Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Received-SPF: Pass (protection.outlook.com: domain of nvidia.com designates
 216.228.117.161 as permitted sender) receiver=protection.outlook.com;
 client-ip=216.228.117.161; helo=mail.nvidia.com; pr=C
Message-ID: <a679502e-0945-c834-9c0a-66cb7b194069@nvidia.com>
Date:   Thu, 8 Dec 2022 11:56:01 -0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.5.1
Subject: Re: [PATCH mm-unstable] mm: clarify folio_set_compound_order() zero
 support
Content-Language: en-US
To:     Matthew Wilcox <willy@infradead.org>,
        Sidhartha Kumar <sidhartha.kumar@oracle.com>
CC:     <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        <akpm@linux-foundation.org>, <songmuchun@bytedance.com>,
        <mike.kravetz@oracle.com>, <tsahu@linux.ibm.com>,
        <david@redhat.com>
References: <20221207223731.32784-1-sidhartha.kumar@oracle.com>
 <92965844-c430-8b8e-d9f1-705d7578bceb@nvidia.com>
 <ec8f46ca-9ea6-4567-2038-22f6d3000ed5@oracle.com>
 <d17530ad-8e12-8069-d619-a2d72fe80e15@nvidia.com>
 <0187f9c2-e80a-9cde-68bc-c9bdbd96b6fe@oracle.com>
 <Y5I78soNmAFv7pi8@casper.infradead.org>
From:   John Hubbard <jhubbard@nvidia.com>
In-Reply-To: <Y5I78soNmAFv7pi8@casper.infradead.org>
Content-Type: text/plain; charset="UTF-8"; format=flowed
Content-Transfer-Encoding: 7bit
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 08 Dec 2022 19:56:07.4160
 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: f8a6a2f1-fb85-4f09-a0eb-08dad9563f25
X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a
X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.161];Helo=[mail.nvidia.com]
X-MS-Exchange-CrossTenant-AuthSource: BN8NAM11FT032.eop-nam11.prod.protection.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Anonymous
X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem
X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA0PR12MB4542
X-Spam-Status: No, score=-1.4 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,
        DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FORGED_SPF_HELO,
        NICE_REPLY_A,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_PASS,
        SPF_NONE autolearn=no autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 12/8/22 11:33, Matthew Wilcox wrote:
> None of the above!
> 
> Whatever we're calling this function *it does not belong* in mm.h.
> Anything outside the MM calling it is going to be a disaster -- can you
> imagine what will happen if a filesystem or device driver is handed a
> folio and decides "Oh, I'll just change the size of this folio"?  It is
> an attractive nuisance and should be confined to mm/internal.h *at best*.
> 
> Equally, we *must not have* separate folio_set_order() and
> folio_set_nr_pages().  These are the same thing!  They must be kept
> in sync!  If we are to have a folio_set_order() instead of open-coding
> it, then it should also update nr_pages.
> 
> So, given that this is now an internal-to-mm, if not internal-to-hugetlb
> function, I see no reason that it should not handle the case of 0.
> I haven't studied what hugetlb_dissolve does, or why it can't use the
> standard split_folio(), but I'm sure there's a good reason.

Sure, perhaps I overreacted to the "abuse of this function" comment, and
the whole thing could be fixed up with a clean name, hiding it from
the public mm.h API, and...removing comments about abuse! haha :)


thanks,
-- 
John Hubbard
NVIDIA