Received-SPF: pass (google.com: domain of linux-kernel+bounces-67684-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Date: Thu, 15 Feb 2024 16:08:47 -0500
From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To: Chuck Lever <chuck.lever@oracle.com>
Cc: Jan Kara <jack@suse.cz>, Chuck Lever <cel@kernel.org>,
        viro@zeniv.linux.org.uk, brauner@kernel.org, hughd@google.com,
        akpm@linux-foundation.org, oliver.sang@intel.com, feng.tang@intel.com,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        maple-tree@lists.infradead.org, linux-mm@kvack.org, lkp@intel.com
Subject: Re: [PATCH RFC 7/7] libfs: Re-arrange locking in offset_iterate_dir()
Message-ID: <20240215210847.u3rnmvt5v2ay7zzq@revolver>
Mail-Followup-To: "Liam R. Howlett" <Liam.Howlett@Oracle.com>,
	Chuck Lever <chuck.lever@oracle.com>, Jan Kara <jack@suse.cz>,
	Chuck Lever <cel@kernel.org>, viro@zeniv.linux.org.uk,
	brauner@kernel.org, hughd@google.com, akpm@linux-foundation.org,
	oliver.sang@intel.com, feng.tang@intel.com,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	maple-tree@lists.infradead.org, linux-mm@kvack.org, lkp@intel.com
References: <170785993027.11135.8830043889278631735.stgit@91.116.238.104.host.secureserver.net>
 <170786028847.11135.14775608389430603086.stgit@91.116.238.104.host.secureserver.net>
 <20240215131638.cxipaxanhidb3pev@quack3>
 <20240215170008.22eisfyzumn5pw3f@revolver>
 <Zc5MnXdxASGiE3lK@tissot.1015granger.net>
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <Zc5MnXdxASGiE3lK@tissot.1015granger.net>
User-Agent: NeoMutt/20220429
Precedence: bulk
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: 
	=?us-ascii?Q?6HJSWzFIlivdblqxrKEOxReBASaNJsznQAzEXvCuxWBaX5Q6GXF4DGeo6WEH?=
 =?us-ascii?Q?0QdAJ1oUe80IXiaipQ6RgHZwWaHoXeoBdFj/W9iME3kL1mUl+Nazm/r1KY2Y?=
 =?us-ascii?Q?FYQB4YfIfhnkh7P6lC9VPNHEigAtji3P7kmv5WhBRfXXUjvFwiOvYF5NkrOp?=
 =?us-ascii?Q?qkU4JTk0ZBltXZKReE3RrKLi3Pr+ZcBHmlwvogePqebx8MG4pWGArMWSiwan?=
 =?us-ascii?Q?0VSbllu/x7b3NpS6bgwabwZ4qETulNlFwPJEfXGpAracLyUxeabHuFB7BAz4?=
 =?us-ascii?Q?kJQIx8JhxERhaohM5cOEbnarTRje7AyGqqNgxkBklAKYchB4swmRgpn01dyK?=
 =?us-ascii?Q?VifsTwaQ1dIRaqSXccpHjBfVA0UuIk9vKOL3c/q/w0g2qw+mjyMyVywu0msE?=
 =?us-ascii?Q?ZT5q15SIuiSkIHtL/cV6QhChnftHWx2eb1p4fJKibbP94mrrfKTnpohFWty0?=
 =?us-ascii?Q?ByDxVBgdTbzhAgunIC6GVAbwL25SZu8zdTW3gBJI7cp8JRMQ8uLV0PUt6z+p?=
 =?us-ascii?Q?o42gfg4xkWdNBfcGxMpOCrZBAui1h6VVXzysGCMKuxVXeLDrHZ8knToQ+94N?=
 =?us-ascii?Q?RTiPISRwpUfhUZGzEm8MeepAD5f2tNz754Ude8AmVi8GXGReiu4l5DyKVnp/?=
 =?us-ascii?Q?dSdNn73RcvRY2FYsojnLd/HvDnp/WsalI/cjLmGwX4Tx9a/b9IkmSf/nMsF2?=
 =?us-ascii?Q?kiMWXPPjiLoME8u4ITwvjzwsFslQ6A46gi885z97TaxE4SluLycEukla5eDm?=
 =?us-ascii?Q?eY9+PTMjuncOZzqjj8PLiob/csWQH8GPSe5sx6pfSybo71vVFfw2MPByvsWh?=
 =?us-ascii?Q?uWzxIN3y0BRj8IxEva3/nQTfeT/CDCbnkZsZ/JEGgSOX563QJhpmJguX2Iif?=
 =?us-ascii?Q?qTA8JTg1d+Q58Tp4+hg1sw5vgmMzptZpRBx/mDok00/Pou8yfNHzpLyWo6M2?=
 =?us-ascii?Q?tYKB0rExM1/PwbVY2DDsDQbzS1jHzSx2/HVmERutLnlHW4Zx0loHwjjDbkiu?=
 =?us-ascii?Q?34ka1yhGx8hwdjXt26VMAoH1w4d1y0bDQbR1J/lBKGt0eypUyR6hba0pXnHO?=
 =?us-ascii?Q?XYR+60syB5F4RqVCkHX+en4+FMkVbRYrrEO7Oj5TZQ9mrfYzw2RyMO9bnORN?=
 =?us-ascii?Q?63nrNEE1NstEbFjAQGvCIxO6q9kPMN6+1ypdqX6qhsaI+Hlep5an7eqtt8xq?=
 =?us-ascii?Q?SlS9BZiEh1JSaeGUN06cOqPSHtFPGqtKzJgoy2s7jbGXiv3zFkcR+By1b7uL?=
 =?us-ascii?Q?o8VI3BiBAMFPZenLpwvwbOhBC1d3ifm3OWfvF/TwNwTP8APQGLg2KBSJrgi7?=
 =?us-ascii?Q?ePHX+/yoXDgr6w/1Vg5J1An9xJCaUDWdhrwGBYPIz+nDMAMHZXC/B24W69zI?=
 =?us-ascii?Q?fbay4h9qatt65/EL1TERgvjk8B0oCU33KpG9pj46BFtau+8J6OdcQ3QBJWcf?=
 =?us-ascii?Q?HNiz43c+als7nqa3PKv7Vw48SFyWrUeIvNPc/sZmWaPnIgdw6Ql6ac1maZum?=
 =?us-ascii?Q?8hacy0jO9/rB2sLvU8bYHbnZeS7t3c39vC5vQVVHX6pE9LJIp1Ty1fqDDij/?=
 =?us-ascii?Q?HaFREdu9tEpm1J60Ps1vxdXAn9RBhmKaXBzdzZV8+rOo3ND7tmyhev+CJptk?=
 =?us-ascii?Q?aw=3D=3D?=
X-MS-Exchange-AntiSpam-ExternalHop-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-ExternalHop-MessageData-0: 
	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
X-OriginatorOrg: oracle.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 6473cf27-04e6-4fe9-7228-08dc2e6a4e66
X-MS-Exchange-CrossTenant-AuthSource: LV8PR10MB7943.namprd10.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 15 Feb 2024 21:08:49.7823
 (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: aydEhWQ3vqsfGMbubDF4O0tZiG1PujP2RYpbVTAId5M/P1cj9XKXRLq0F6yMneC+BXGa7pFFoNroz3DXAclZwQ==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR10MB4236
X-Proofpoint-Virus-Version: vendor=baseguard
 engine=ICAP:2.0.272,Aquarius:18.0.1011,Hydra:6.0.619,FMLib:17.11.176.26
 definitions=2024-02-15_20,2024-02-14_01,2023-05-22_02
X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 malwarescore=0 mlxscore=0 spamscore=0
 adultscore=0 phishscore=0 bulkscore=0 mlxlogscore=963 suspectscore=0
 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2311290000
 definitions=main-2402150168
X-Proofpoint-ORIG-GUID: 33f0wMru65IlyBrLpdDMOK6lObSnm4ub
X-Proofpoint-GUID: 33f0wMru65IlyBrLpdDMOK6lObSnm4ub

* Chuck Lever <chuck.lever@oracle.com> [240215 12:40]:
> On Thu, Feb 15, 2024 at 12:00:08PM -0500, Liam R. Howlett wrote:
> > * Jan Kara <jack@suse.cz> [240215 08:16]:
> > > On Tue 13-02-24 16:38:08, Chuck Lever wrote:
> > > > From: Chuck Lever <chuck.lever@oracle.com>
> > > > 
> > > > Liam says that, unlike with xarray, once the RCU read lock is
> > > > released ma_state is not safe to re-use for the next mas_find() call.
> > > > But the RCU read lock has to be released on each loop iteration so
> > > > that dput() can be called safely.
> > > > 
> > > > Thus we are forced to walk the offset tree with fresh state for each
> > > > directory entry. mt_find() can do this for us, though it might be a
> > > > little less efficient than maintaining ma_state locally.
> > > > 
> > > > Since offset_iterate_dir() doesn't build ma_state locally any more,
> > > > there's no longer a strong need for offset_find_next(). Clean up by
> > > > rolling these two helpers together.
> > > > 
> > > > Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> > > 
> > > Well, in general I think even xas_next_entry() is not safe to use how
> > > offset_find_next() was using it. Once you drop rcu_read_lock(),
> > > xas->xa_node could go stale. But since you're holding inode->i_rwsem when
> > > using offset_find_next() you should be protected from concurrent
> > > modifications of the mapping (whatever the underlying data structure is) -
> > > that's what makes xas_next_entry() safe AFAIU. Isn't that enough for the
> > > maple tree? Am I missing something?
> > 
> > If you are stopping, you should be pausing the iteration.  Although this
> > works today, it's not how it should be used because if we make changes
> > (ie: compaction requires movement of data), then you may end up with a
> > UAF issue.  We'd have no way of knowing you are depending on the tree
> > structure to remain consistent.
> > 
> > IOW the inode->i_rwsem is protecting writes of data but not the
> > structure holding the data.
> > 
> > This is true for both xarray and maple tree.
> 
> Would it be appropriate to reorder this series so 7/7 comes before
> the transition to use Maple Tree?

I think it would, yes.

Thanks,
Liam