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;
Date:   Thu, 8 Dec 2022 12:01:15 -0800
From:   Mike Kravetz <mike.kravetz@oracle.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Sidhartha Kumar <sidhartha.kumar@oracle.com>,
        John Hubbard <jhubbard@nvidia.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, songmuchun@bytedance.com,
        tsahu@linux.ibm.com, david@redhat.com
Subject: Re: [PATCH mm-unstable] mm: clarify folio_set_compound_order() zero
 support
Message-ID: <Y5JCi3h8bUzLf3cu@monkey>
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>
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Y5I78soNmAFv7pi8@casper.infradead.org>
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?oTq45sw3fA57AtuYrZYxXNryJnpF+CrKrEaoCBOqg4rFR/7yoOdJAPtolGSn?=
 =?us-ascii?Q?aFrzVH1QGwmg1Mo47ukUTx7LKgvziRbYtadiqLQRTGxgnP6wCTzV/lN5KLn/?=
 =?us-ascii?Q?MgdSr8wzQKPNHNEKBhEZHzQc/BOXaYGbOyHBlc2OfOJCFyFK6O+SXDhAP+GV?=
 =?us-ascii?Q?Tntvmp4Zzafx+1ahvc3ygjd+7DlG1rImGYt0ZVfRFIXIl4/RuHsAwXeSqI3O?=
 =?us-ascii?Q?siqhsad6WyF8rPzBysmv5lwFoN32lGQSy+q/YAGPebiQ7rGnZmLQPcRCGoGY?=
 =?us-ascii?Q?/+M6BvLT1pbyjRkW8nyAWsHfaL5umonawnAje3FbT8VrOmaC1x6nPEMtgvoR?=
 =?us-ascii?Q?UDFnurJucPxDqM7vFitRBv9qc3HCpO5QsrfAMNdpYUzXWyz/Ck05jh6ABokB?=
 =?us-ascii?Q?I1qk5Ak+QuurKOUwdUsf5uSECVcGz4pVG+mbCKsvCBqzFTEQzoDbVhs0n3gd?=
 =?us-ascii?Q?j6gsvm88wQgQiNP7+iGe1KHd02qeaSva+eLCLYW5iRSqLPnZEREJ+aFOuaox?=
 =?us-ascii?Q?yF2dnGR+1pF4TGxl18Htce65U/BJtO5jSVNdsjG7A/gu9dMoq0QL93IcnGUh?=
 =?us-ascii?Q?ewrgzkNVfY9VQeVRBQeOvns8ZxG0fNLJFydMihfXaYrZpRuLo9CPTWXt7d8r?=
 =?us-ascii?Q?A0g6+63t4AAAaEs+qwXHS39YMSvv/RldjyvWk+TW+SeyXIFXGE3q1FRBoTVO?=
 =?us-ascii?Q?aENNEFrHD1mcRwRqUkKBGEVqpRyyg18o7S51RreFDhm6UBLYn1aOcVK1taWz?=
 =?us-ascii?Q?uCJgeuFCjDYySi6J2PwRNbKkFN9dOV8uFx+tNthqGodS927k44HIVcjxyjaN?=
 =?us-ascii?Q?ceMMEWqCc4Gvtom28ebzGmv1QyCcBS55CjOy9R23q/f6/i81VlaqW6So606Y?=
 =?us-ascii?Q?zwxx2AFwZdlAdrzly7KhKTBJ+AWZwlrb2qMSKXnuFmlrjeQxC9TJh56xcDN3?=
 =?us-ascii?Q?3KYvnhgj7af0vLJ4cIGtO52wDxJokWtLCkNpt0b0d6EA+F3ru8nZqBSia3Wl?=
 =?us-ascii?Q?9Zlx3XtTQ18LvvWoznEPLXh0P44SUoxYZIfv8h45WTC6tTx6LmtzvYMylsFm?=
 =?us-ascii?Q?pRZghF+QAgTJfeRXDgrNG14rnCu+2N2HNVIc9vBC+b6jhe8Ozt+uqe2crYIN?=
 =?us-ascii?Q?EMIHDwIQvuFd56DKUCN1ZHw8BLa+cExvv9cF7QBMVrEYa3USGuK4uAg0uQFK?=
 =?us-ascii?Q?rtT7ACQS6fgDoDY3cpq1CuVLm0T2KggRLeYibEK3v3s8yq6lrLnS6D8pxbDq?=
 =?us-ascii?Q?R5/4cRPWC+11BBG4t/Um0tgs5jMMhNKBIvUAgGUvBmKy2qk13W0TZD7myA3O?=
 =?us-ascii?Q?Knm73EwvLdRos9wn7aHBs4c4cuStegkWBv6rtSKZyU3ca2y6h3QMsNqI8R2e?=
 =?us-ascii?Q?8UkWlWg+xB25KCr5lw+FYpTTbIWxo7EBgodWrx4H3xmanQWxMHSKplFPbW3I?=
 =?us-ascii?Q?IgFpJW+P4Tb4+kmEPo05XkdWu0Tu8+jgWhojdEnJvZKzndTHXZIYnSssSr1+?=
 =?us-ascii?Q?boZyFdiasaYEY68vnpvYDXuNbnS8eVs4vzHVOWyFTZg5cjT2/bPVStb4Wivi?=
 =?us-ascii?Q?iGUDIAew/XA58Gd/mHXA7otDmIqDoxYrYgyOY9m0zfpNIJB6ui98m4RQ+eV5?=
 =?us-ascii?Q?+Q=3D=3D?=
X-OriginatorOrg: oracle.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 100a85b5-85fc-44aa-350a-08dad956f7d4
X-MS-Exchange-CrossTenant-AuthSource: BY5PR10MB4196.namprd10.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 08 Dec 2022 20:01:17.6828
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: dtBSrUdrP9uylq9TNWqHUl340szalFj7u9ubj5HB/v9g70ERdcOXC+AuWv0wzqUZxiDczD7iQwHpltrHRQQxrA==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: CO1PR10MB4740
X-Proofpoint-Virus-Version: vendor=baseguard
 engine=ICAP:2.0.205,Aquarius:18.0.923,Hydra:6.0.545,FMLib:17.11.122.1
 definitions=2022-12-08_11,2022-12-08_01,2022-06-22_01
X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=932 bulkscore=0
 suspectscore=0 phishscore=0 malwarescore=0 mlxscore=0 spamscore=0
 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1
 engine=8.12.0-2210170000 definitions=main-2212080166
X-Proofpoint-ORIG-GUID: sGPOXi5xc3mS1UANPsodj5sfqx1KG9AX
X-Proofpoint-GUID: sGPOXi5xc3mS1UANPsodj5sfqx1KG9AX
X-Spam-Status: No, score=-2.8 required=5.0 tests=BAYES_00,DKIM_SIGNED,
        DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_LOW,
        RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE autolearn=ham
        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 12/08/22 19:33, Matthew Wilcox wrote:
> On Thu, Dec 08, 2022 at 10:06:07AM -0800, Sidhartha Kumar wrote:
> > On 12/7/22 6:27 PM, John Hubbard wrote:
> > > On 12/7/22 17:42, Sidhartha Kumar wrote:
> > This works for me, I will take this approach along with Muchun's feedback
> > about a wrapper function so as not to touch _folio_order directly and send
> > out a new version.
> > 
> > One question I have is if I should then get rid of
> > folio_set_compound_order() as hugetlb is the only compound page user I've
> > converted to folios so far and its use can be replaced by the suggested
> > folio_set_nr_pages() and folio_set_order().
> > 
> > Hugetlb also has one has one call to folio_set_compound_order() with a
> > non-zero order, should I replace this with a call to folio_set_order() and
> > folio_set_nr_pages() as well, or keep folio_set_compound_order() and remove
> > zero order support and the comment. Please let me know which approach you
> > would prefer.
> 
> 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*.

I suspect it was placed in mm.h as it is the 'folio version' of
set_compound_order which resides in mm.h.  But, no need to repeat that
unfortunate placement.

> 
> 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.

Ok.  Agree.

> 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.

The hugetlb code is changing the compound page/folio it created from a set of
individual pages back to individual pages so they can be returned to the
low level allocator.  Somewhat like what page_alloc/page_free do.  split_folio
is overkill.  split_page would be a closer match.

It makes perfect sense to put the function in mm internal.h.

Thanks,
-- 
Mike Kravetz