From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f182.google.com (mail-pf0-f182.google.com [209.85.192.182]) by kanga.kvack.org (Postfix) with ESMTP id E08F46B0005 for ; Tue, 16 Feb 2016 10:30:04 -0500 (EST) Received: by mail-pf0-f182.google.com with SMTP id e127so107026097pfe.3 for ; Tue, 16 Feb 2016 07:30:04 -0800 (PST) Received: from mga01.intel.com (mga01.intel.com. [192.55.52.88]) by mx.google.com with ESMTP id s80si51823794pfi.55.2016.02.16.07.30.04 for ; Tue, 16 Feb 2016 07:30:04 -0800 (PST) Subject: Re: [PATCHv2 04/28] mm: make remove_migration_ptes() beyond mm/migration.c References: <1455200516-132137-1-git-send-email-kirill.shutemov@linux.intel.com> <1455200516-132137-5-git-send-email-kirill.shutemov@linux.intel.com> <56BE0E62.60806@intel.com> <20160216095428.GB46557@black.fi.intel.com> From: Dave Hansen Message-ID: <56C34073.6010208@intel.com> Date: Tue, 16 Feb 2016 07:29:55 -0800 MIME-Version: 1.0 In-Reply-To: <20160216095428.GB46557@black.fi.intel.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Hugh Dickins , Andrea Arcangeli , Andrew Morton , Vlastimil Babka , Christoph Lameter , Naoya Horiguchi , Jerome Marchand , Yang Shi , Sasha Levin , linux-kernel@vger.kernel.org, linux-mm@kvack.org On 02/16/2016 01:54 AM, Kirill A. Shutemov wrote: > On Fri, Feb 12, 2016 at 08:54:58AM -0800, Dave Hansen wrote: >> On 02/11/2016 06:21 AM, Kirill A. Shutemov wrote >>> We also shouldn't try to mlock() pte-mapped huge pages: pte-mapeed THP >>> pages are never mlocked. >> >> That's kinda subtle. Can you explain more? >> >> If we did the following: >> >> ptr = mmap(NULL, 512*PAGE_SIZE, ...); >> mlock(ptr, 512*PAGE_SIZE); >> fork(); >> munmap(ptr + 100 * PAGE_SIZE, PAGE_SIZE); >> >> I'd expect to get two processes, each mapping the same compound THP, one >> with a PMD and the other with 511 ptes and one hole. Is there something >> different that goes on? > > I'm not sure what exactly you want to ask with this code, but it will have > the following result: > > - After fork() process will split the pmd in munlock(). For file thp > split pmd, means clear it out. Mapping split_huge_pmd() would munlock > the page as we do for anon thp; > > - In child process the page is never mapped as mlock() is not inherited > and we don't copy page tables for shared VMA as they can re-faulted > later; Huh, I didn't realize we don't inherit mlock() across fork(). Learn something every day! > The basic semantic for mlock()ed file THP would be the same as for anon > THP: we only keep the page mlocked as long as it's mapped only with PMDs. > This way it's relatively simple to make sure that we don't leak mlocked > pages. Ahh, I forgot about that bit. Could you add some of that description to the changelog so I don't forget again? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org