGNATURE-----

--=_MailMate_80997A0A-BCA7-4724-8B72-2A1A1D67B3D7_=--