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Thu, 20 May 2021 19:06:37 +0000 From: Zi Yan To: Peter Xu Cc: "Aneesh Kumar K.V" , Nathan Chancellor , linux-mm@kvack.org, akpm@linux-foundation.org, mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org, kaleshsingh@google.com, npiggin@gmail.com, joel@joelfernandes.org, Christophe Leroy Subject: Re: [PATCH v5 3/9] mm/mremap: Use pmd/pud_poplulate to update page table entries Date: Thu, 20 May 2021 15:06:30 -0400 X-Mailer: MailMate (1.14r5806) Message-ID: <4CE7132C-3800-456B-91DA-613391361B94@nvidia.com> In-Reply-To: References: <20210422054323.150993-1-aneesh.kumar@linux.ibm.com> <20210422054323.150993-4-aneesh.kumar@linux.ibm.com> <87mtsrqqk0.fsf@linux.ibm.com> <6e0dbb76-2b33-53f1-246e-30cec2b871e2@linux.ibm.com> <87o8d5le4q.fsf@linux.ibm.com> Content-Type: multipart/signed; boundary="=_MailMate_DE17EC09-DF1F-4138-802E-E02A60908704_="; micalg=pgp-sha512; protocol="application/pgp-signature" X-Originating-IP: [216.228.112.21] X-ClientProxiedBy: MN2PR16CA0038.namprd16.prod.outlook.com (2603:10b6:208:234::7) To MN2PR12MB3823.namprd12.prod.outlook.com (2603:10b6:208:168::26) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [10.2.60.96] (216.228.112.21) by MN2PR16CA0038.namprd16.prod.outlook.com (2603:10b6:208:234::7) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4150.23 via Frontend Transport; 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dkim=pass header.d=Nvidia.com header.s=selector2 header.b=QVhP6oL6; spf=none (imf04.hostedemail.com: domain of ziy@nvidia.com has no SPF policy when checking 40.107.236.68) smtp.mailfrom=ziy@nvidia.com; dmarc=pass (policy=none) header.from=nvidia.com X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: E88432C8 X-Stat-Signature: 4ixjq3x1cua8ig5r17w9mj7a7fne8owd X-HE-Tag: 1621537597-182922 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: --=_MailMate_DE17EC09-DF1F-4138-802E-E02A60908704_= Content-Type: text/plain; charset=UTF-8; markup=markdown Content-Transfer-Encoding: quoted-printable On 20 May 2021, at 10:57, Peter Xu wrote: > On Thu, May 20, 2021 at 07:07:57PM +0530, Aneesh Kumar K.V wrote: >> "Aneesh Kumar K.V" writes: >> >>> On 5/20/21 6:16 PM, Peter Xu wrote: >>>> On Thu, May 20, 2021 at 01:56:54PM +0530, Aneesh Kumar K.V wrote: >>>>>> This seems to work at least for my userfaultfd test on shmem, howe= ver I don't >>>>>> fully understand the commit message [1] on: How do we guarantee we= 're not >>>>>> moving a thp pte? >>>>>> >>>>> >>>>> move_page_tables() checks for pmd_trans_huge() and ends up calling >>>>> move_huge_pmd if it is a THP entry. >>>> >>>> Sorry to be unclear: what if a huge pud thp? >>>> >>> >>> I am still checking. Looking at the code before commit >>> c49dd340180260c6239e453263a9a244da9a7c85, I don't see kernel handling= >>> huge pud thp. I haven't studied huge pud thp enough to understand >>> whether c49dd340180260c6239e453263a9a244da9a7c85 intent to add that >>> support. >>> >>> We can do a move_huge_pud() like we do for huge pmd thp. But I am not= >>> sure whether we handle those VMA's earlier and restrict mremap on the= m? >> >> something like this? (not even compile tested). I am still not sure >> whether this is really needed or we handle DAX VMA's in some other for= m. > > Yeah maybe (you may want to at least drop that extra "case HPAGE_PUD").= > > It's just that if with CONFIG_HAVE_MOVE_PUD (x86 and arm64 enables it b= y > default so far) it does seem to work even with huge pud, while after th= is patch > it seems to be not working anymore, even with your follow up fix. > > Indeed I saw CONFIG_HAVE_MOVE_PUD is introduced a few months ago so bre= aking > someone seems to be unlikely, perhaps no real user yet to mremap() a hu= ge pud > for dax or whatever backend? > > Ideally maybe rework this patch (or series?) and repost it for a better= review? > Agree the risk seems low. I'll leave that to you and Andrew to decide.= =2E It seems that the mremap function for 1GB DAX THP was not added when 1GB = DAX THP was implemented[1]. I guess no one is using mremap on 1GB DAX THP. Maybe = we want to at least add a warning or VM_BUG_ON to catch this or use Aneesh=E2=80=99= s move_huge_pud() to handle the situation properly? [1] https://lore.kernel.org/linux-ext4/148545012634.17912.139517636064103= 03827.stgit@djiang5-desk3.ch.intel.com/ =E2=80=94 Best Regards, Yan, Zi --=_MailMate_DE17EC09-DF1F-4138-802E-E02A60908704_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEEh7yFAW3gwjwQ4C9anbJR82th+ooFAmCmszYPHHppeUBudmlk aWEuY29tAAoJEJ2yUfNrYfqK9BAP/AgWiKf2ZnYLRxNij+dYb/HHzHxhArqJiM5h ejuk5rlSM8O/yeSl8GvWRYTU7SUhw5wAmSbr3R9hXdJM8M5GFfVDk1JwGs9XMD3R H0hIsPqFvLzEkGhVBIoBdkqb9Xy+QSEfS6/bkqaYhKKbSln9GkC/o0ygjS15FnD+ PY7++lmQIA8XgP6COGCDbWhaUB5I2elVe7oQbaBZ/q9m0FgHA4BhbwvZhMq0f7ze M7hAOFRTlXbMtjbfaypYGYPMMSVyvvtOih62hsy6rVSDwNpq2ezAveC1KWnXIOYZ 77qCx5vNd2xx5nzkplviIA6GpV1GxEGm9/wgcItU5JHM9wgX5RDe9vxm1fiLP5FJ kkNX4gq9lEv2OEanhboq6X+h/dUj84m7oj2Xu+3h4vRs0IzOoQFcZt4Ur9OdU0Fh rHmNxkgGjBd3dhLAVsyT6TxlRU6UQYULcWLNicB/B1comePITECALuM1TGC0/J12 fO0/DGLDLgqgoS6Zd7Rhe7BYGV+wMAm/H/BqoNYNQyHObHHuDkejMJ96DtA76FL0 oCCvl26lG8C9FtLPc4I7fdH+dZL5gUGlWevnUNPycG8aNrV5eJ5PfcOwmlCgyNDQ Sb1MbYIx1rg7ugxW4UbLiUV2Z+ufa9dxx6KFWFt4BGIrtiAIeAEwP2BISbD+I37E U//1qIkE =vRDk -----END PGP SIGNATURE----- --=_MailMate_DE17EC09-DF1F-4138-802E-E02A60908704_=-- 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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MSGID_FROM_MTA_HEADER,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 D7F4AC43461 for ; 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charset=UTF-8; markup=markdown Content-Transfer-Encoding: quoted-printable On 20 May 2021, at 10:57, Peter Xu wrote: > On Thu, May 20, 2021 at 07:07:57PM +0530, Aneesh Kumar K.V wrote: >> "Aneesh Kumar K.V" writes: >> >>> On 5/20/21 6:16 PM, Peter Xu wrote: >>>> On Thu, May 20, 2021 at 01:56:54PM +0530, Aneesh Kumar K.V wrote: >>>>>> This seems to work at least for my userfaultfd test on shmem, howe= ver I don't >>>>>> fully understand the commit message [1] on: How do we guarantee we= 're not >>>>>> moving a thp pte? >>>>>> >>>>> >>>>> move_page_tables() checks for pmd_trans_huge() and ends up calling >>>>> move_huge_pmd if it is a THP entry. >>>> >>>> Sorry to be unclear: what if a huge pud thp? >>>> >>> >>> I am still checking. Looking at the code before commit >>> c49dd340180260c6239e453263a9a244da9a7c85, I don't see kernel handling= >>> huge pud thp. I haven't studied huge pud thp enough to understand >>> whether c49dd340180260c6239e453263a9a244da9a7c85 intent to add that >>> support. >>> >>> We can do a move_huge_pud() like we do for huge pmd thp. But I am not= >>> sure whether we handle those VMA's earlier and restrict mremap on the= m? >> >> something like this? (not even compile tested). I am still not sure >> whether this is really needed or we handle DAX VMA's in some other for= m. > > Yeah maybe (you may want to at least drop that extra "case HPAGE_PUD").= > > It's just that if with CONFIG_HAVE_MOVE_PUD (x86 and arm64 enables it b= y > default so far) it does seem to work even with huge pud, while after th= is patch > it seems to be not working anymore, even with your follow up fix. > > Indeed I saw CONFIG_HAVE_MOVE_PUD is introduced a few months ago so bre= aking > someone seems to be unlikely, perhaps no real user yet to mremap() a hu= ge pud > for dax or whatever backend? > > Ideally maybe rework this patch (or series?) and repost it for a better= review? > Agree the risk seems low. I'll leave that to you and Andrew to decide.= =2E It seems that the mremap function for 1GB DAX THP was not added when 1GB = DAX THP was implemented[1]. I guess no one is using mremap on 1GB DAX THP. Maybe = we want to at least add a warning or VM_BUG_ON to catch this or use Aneesh=E2=80=99= s move_huge_pud() to handle the situation properly? [1] https://lore.kernel.org/linux-ext4/148545012634.17912.139517636064103= 03827.stgit@djiang5-desk3.ch.intel.com/ =E2=80=94 Best Regards, Yan, Zi --=_MailMate_DE17EC09-DF1F-4138-802E-E02A60908704_= Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQJDBAEBCgAtFiEEh7yFAW3gwjwQ4C9anbJR82th+ooFAmCmszYPHHppeUBudmlk aWEuY29tAAoJEJ2yUfNrYfqK9BAP/AgWiKf2ZnYLRxNij+dYb/HHzHxhArqJiM5h ejuk5rlSM8O/yeSl8GvWRYTU7SUhw5wAmSbr3R9hXdJM8M5GFfVDk1JwGs9XMD3R H0hIsPqFvLzEkGhVBIoBdkqb9Xy+QSEfS6/bkqaYhKKbSln9GkC/o0ygjS15FnD+ PY7++lmQIA8XgP6COGCDbWhaUB5I2elVe7oQbaBZ/q9m0FgHA4BhbwvZhMq0f7ze M7hAOFRTlXbMtjbfaypYGYPMMSVyvvtOih62hsy6rVSDwNpq2ezAveC1KWnXIOYZ 77qCx5vNd2xx5nzkplviIA6GpV1GxEGm9/wgcItU5JHM9wgX5RDe9vxm1fiLP5FJ kkNX4gq9lEv2OEanhboq6X+h/dUj84m7oj2Xu+3h4vRs0IzOoQFcZt4Ur9OdU0Fh rHmNxkgGjBd3dhLAVsyT6TxlRU6UQYULcWLNicB/B1comePITECALuM1TGC0/J12 fO0/DGLDLgqgoS6Zd7Rhe7BYGV+wMAm/H/BqoNYNQyHObHHuDkejMJ96DtA76FL0 oCCvl26lG8C9FtLPc4I7fdH+dZL5gUGlWevnUNPycG8aNrV5eJ5PfcOwmlCgyNDQ Sb1MbYIx1rg7ugxW4UbLiUV2Z+ufa9dxx6KFWFt4BGIrtiAIeAEwP2BISbD+I37E U//1qIkE =vRDk -----END PGP SIGNATURE----- --=_MailMate_DE17EC09-DF1F-4138-802E-E02A60908704_=--