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;
From:   Zi Yan <ziy@nvidia.com>
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org,
        Shuah Khan <shuah@kernel.org>, Yang Shi <shy828301@gmail.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-kselftest@vger.kernel.org
Subject: Re: [RFC PATCH 4/5] mm: truncate: split huge page cache page to a non-zero order if possible.
Date:   Tue, 22 Mar 2022 10:19:35 -0400
Message-ID: <B5F3E897-7BBF-422E-AF62-9E4AF50F655C@nvidia.com>
In-Reply-To: <Yjj9FaoChB3u0Gbh@carbon.dhcp.thefacebook.com>
References: <20220321142128.2471199-1-zi.yan@sent.com>
 <20220321142128.2471199-5-zi.yan@sent.com>
 <Yjj9FaoChB3u0Gbh@carbon.dhcp.thefacebook.com>
Content-Type: multipart/signed;
 boundary="=_MailMate_35D1A03D-315B-42D0-8538-3E47809BB42E_=";
 micalg=pgp-sha512; protocol="application/pgp-signature"
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?g0gzXN1veKgmX1ChY6eEodiUWPChH4BLNh7mNYfIfQYa/l+uajqU59Oy8nlD?=
 =?us-ascii?Q?C9qtK9J5IM/hEjQZyH9KIOUlUOGRDbxfUE9F6WFfmG85z8KeDmV1j09olD/w?=
 =?us-ascii?Q?WDxD8sdsEJLLQTpel2sq1TYoIFPyU/yhlgxdxazXmxV+JZ/rpzSUspRMT9Oz?=
 =?us-ascii?Q?bGB76TLtN92RGqR0lBvFYqmwYFHMruRgXIn4bea4OuG9CfX5WcsJeLKDTsoO?=
 =?us-ascii?Q?McW6+p8M0wTzHaWnMvYFR0RHkleJM/RtniPIxct+w1X0D/FdF9TZhwzUG8QI?=
 =?us-ascii?Q?S6H3RcjDzZAo9mVh48MIuVTexd3nj+OwOo0uWYA0xVx4n+IOQGPC1KToGLZB?=
 =?us-ascii?Q?/WPaEkXpp8QHw0cOM7xvyuO4ih7g2rPdEV9FYg/AB3KGI42DEdKyC+urJvEN?=
 =?us-ascii?Q?uTZlQlXP9zheAVeKBZC2BzlGEo0epcTD08s8j1EHgrnN22cD15b+nFusNSvG?=
 =?us-ascii?Q?o1VjLp7EsBwF7ASteGImDu99x88kPOvLUn2a/KjcWxkDmM2m5txMaS0wcLFi?=
 =?us-ascii?Q?GVxZIuomR8dlAsoGVtLbWbB1R1UNSmttQL5yiblbvparSezW33fT6A2CwfPS?=
 =?us-ascii?Q?FUiQEmJjsyd+OQb4ukJwVqwnMPrDB4jS7XxIeyNCaCr6c+erfCGxmDYDDSGO?=
 =?us-ascii?Q?GlnNBrRpC2PJ0sGgSegyNvmV7ha/0k5tJuIuAd0SLWtkA07jDhBc6HtVSmnS?=
 =?us-ascii?Q?ECyWFq2t/b7hrOwK9LrcllqvhI/+cMvHbYeIaIoYF7LipSbiJvBYsx4Nli0K?=
 =?us-ascii?Q?4/cPeLlnnXKzJOJ8AALDMsfhq+kA8SVSKay1rG1X8jLTsC7+6fVJX9+j//ED?=
 =?us-ascii?Q?Y+92FHanznh7Fjs6NTubhXlSQTBd7pqANy4Czgn4oLQLVzPrsuXRuClD/s6W?=
 =?us-ascii?Q?ao6PCe6r3MyEKiWJM0ajdgYpFQRWgYgZSrGgS9QNavuH/8FsBJiX78I9Dkvv?=
 =?us-ascii?Q?6lVQhxfFX7WJsUSQsnv7ZzNLl7vvfGfCOzJKJ0/Rzb1Hpwf22cBu9Xj2uqfv?=
 =?us-ascii?Q?ITuhtNrPY8CsmGbNoYM2NKuCW729Z2bPisE9jw72q2PeUdXBQPpXQfeAhIv5?=
 =?us-ascii?Q?dBh/r10Mnd0bFWhwf+TzG5swp+ksf9XBcgrYFEhgz4jsnQNg2iHnxdFx7RUC?=
 =?us-ascii?Q?dn2a6lkIO5XXpgobvXyN1ePRJ1wARRnbAGw7quRjqTP9nai1K+HHG1h4QJ1S?=
 =?us-ascii?Q?TinUz35oikbpnwZHxkuUQ0tghkQpMancOFBm1a2jP0rxwspoOn45VnQh4ZV/?=
 =?us-ascii?Q?f6m9Cb0sH4NmAQIYNYhtxDCJwl6v7SIygnuUvhNxfUmo4kWvBW949U9pHj1M?=
 =?us-ascii?Q?8O3O0OuDQpJl+3cVyMkvZ/xamM0ZimggGj5PXeqW9a16JvjcP54k9VF0M3/y?=
 =?us-ascii?Q?fJ3fd13pnwUx8xEGn4PS5Uxl1odA8qaK6ryDflYSYBRkwNax2ZMEoHeSGDW1?=
 =?us-ascii?Q?Ot8pmmo/uutzuYYJaON4iCDly9epJgkl?=
X-OriginatorOrg: Nvidia.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 802ecee5-96e9-4bd6-fcc4-08da0c0eff3e
X-MS-Exchange-CrossTenant-AuthSource: MN2PR12MB3823.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 Mar 2022 14:19:37.8989
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: GgWwr4ycQ0OIWqgJr+Zsy+qk4MwlG+ZNV8iNit3j1H5rddeKc4MSjOURu3ITh7bz
X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN1PR12MB2415
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,
        DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FORGED_SPF_HELO,
        RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_PASS,SPF_NONE,
        T_SCC_BODY_TEXT_LINE 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

--=_MailMate_35D1A03D-315B-42D0-8538-3E47809BB42E_=
Content-Type: text/plain

On 21 Mar 2022, at 18:32, Roman Gushchin wrote:

> On Mon, Mar 21, 2022 at 10:21:27AM -0400, Zi Yan wrote:
>> From: Zi Yan <ziy@nvidia.com>
>>
>> To minimize the number of pages after a huge page truncation, we do not
>> need to split it all the way down to order-0. The huge page has at most
>> three parts, the part before offset, the part to be truncated, the part
>> remaining at the end. Find the greatest common power of two multiplier of
>> the non-zero values of them as the new order, so we can split the huge
>> page to this order and keep the remaining pages as large and as few as
>> possible.
>
> Would you mind please to describe the algorithm in more details?

Sure.

During truncation, there can be three parts in a huge page:
1. the _offset_ from the beginning of the huge page,
2. the _length_ of the to-be-truncated part,
3. the _remaining_ part after the to-be-truncated part.

the size of the split huge page need to be the greatest common divisor
of the non-zero ones of three after being rounded down to power of two.

OK, I actually find there is a gcd function. I think the algorithm can
simplified to

new_order = ilog2(gcd(gcd(offset, length), remaining)) - PAGE_SHIFT;

I will update the code, the commit message, and the comment in the next
version.

Thank you for the comment.

--
Best Regards,
Yan, Zi

--=_MailMate_35D1A03D-315B-42D0-8538-3E47809BB42E_=
Content-Description: OpenPGP digital signature
Content-Disposition: attachment; filename="signature.asc"
Content-Type: application/pgp-signature; name="signature.asc"

-----BEGIN PGP SIGNATURE-----
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=etv0
-----END PGP SIGNATURE-----

--=_MailMate_35D1A03D-315B-42D0-8538-3E47809BB42E_=--