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[2003:d8:2f27:6d00:7b96:afc9:83d0:5bd]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4613f3c69d5sm39701795e9.24.2025.09.17.07.52.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 17 Sep 2025 07:52:45 -0700 (PDT) Message-ID: Date: Wed, 17 Sep 2025 16:52:40 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 01/11] filemap: Pass address_space mapping to ->free_folio() To: Hugh Dickins , "Roy, Patrick" Cc: "Thomson, Jack" , "Kalyazin, Nikita" , "Cali, Marco" , "derekmn@amazon.co.uk" , Elliot Berman , "willy@infradead.org" , "corbet@lwn.net" , "pbonzini@redhat.com" , "maz@kernel.org" , "oliver.upton@linux.dev" , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "catalin.marinas@arm.com" , "will@kernel.org" , "chenhuacai@kernel.org" , "kernel@xen0n.name" , "paul.walmsley@sifive.com" , "palmer@dabbelt.com" , "aou@eecs.berkeley.edu" , "alex@ghiti.fr" , "agordeev@linux.ibm.com" , "gerald.schaefer@linux.ibm.com" , "hca@linux.ibm.com" , "gor@linux.ibm.com" , "borntraeger@linux.ibm.com" , "svens@linux.ibm.com" , "dave.hansen@linux.intel.com" , "luto@kernel.org" , "peterz@infradead.org" , "tglx@linutronix.de" , "mingo@redhat.com" , "bp@alien8.de" , "x86@kernel.org" , "hpa@zytor.com" , "trondmy@kernel.org" , "anna@kernel.org" , "hubcap@omnibond.com" , "martin@omnibond.com" , "viro@zeniv.linux.org.uk" , "brauner@kernel.org" , "jack@suse.cz" , "akpm@linux-foundation.org" , "lorenzo.stoakes@oracle.com" , "Liam.Howlett@oracle.com" , "vbabka@suse.cz" , "rppt@kernel.org" , "surenb@google.com" , "mhocko@suse.com" , "ast@kernel.org" , "daniel@iogearbox.net" , "andrii@kernel.org" , "martin.lau@linux.dev" , "eddyz87@gmail.com" , "song@kernel.org" , "yonghong.song@linux.dev" , "john.fastabend@gmail.com" , "kpsingh@kernel.org" , "sdf@fomichev.me" , "haoluo@google.com" , "jolsa@kernel.org" , "jgg@ziepe.ca" , "jhubbard@nvidia.com" , "peterx@redhat.com" , "jannh@google.com" , "pfalcato@suse.de" , "axelrasmussen@google.com" , "yuanchu@google.com" , "weixugc@google.com" , "hannes@cmpxchg.org" , "zhengqi.arch@bytedance.com" , "shakeel.butt@linux.dev" , "shuah@kernel.org" , "seanjc@google.com" , "linux-fsdevel@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "kvmarm@lists.linux.dev" , "loongarch@lists.linux.dev" , "linux-riscv@lists.infradead.org" , "linux-s390@vger.kernel.org" , "linux-nfs@vger.kernel.org" , "devel@lists.orangefs.org" , "linux-mm@kvack.org" , "bpf@vger.kernel.org" , "linux-kselftest@vger.kernel.org" References: <20250912091708.17502-1-roypat@amazon.co.uk> <20250912091708.17502-2-roypat@amazon.co.uk> <7c2677e1-daf7-3b49-0a04-1efdf451379a@google.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 16.09.25 08:23, Hugh Dickins wrote: > On Fri, 12 Sep 2025, Roy, Patrick wrote: > >> From: Elliot Berman >> >> When guest_memfd removes memory from the host kernel's direct map, >> direct map entries must be restored before the memory is freed again. To >> do so, ->free_folio() needs to know whether a gmem folio was direct map >> removed in the first place though. While possible to keep track of this >> information on each individual folio (e.g. via page flags), direct map >> removal is an all-or-nothing property of the entire guest_memfd, so it >> is less error prone to just check the flag stored in the gmem inode's >> private data. However, by the time ->free_folio() is called, >> folio->mapping might be cleared. To still allow access to the address >> space from which the folio was just removed, pass it in as an additional >> argument to ->free_folio, as the mapping is well-known to all callers. >> >> Link: https://lore.kernel.org/all/15f665b4-2d33-41ca-ac50-fafe24ade32f@redhat.com/ >> Suggested-by: David Hildenbrand >> Acked-by: David Hildenbrand >> Signed-off-by: Elliot Berman >> [patrick: rewrite shortlog for new usecase] >> Signed-off-by: Patrick Roy >> --- >> Documentation/filesystems/locking.rst | 2 +- >> fs/nfs/dir.c | 11 ++++++----- >> fs/orangefs/inode.c | 3 ++- >> include/linux/fs.h | 2 +- >> mm/filemap.c | 9 +++++---- >> mm/secretmem.c | 3 ++- >> mm/vmscan.c | 4 ++-- >> virt/kvm/guest_memfd.c | 3 ++- >> 8 files changed, 21 insertions(+), 16 deletions(-) >> >> diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst >> index aa287ccdac2f..74c97287ec40 100644 >> --- a/Documentation/filesystems/locking.rst >> +++ b/Documentation/filesystems/locking.rst >> @@ -262,7 +262,7 @@ prototypes:: >> sector_t (*bmap)(struct address_space *, sector_t); >> void (*invalidate_folio) (struct folio *, size_t start, size_t len); >> bool (*release_folio)(struct folio *, gfp_t); >> - void (*free_folio)(struct folio *); >> + void (*free_folio)(struct address_space *, struct folio *); >> int (*direct_IO)(struct kiocb *, struct iov_iter *iter); >> int (*migrate_folio)(struct address_space *, struct folio *dst, >> struct folio *src, enum migrate_mode); > > Beware, that is against the intent of free_folio(). > > Since its 2.6.37 origin in 6072d13c4293 ("Call the filesystem back > whenever a page is removed from the page cache"), freepage() or > free_folio() has intentionally NOT taken a struct address_space *mapping, > because that structure may already be freed by the time free_folio() is > called, if the last folio holding it has now been freed. Thanks for noticing that Hugh, very good point! > > Maybe something has changed since then, or maybe it happens to be safe > just in the context in which you want to use it; but it is against the > principle of free_folio(). (Maybe an rcu_read_lock() could be added > in __remove_mapping() to make it safe nowadays? maybe not welcome.) Let me dig into the callers: 1) filemap_free_folio() filemap_free_folio() looks up the callback through mapping->a_ops->free_folio. Nothing happens in-between that lookup and the callback so we should be good. 2) replace_page_cache_folio() replace_page_cache_folio() similarly looks up the callback through mapping->a_ops->free_folio. We do some operations afterwards, but essentially store the new folio in the page cache and remove the old one. The only caller is fuse_try_move_folio(), and IIUC both folios are locked, preventing concurrent truncation and the mapping going away. 3) __remove_mapping() __remove_mapping() also looks up the callback through mapping->a_ops->free_folio. Before we call free_folio() we remove the folio from the pagecache (__filemap_remove_folio) to then drop locks and call free_folio(). We're only holding the folio lock at that point. So yes I agree, truncate_inode_pages_final() could be racing with __remove_mapping().c That's probably exactly what the docs describe regarding reclaim. rcu_read_lock() should indeed work, or some other mechanism that keeps truncate_inode_pages_final() from succeeding in this racy situation. Alternatively I guess we would have to use another callback. -- Cheers David / dhildenb