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[23.128.96.18]) by mx.google.com with ESMTP id g3si4952554edp.527.2021.04.01.12.30.23; Thu, 01 Apr 2021 12:30:45 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@google.com header.s=20161025 header.b=ZHtLYWtQ; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=google.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235271AbhDAT3a (ORCPT + 99 others); Thu, 1 Apr 2021 15:29:30 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53556 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235465AbhDAT3M (ORCPT ); Thu, 1 Apr 2021 15:29:12 -0400 Received: from mail-io1-xd2a.google.com (mail-io1-xd2a.google.com [IPv6:2607:f8b0:4864:20::d2a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E2D51C0613B0 for ; Thu, 1 Apr 2021 12:21:13 -0700 (PDT) Received: by mail-io1-xd2a.google.com with SMTP id k25so3281415iob.6 for ; Thu, 01 Apr 2021 12:21:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=898Op6M40JinO4D3cznUkQtr1L0luSLau7chpWc5uKg=; b=ZHtLYWtQ+FqDVYKZTHwxwCgUSMGSoQwWB3jcqgznkK+uvYCZl71pVwy5ATKWFW+awD RSsxcenvm4NI2KyYWaJUIG6gvaMfET9w4iUr92BU7UnnqyKaC+Ju011n6FIZIKYDawUS RfPx0iEgWjQkSsfvtpGU74dXu9GoGZoyW0RremDQPcGSRotsSgTYpyhipkTT3zuFwni9 sjiPRhgr49+7DQJBGGd7WW1reLrFWvr/qNBKcAA2mu09pJ2RM+Gdf2ROLzB5obPcnu9S RvCPLxzmOd4AN2AX3Z8HVq/gV/3V7zY244fX74qyZGgWcN/dIAA+Fg3WJlQfpEJEuMwR wlBA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=898Op6M40JinO4D3cznUkQtr1L0luSLau7chpWc5uKg=; b=Y9qW5SmHtCS1ZPCTUomQXiyaLufBErdvY6nV+W1LZWcbZlXG9ljCZqgN2kNIJ5L0Qn uX1ju7nDC6SSPMfueYXydEK4D8dfubcT4pMGhnbX8W/VZEzMm4lrKVC8W4oEJOkWtfB5 AxneTPxUYHYnMhLQjvZUKU0lyCyAgtILFr1ldiVB6QDgyfC8W6u86tYL49ecaHVYGqW4 ZWGfEdRt3rnzE+zLJ//CBWmQd7qNjd5mqqboXofU0I5wFi3DoLnxR/ssBrZ2rinrMpwI KKebG7mJ9c3Wu8hSqRgVySyOZ5zlWFHEGmjNHq736LaNmPj7QPsbxtccZ4C2qTZtMbWH z/4w== X-Gm-Message-State: AOAM533zLQj3yzw09tMxzLAXvCixhNER9+5v2ktzwZZiWFk8Ez+Nwul8 a6hmrzarJXFYXaeN6CG2712dkQEIFLn2CWpuXcnrpj87JgY= X-Received: by 2002:a05:6638:388e:: with SMTP id b14mr9630881jav.62.1617304872817; Thu, 01 Apr 2021 12:21:12 -0700 (PDT) MIME-Version: 1.0 References: <20210326000805.2518-1-apopple@nvidia.com> <23784464.epyy5R1Yul@nvdebian> <20210331115746.GA1463678@nvidia.com> <2557539.O4bb4zRkYN@nvdebian> In-Reply-To: <2557539.O4bb4zRkYN@nvdebian> From: Shakeel Butt Date: Thu, 1 Apr 2021 12:21:01 -0700 Message-ID: Subject: Re: [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to_unmap To: Alistair Popple , Hugh Dickins Cc: Jason Gunthorpe , John Hubbard , Linux MM , nouveau@lists.freedesktop.org, bskeggs@redhat.com, Andrew Morton , linux-doc@vger.kernel.org, LKML , kvm-ppc@vger.kernel.org, dri-devel@lists.freedesktop.org, rcampbell@nvidia.com, =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , Christoph Hellwig , daniel@ffwll.ch, Matthew Wilcox , Christoph Hellwig Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org CC: Hugh Dickins On Wed, Mar 31, 2021 at 9:37 PM Alistair Popple wrote: > > On Wednesday, 31 March 2021 10:57:46 PM AEDT Jason Gunthorpe wrote: > > On Wed, Mar 31, 2021 at 03:15:47PM +1100, Alistair Popple wrote: > > > On Wednesday, 31 March 2021 2:56:38 PM AEDT John Hubbard wrote: > > > > On 3/30/21 3:56 PM, Alistair Popple wrote: > > > > ... > > > > >> +1 for renaming "munlock*" items to "mlock*", where applicable. good > > > grief. > > > > > > > > > > At least the situation was weird enough to prompt further > investigation :) > > > > > > > > > > Renaming to mlock* doesn't feel like the right solution to me either > > > though. I > > > > > am not sure if you saw me responding to myself earlier but I am > thinking > > > > > renaming try_to_munlock() -> page_mlocked() and try_to_munlock_one() - > > > > > > > page_mlock_one() might be better. Thoughts? > > > > > > > > > > > > > Quite confused by this naming idea. Because: try_to_munlock() returns > > > > void, so a boolean-style name such as "page_mlocked()" is already not a > > > > good fit. > > > > > > > > Even more important, though, is that try_to_munlock() is mlock-ing the > > > > page, right? Is there some subtle point I'm missing? It really is doing > > > > an mlock to the best of my knowledge here. Although the kerneldoc > > > > comment for try_to_munlock() seems questionable too: > > > > > > It's mlocking the page if it turns out it still needs to be locked after > > > unlocking it. But I don't think you're missing anything. > > > > It is really searching all VMA's to see if the VMA flag is set and if > > any are found then it mlocks the page. > > > > But presenting this rountine in its simplified form raises lots of > > questions: > > > > - What locking is being used to read the VMA flag? > > - Why do we need to manipulate global struct page flags under the > > page table locks of a single VMA? > > I was wondering that and questioned it in an earlier version of this series. I > have done some digging and the commit log for b87537d9e2fe ("mm: rmap use pte > lock not mmap_sem to set PageMlocked") provides the original justification. > > It's fairly long so I won't quote it here but the summary seems to be that > among other things the combination of page lock and ptl makes this safe. I > have yet to verify if everything there still holds and is sensible, but the > last paragraph certainly is :-) > > "Stopped short of separating try_to_munlock_one() from try_to_munmap_one() > on this occasion, but that's probably the sensible next step - with a > rename, given that try_to_munlock()'s business is to try to set Mlocked." > > > - Why do we need to check for huge pages inside the VMA loop, not > > before going to the rmap? PageTransCompoundHead() is not sensitive to > > the PTEs. (and what happens if the huge page breaks up concurrently?) > > - Why do we clear the mlock bit then run around to try and set it? > > I don't have an answer for that as I'm not (yet) across all the mlock code > paths, but I'm hoping this patch at least won't change anything. > It would be good to ask the person who has the most answers? Hugh, the thread started at https://lore.kernel.org/dri-devel/20210326000805.2518-4-apopple@nvidia.com/