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Sun, 27 Sep 2026 07:02:07 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id iL1nC2+/uGo/dgAAD6G6ig (envelope-from ); Sun, 27 Sep 2026 07:02:07 +0000 From: Gabriel Krisman Bertazi To: Daniel Tang , =?utf-8?Q?Andr=C3=A9?= Almeida , Christian Brauner , linux-fsdevel@vger.kernel.org Cc: Alexander Viro , Jan Kara , Theodore Ts'o , Andreas Dilger , Hugh Dickins , Andrew Morton , Jonathan Corbet , smcv@collabora.com, kernel-dev@igalia.com, linux-kernel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-mm@kvack.org, linux-doc@vger.kernel.org Subject: Re: [BUG] Is casefold unreported in /proc/mounts? In-Reply-To: <5193732.31r3eYUQgx@daniel-desktop3> Organization: SUSE References: <5193732.31r3eYUQgx@daniel-desktop3> Date: Sun, 27 Sep 2026 03:02:01 -0400 Message-ID: <877bk7yy6u.fsf@mailhost.krisman.be> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="=-=-=" X-Rspamd-Action: no action X-Rspamd-Server: rspam06 X-Stat-Signature: 9shqbatb58rd7hhgn47up5uweaxh5jcf X-Rspam-User: X-Rspamd-Queue-Id: C660D140002 X-HE-Tag: 1790492537-602551 X-HE-Meta: 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 lV9SSs2F /uzWAfFLFu23qwvxxNXdHOOyeMo2OuGZ4wisEdQ+fmKxhp4rZGte+hWIDihzBDcQIrrXxUkPXe4teUaUcwd9GtLNNl2BFyskS5wrAUC5C21qQmT4pmKonSuE5+PxRPW7l+Jreiu8cpDPA1ihjidXQqPpr4GKVburM6gt5f7SxnztwlaQXXvKIXOfCWV6+APeU936pAVYjiQCfhdE8O0yNm01yqWeTQ840Taq/kY/qMAXww7v3HP+KGRnGA6+Wj0iO3KbgKcvuDhNMN7zGPnwQJQoMhpCB7wzdG4jbdFKA9+cQx+R24WcUVLvEJQNWGz9kL2DoIIsEkT7rWWQVf62s7xOF8EtX7UIISSq6e9LS2imrlRgo9XjcttSoGsjQOPvKQBPbZepqFKpBReWqUNEv8Twx8aqkaFNq5bbiVAyvznjDrKUK2liFLujFqxeyBBjB5scvnpc3JRj2RjhwRjs9zThCMGvv4tdFxBlz7Km2bAPgDoaolLOgQqp+l7yUoaCFVDY0HFJBlUlhczmS2wfiRHsSE5dcpNEAsShfgVLQNoJ+Mq13OsCQ3yrWYw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: --=-=-= Content-Type: text/plain Daniel Tang writes: > Upon successfully mounting via `sudo mount -t tmpfs -o casefold test /mnt`, I > cannot find a read-only way to determine whether this option is enabled. > `mount`, `cat /proc/mounts`, and `cat /proc/mountinfo` only display "rw", > "relatime", and "inode64", never "casefold" for both with and without > `-o casefold`. The only way on 7.0.0-34-generic to test I found is > `mkdir test ; chattr +F test ; echo $?`, but that isn't atomic. Which command > can determine, without writing anything, whether someone or some program > specified `-o casefold`? If absent, I suggest adding the missing info to > `/proc/mounts`, likely `mm/shmem.c:shmem_show_options`. Yeah. For device-backed filesystems it is not a mount parameter, but it is for shmem/ovl and should show in /proc/mounts. Either way, there are two ways to check: 1) Parse dmesg after the mount. It is easily overwritten and probably worse than just trying; 2) better way: do FS_IOC_SETFLAGS on the mountpoint (with chattr +F). If casefold is supported, you'll get -ENOTEMPTY, otherwise you get -EOPNOTSUPP. Since it is the root of the filesystem and a single syscall, unless its remounted from under you (in which case /proc/mounts doesn't help either), you are safe. It is worth mentioning you can check kernel support with /sys/fs/*/features/casefold. We probably should add the same for /sys/fs/ext4//. This fixes /proc/mounts for shmem: --=-=-= Content-Type: text/x-patch Content-Disposition: inline; filename=0001-shmem-Report-casefold-setting-in-mount-options.patch >From c4b198424fac49e10e32553b223b7615febc6513 Mon Sep 17 00:00:00 2001 From: Gabriel Krisman Bertazi Date: Sun, 27 Sep 2026 02:49:26 -0400 Subject: [PATCH] shmem: Report casefold setting in mount options For not device-backed filesystems, casefolding is a mount option. Report the current encoding and casefolding flags in /proc/mounts so userspace can easily check if casefolding is supported. Reported-by: Daniel Tang Link: https://lore.kernel.org/linux-fsdevel/5193732.31r3eYUQgx@daniel-desktop3/#R Signed-off-by: Gabriel Krisman Bertazi --- mm/shmem.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/mm/shmem.c b/mm/shmem.c index 897fa2b61346..2b93fcbf5127 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -4916,6 +4916,17 @@ static int shmem_show_options(struct seq_file *seq, struct dentry *root) if (sbinfo->qlimits.grpquota_ihardlimit) seq_printf(seq, ",grpquota_inode_hardlimit=%lld", sbinfo->qlimits.grpquota_ihardlimit); +#endif +#if IS_ENABLED(CONFIG_UNICODE) + if (root->d_sb->s_encoding) { + unsigned int version = root->d_sb->s_encoding->version; + + seq_printf(seq, ",casefold=utf8-%u.%u.%u", + unicode_major(version), unicode_minor(version), + unicode_rev(version)); + if (sb_has_strict_encoding(root->d_sb)) + seq_puts(seq, ",strict_encoding"); + } #endif return 0; } -- 2.55.0 --=-=-=--