From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-13.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 31F86C433B4 for ; Thu, 1 Apr 2021 19:29:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0FCA160241 for ; Thu, 1 Apr 2021 19:29:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235313AbhDAT3a (ORCPT ); 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-Google-Smtp-Source: ABdhPJxEg/kpAy++MLYpSkRG9zcOD0RT02x3wwPiTJxd0kzs4URGTpC5xS4fYjGbFYzdClWgvc/xicYjAnIIn52b/Ng= 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/ From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.5 required=3.0 tests=BAYES_00,DKIM_ADSP_CUSTOM_MED, DKIM_INVALID,DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CDF7FC433ED for ; Thu, 1 Apr 2021 22:28:17 +0000 (UTC) Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 6AAD36108F for ; Thu, 1 Apr 2021 22:28:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6AAD36108F Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=nouveau-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id ECBCF6E02D; Thu, 1 Apr 2021 22:28:14 +0000 (UTC) Received: from mail-io1-xd2c.google.com (mail-io1-xd2c.google.com [IPv6:2607:f8b0:4864:20::d2c]) by gabe.freedesktop.org (Postfix) with ESMTPS id A596F6ECFF for ; Thu, 1 Apr 2021 19:21:13 +0000 (UTC) Received: by mail-io1-xd2c.google.com with SMTP id b10so3284840iot.4 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=GIq+lCH0+tP0pCQJQTM1tkvw5VHuAxhlJ+VfArRJEKi1p5IqGFqXZXkEMJKIpbA4i8 GBwzab7th/0vuF4SiWzKUhKmIhOYk9BR+Y+HPpYV7c/JVDcSGaREiMtH1ky+ACXp1jtB ZzUDuIaxuu4J++KF3ZeUxfsZV9mSJpkJL7ttaOsjiv5pTgE0P83ic+Fzl6Z0b/FLG7KO LbwQFKf4VcL5XUIEpCRwO74htKRvY9hrVyRWhCUgm8TsKqQtRLmODoV846ggiCcA9ocI X82ZS/Y72Tsbf438QBeQLbIfQvRdMAWK3lHZxTxCbi9yPabAL8vD3teojTK0Hs0TIrcU MqyQ== X-Gm-Message-State: AOAM533mafrTBZ3UxFh3Yi1lJXreaSiVvY+maxcrihGS3U63J8+8DxAd ct9QiZilrw+heKVabknO53A4gSdGYz14LYpR/J2VpA== X-Google-Smtp-Source: ABdhPJxEg/kpAy++MLYpSkRG9zcOD0RT02x3wwPiTJxd0kzs4URGTpC5xS4fYjGbFYzdClWgvc/xicYjAnIIn52b/Ng= 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: To: Alistair Popple , Hugh Dickins X-Mailman-Approved-At: Thu, 01 Apr 2021 22:28:13 +0000 Subject: Re: [Nouveau] [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to_unmap X-BeenThere: nouveau@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Nouveau development list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: rcampbell@nvidia.com, Matthew Wilcox , daniel@ffwll.ch, linux-doc@vger.kernel.org, nouveau@lists.freedesktop.org, dri-devel@lists.freedesktop.org, LKML , kvm-ppc@vger.kernel.org, Christoph Hellwig , Linux MM , bskeggs@redhat.com, Jason Gunthorpe , Andrew Morton , Christoph Hellwig Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: nouveau-bounces@lists.freedesktop.org Sender: "Nouveau" 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/ _______________________________________________ Nouveau mailing list Nouveau@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/nouveau From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.5 required=3.0 tests=BAYES_00,DKIM_ADSP_CUSTOM_MED, DKIM_INVALID,DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9AE05C43460 for ; Fri, 2 Apr 2021 09:30:10 +0000 (UTC) Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 3D34261103 for ; Fri, 2 Apr 2021 09:30:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3D34261103 Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=dri-devel-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 4DE566E087; Fri, 2 Apr 2021 09:30:09 +0000 (UTC) Received: from mail-io1-xd35.google.com (mail-io1-xd35.google.com [IPv6:2607:f8b0:4864:20::d35]) by gabe.freedesktop.org (Postfix) with ESMTPS id ABE036ED01 for ; Thu, 1 Apr 2021 19:21:13 +0000 (UTC) Received: by mail-io1-xd35.google.com with SMTP id e8so3284798iok.5 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=IyFLMQkamjHVQblL07rIK7TyzbwTHa1l1/Nm2Giji8dgScZSuexWVRBBOtTPvt1jkN U3ClPHJD9kjWyVl2BYu5FZncMGuXdUkenwHpVopPVqNdwroDimTp7ZAX/Gr3GFDBg6ck MpF84DtV6E5EVSFnxgEf85u0hubehI8M6kIVz1blDIR5MfO8NhIBM3Yf1MmJnnX2opDL eNYz49T4m77wxzQlPP+ViySpzPSVcePN9IZaVWiITosBX9uGCppM9rdDgzioMgl4R4cu N1lQHTGtz8Bt+Se7WGJF68PQAVwByGDziezlii7uN3Agbu3VgXzM7/JTW5s0KOivsrf1 PAew== X-Gm-Message-State: AOAM533dYleTefnISHwzZfPKrtdMJ8NDbEzsCZN4S2hQzawPD/FmOQ+X ZeYf8/3ZOH7t3hBoBRQKiL3IK/3dC7palm+s9Exu0Q== X-Google-Smtp-Source: ABdhPJxEg/kpAy++MLYpSkRG9zcOD0RT02x3wwPiTJxd0kzs4URGTpC5xS4fYjGbFYzdClWgvc/xicYjAnIIn52b/Ng= 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 X-Mailman-Approved-At: Fri, 02 Apr 2021 09:30:09 +0000 X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: rcampbell@nvidia.com, Matthew Wilcox , linux-doc@vger.kernel.org, nouveau@lists.freedesktop.org, dri-devel@lists.freedesktop.org, LKML , kvm-ppc@vger.kernel.org, Christoph Hellwig , Linux MM , =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= , bskeggs@redhat.com, Jason Gunthorpe , John Hubbard , Andrew Morton , Christoph Hellwig Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" 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/ _______________________________________________ dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel From mboxrd@z Thu Jan 1 00:00:00 1970 From: Shakeel Butt Date: Thu, 01 Apr 2021 19:21:01 +0000 Subject: Re: [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to_unmap Message-Id: List-Id: References: <20210326000805.2518-1-apopple@nvidia.com> <23784464.epyy5R1Yul@nvdebian> <20210331115746.GA1463678@nvidia.com> <2557539.O4bb4zRkYN@nvdebian> In-Reply-To: <2557539.O4bb4zRkYN@nvdebian> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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 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/