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Howlett" , Vlastimil Babka , Shakeel Butt , David Hildenbrand , Rik van Riel , Harry Yoo , Jann Horn , Mike Rapoport , Michal Hocko , Pedro Falcato , Chris Li , Barry Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 3/8] mm/rmap: remove unnecessary root lock dance in anon_vma clone, unmap Message-ID: <2808d8d8-e772-4210-ada4-2b39d30c7029@lucifer.local> References: <9b5a438237e4ea3b19a986a4f8fddb8848cba54a.1767711638.git.lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: LNXP265CA0047.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:5c::35) To DM4PR10MB8218.namprd10.prod.outlook.com (2603:10b6:8:1cc::16) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR10MB8218:EE_|SN7PR10MB6288:EE_ X-MS-Office365-Filtering-Correlation-Id: 59f15ddb-d2d5-4f50-ea35-08de5395e791 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 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 IKlOEF2v 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Jan 14, 2026 at 08:55:58AM -0800, Suren Baghdasaryan wrote: > On Tue, Jan 6, 2026 at 7:04 AM Lorenzo Stoakes > wrote: > > > > The root anon_vma of all anon_vma's linked to a VMA must by definition be > > the same - a VMA and all of its descendants/ancestors must exist in the > > same CoW chain. > > > > Commit bb4aa39676f7 ("mm: avoid repeated anon_vma lock/unlock sequences in > > anon_vma_clone()") introduced paranoid checking of the root anon_vma > > remaining the same throughout all AVC's in 2011. > > > > I think 15 years later we can safely assume that this is always the case. > > > > Additionally, since unfaulted VMAs being cloned from or unlinked are > > no-op's, we can simply lock the anon_vma's associated with this rather than > > doing any specific dance around this. > > > > This removes unnecessary checks and makes it clear that the root anon_vma > > is shared between all anon_vma's in a given VMA's anon_vma_chain. > > > > Signed-off-by: Lorenzo Stoakes > > --- > > mm/rmap.c | 51 +++++++++++++++------------------------------------ > > 1 file changed, 15 insertions(+), 36 deletions(-) > > > > diff --git a/mm/rmap.c b/mm/rmap.c > > index de2cbe860566..6ac42671bedd 100644 > > --- a/mm/rmap.c > > +++ b/mm/rmap.c > > @@ -232,32 +232,6 @@ int __anon_vma_prepare(struct vm_area_struct *vma) > > return -ENOMEM; > > } > > > > -/* > > - * This is a useful helper function for locking the anon_vma root as > > - * we traverse the vma->anon_vma_chain, looping over anon_vma's that > > - * have the same vma. > > - * > > - * Such anon_vma's should have the same root, so you'd expect to see > > - * just a single mutex_lock for the whole traversal. > > - */ > > -static inline struct anon_vma *lock_anon_vma_root(struct anon_vma *root, struct anon_vma *anon_vma) > > -{ > > - struct anon_vma *new_root = anon_vma->root; > > - if (new_root != root) { > > - if (WARN_ON_ONCE(root)) > > - up_write(&root->rwsem); > > - root = new_root; > > - down_write(&root->rwsem); > > - } > > - return root; > > -} > > - > > -static inline void unlock_anon_vma_root(struct anon_vma *root) > > -{ > > - if (root) > > - up_write(&root->rwsem); > > -} > > - > > static void check_anon_vma_clone(struct vm_area_struct *dst, > > struct vm_area_struct *src) > > { > > @@ -310,26 +284,28 @@ static void cleanup_partial_anon_vmas(struct vm_area_struct *vma); > > int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) > > { > > struct anon_vma_chain *avc, *pavc; > > - struct anon_vma *root = NULL; > > > > check_anon_vma_clone(dst, src); > > > > if (!src->anon_vma) > > return 0; > > > > + check_anon_vma_clone(dst, src); > > check_anon_vma_clone() was already called 4 lines above. I'm guessing > this was accidentally carried over from the previous version? > Yeah I don't know why this is here, in the tree it isn't, maybe Andrew noticed and elided? :) But yeah the check above is correct so current in-tree state is correct. Thanks, Lorenzo