Date:   Mon, 6 Feb 2023 09:14:02 -0400
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Alistair Popple <apopple@nvidia.com>
Cc:     linux-mm@kvack.org, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, jhubbard@nvidia.com,
        tjmercier@google.com, hannes@cmpxchg.org, surenb@google.com,
        mkoutny@suse.com, daniel@ffwll.ch, netdev@vger.kernel.org,
        linux-rdma@vger.kernel.org, rds-devel@oss.oracle.com
Subject: Re: [RFC PATCH 10/19] net: skb: Switch to using vm_account
Message-ID: <Y+D9Gkuo1/l0Ty9W@nvidia.com>
References: <cover.f52b9eb2792bccb8a9ecd6bc95055705cfe2ae03.1674538665.git-series.apopple@nvidia.com>
 <9b54eef0b41b678cc5f318bd5ae0917bba5b8e21.1674538665.git-series.apopple@nvidia.com>
 <Y8/wWTOOjyfGBrP0@nvidia.com>
 <87pmawz2ma.fsf@nvidia.com>
 <878rhbflcs.fsf@nvidia.com>
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <878rhbflcs.fsf@nvidia.com>
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?8mHaLS4zGeYb37aoHdj4xT8M3Pb27KE5URaoNqfGsujMlNEE4dIuKiRC50v5?=
 =?us-ascii?Q?GTuaSFWugosaxbdBhHXbthnlDtkInlD8fDH4AOouirzAxtQbxE8TyFW89Brm?=
 =?us-ascii?Q?dDSZq3SLBCKVuud6ad7ivA24MW9ZXtqHbZcsV/t6crfqsqndi8yQZs9N++AX?=
 =?us-ascii?Q?3hv60wlP4dKgMyDR7APYUMeKXDEHWFuhnzHILoqaJK0bTYJ9libCMnM1N3Wb?=
 =?us-ascii?Q?PyJg7iuAFiZZ7fdb68uhtzQ0aIcWhrN5fef75RHRbH+y4TIkcPedTbHXigE5?=
 =?us-ascii?Q?4904/wnxreHBfkOJ4FcpdRZEkDt7PxTwYXN+eOZE4Cg8p6Fxx9FYUicAbCMd?=
 =?us-ascii?Q?HnneB7zUzgDPk+hKBOJEl8fGQ3oBenvho088PduQlnYFP3KVXoTQAeY49Unl?=
 =?us-ascii?Q?dsMclQ7B4GXyDtVfUJGlbs/KprlrXHsqCv+s8QWN4oyv299S1BL4zCguW7hk?=
 =?us-ascii?Q?9XlJqix/HlEu+Or8326rhN6uJrXpgoL1B+38kHeyDjikuXbz1Ew5q0MNUOZ0?=
 =?us-ascii?Q?0YTcePCkPdQ7enRJdHPNLnyTJxS6KG/Q/YxRoN9aehYCz3grtKTvO3SBY0xP?=
 =?us-ascii?Q?qG1B1EhpAUoU/hh5CTFSSYNfbznsWRvSCAz8U1tGVRsiBjBSQfhwm+55XPlt?=
 =?us-ascii?Q?aUYxy73b4sCdsi2LYl2fe5dZE/Aj8xd0Yaow7W6Qt67qty22dVEoa6M5vduw?=
 =?us-ascii?Q?iHV4d8XZBurgDFCs5Nl7ffCrnDM6W0kTihj9LOGP5tAXFWK81k/MVyDsoKrs?=
 =?us-ascii?Q?/X/uUXQNFLjd/fCoMRG7KQsTSvSt0RqNSdgTf2FPaLbMRMSMiesR5xKuDsXy?=
 =?us-ascii?Q?1xakDJ/ZkKKA0tUIG8uf+5zvBLN2IRWjr2MtoGvUCyrGaFxw26nj8q9fi5ud?=
 =?us-ascii?Q?OwZzOigydHXvhjxRXsrqLsMm0Cktwf7ohCzvR7j+dSoaLJ8lMNiwPFJEFUJo?=
 =?us-ascii?Q?Glyo/+JFKbKAJPGwt15IDhYodQGSCGOODpsPxjBfEM1h1ohG6r5fwhTIXABS?=
 =?us-ascii?Q?b7TCjW8XUNft4wsmGfE1VQFoqJOOckC8Qcx1P2TtEyyP+0B0F7x0OcvEQpg1?=
 =?us-ascii?Q?ygceKdM3pfQdBAmXOeldjyH1XViS6w7Ff9DwlRHUMGVce0/oj8d4aC8rVp1C?=
 =?us-ascii?Q?GOpmnvZLnEa8SBVDe6AoYDfizvgWADpIB/CxaYDZyC8dPNS0Yk6rd21rcH7n?=
 =?us-ascii?Q?D7LMbVPvw/txpo4RvA5OwGWla1e4L+pZu0dyVbNmtY7mKUE7WRq8Ot8YzGAF?=
 =?us-ascii?Q?pVQfgspnrU4qb8WYpfplaifOpAGNlnIvnJQQtAqYegjtgBC77qXBab4lQFh8?=
 =?us-ascii?Q?gHnmM46NqbqWWncv093af8xmkQn29KTrU4cF4IEAbnAOUkIigdocxyolm0iN?=
 =?us-ascii?Q?CArjSigb4AaL/0x7QiSu0Y2eh5c+1Nn+0C0130RTiNfmg5nMvVhLEmcTqlKx?=
 =?us-ascii?Q?tXctmsTtXEBQXzMy4Bfj926qfQpimQ7lBSrgcst2KaLtwU957jDWEUCs4w+s?=
 =?us-ascii?Q?2j5oDm60Fr5ClfeV2OcdSBQ1i+YpI+s7Kg2I5WsbHGs3sHdWqPuNbmiPw7zs?=
 =?us-ascii?Q?dHWcAPSUue8KXWjrPVGoG1TwCCGvyJVy0MZgYchB?=
X-OriginatorOrg: Nvidia.com
X-MS-Exchange-CrossTenant-Network-Message-Id: ca40b878-b154-48d8-979b-08db084405cb
X-MS-Exchange-CrossTenant-AuthSource: LV2PR12MB5869.namprd12.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 06 Feb 2023 13:14:05.2490
 (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: 9Zg0nTuywuEszodc7W1RRE1ELc/IUvyF3AqNYnX0bRTZieM6djzHUHIgTKKq76Ls
X-MS-Exchange-Transport-CrossTenantHeadersStamped: MW3PR12MB4380
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Feb 06, 2023 at 03:36:49PM +1100, Alistair Popple wrote:

> >> But then I don't really know how RDS works, Santos?
> >>
> >> Regardless, maybe the vm_account should be stored in the
> >> rds_msg_zcopy_info ?
> >
> > On first glance that looks like a better spot. Thanks for the
> > idea.
> 
> That works fine for RDS but not for skbuff. 

I would definately put the RDS stuff like that..

> We still need a vm_account in the struct sock or somewhere else for
> that. For example in msg_zerocopy_realloc() we only have a struct
> ubuf_info_msgzc available. We can't add a struct vm_account field to
> that because ultimately it is stored in struct sk_buff->ck[] which
> is not large enough to contain ubuf_info_msgzc + vm_account.

Well, AFAICT this is using iov_iter to get the pages and in general
iov_iter - eg as used for O_DIRECT - doesn't charge anything.

If this does somehow allow a userspace to hold pin a page for a long
time then it is already technically wrong because it doesn't use
FOLL_LONGTERM.

Arguably FOLL_LONGTERM should be the key precondition to require
accounting.

So I wonder if it should just be deleted?

Jason