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Shutemov" To: Yang Shi Cc: akpm@linux-foundation.org, Andrea Arcangeli , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" Subject: Re: [PATCHv2 5/8] khugepaged: Allow to callapse a page shared across fork Message-ID: <20200410155613.cxqplrmwckezx3hl@box> References: <20200403112928.19742-1-kirill.shutemov@linux.intel.com> <20200403112928.19742-6-kirill.shutemov@linux.intel.com> <20200408131044.xzlheacvslrbwrja@box> <107630f5-bbde-3f78-23e9-6f6b3113d709@linux.alibaba.com> <61569ead-2bbd-6d18-c04c-3251218a3fcd@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <61569ead-2bbd-6d18-c04c-3251218a3fcd@linux.alibaba.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 09, 2020 at 05:03:00PM -0700, Yang Shi wrote: > > > On 4/8/20 11:51 AM, Yang Shi wrote: > > > > > > On 4/8/20 6:10 AM, Kirill A. Shutemov wrote: > > > 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 > > > > > --- > > > > > ?? 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. > > > > It sounds not easy to trigger since it totally depends on timing, I'm > > wondering we may have to use MADV_PAGEOUT? Something below off the top > > of my head may trigger this? > > > > > > ??? CPU??? ?? A??? ??? ??? ??? ??? ? ??? ? ??? ??? CPU??? B ??? ??? ??? > > ??? ??? CPU??? C > > In parent: > > MADV_HUGEPAGE > > page fault to fill with THP > > fork > > ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? In Child: > > MADV_NOHUGEPAGE > > MADV_DONTNEED (split pmd) > > MADV_PAGEOUT > > ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? -> > > add_to_swap > > ??? ??? ??? ??? ??? ??? ??? ??? khugepaged scan parent and try to > > collapse PTE-mapped > > > > ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? -> > > try_to_unmap > > > > When doing MADV_DONTNEED we need make sure head page is unmapped since > > MADV_PAGEOUT would call page_mapcount(page) to skip shared mapping. > > This can't trigger it since MADV_PAGEOUT would isolate the page from lru so > khugepaged_scan_pmd() can't reach the refcount check at all. > > If try_to_unmap() or pageout() fails, the page may stay in swap cache. But I > didn't think of an easy way to make this happen. Okay, I'll fix it without a test case. -- Kirill A. Shutemov