On 2022/4/7 1:17, Jan Kara wrote: > On Wed 06-04-22 16:45:03, Zhang Yi wrote: >> Symlink's external data block is one kind of metadata block, and now >> that almost all ext4 metadata block's page cache (e.g. directory blocks, >> quota blocks...) belongs to bdev backing inode except the symlink. It >> is essentially worked in data=journal mode like other regular file's >> data block because probably in order to make it simple for generic VFS >> code handling symlinks or some other historical reasons, but the logic >> of creating external data block in ext4_symlink() is complicated. and it >> also make things confused if user do not want to let the filesystem >> worked in data=journal mode. This patch convert the final exceptional >> case and make things clean, move the mapping of the symlink's external >> data block to bdev like any other metadata block does. >> >> Signed-off-by: Zhang Yi <yi.zhang@xxxxxxxxxx> >> --- >> This RFC patch follow the talking of whether if we could unify the >> journal mode of ext4 metadata blocks[1], it stop using the data=journal >> mode for the final exception case of symlink's external data block. Any >> comments are welcome, thanks. >> >> [1]. https://lore.kernel.org/linux-ext4/20220321151141.hypnhr6o4vng2sa6@xxxxxxxxxx/T/#m84b942a6bb838ba60ae8afd906ebbb987a577488 >> >> fs/ext4/inode.c | 9 +--- >> fs/ext4/namei.c | 123 +++++++++++++++++++++------------------------- >> fs/ext4/symlink.c | 44 ++++++++++++++--- >> 3 files changed, 93 insertions(+), 83 deletions(-) > > Hum, we don't save on code but I'd say the result is somewhat more > standard. So I guess this makes some sense. Let's see what Ted thinks... > > Otherwise I've found just one small bug below. > >> @@ -3270,26 +3296,8 @@ static int ext4_symlink(struct user_namespace *mnt_userns, struct inode *dir, >> if (err) >> return err; >> >> - if ((disk_link.len > EXT4_N_BLOCKS * 4)) { >> - /* >> - * For non-fast symlinks, we just allocate inode and put it on >> - * orphan list in the first transaction => we need bitmap, >> - * group descriptor, sb, inode block, quota blocks, and >> - * possibly selinux xattr blocks. >> - */ >> - credits = 4 + EXT4_MAXQUOTAS_INIT_BLOCKS(dir->i_sb) + >> - EXT4_XATTR_TRANS_BLOCKS; >> - } else { >> - /* >> - * Fast symlink. We have to add entry to directory >> - * (EXT4_DATA_TRANS_BLOCKS + EXT4_INDEX_EXTRA_TRANS_BLOCKS), >> - * allocate new inode (bitmap, group descriptor, inode block, >> - * quota blocks, sb is already counted in previous macros). >> - */ >> - credits = EXT4_DATA_TRANS_BLOCKS(dir->i_sb) + >> - EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3; >> - } >> - >> + credits = EXT4_DATA_TRANS_BLOCKS(dir->i_sb) + >> + EXT4_INDEX_EXTRA_TRANS_BLOCKS + 3; > > This does not seem like enough credits - we may need to allocate inode, add > entry to directory, allocate & initialize symlink block. So I think you > need to add 4 for block allocation + init in case of non-fast symlink. And > please keep the comment explaining what is actually counted in the number > of credits... > Thanks for pointing this out, and ext4_mkdir() seems has the same problem too because we also need to allocate one more block to store '.' and '..' entries for a new created empty directory. BTW, look the credits calculation in depth, the definition of EXT4_DATA_TRANS_BLOCKS is weird, the '-2' subtraction looks wrong. > #define EXT4_DATA_TRANS_BLOCKS(sb) (EXT4_SINGLEDATA_TRANS_BLOCKS(sb) + \ > EXT4_XATTR_TRANS_BLOCKS - 2 + \ > EXT4_MAXQUOTAS_TRANS_BLOCKS(sb)) I see the history log, before commit[1], the '-2' subtract the 2 more duplicate counted super block in '3 * EXT3_SINGLEDATA_TRANS_BLOCKS', but after this commit, it seems buggy because we have only count the super block once. It's a long time ago, I'm not sure am I missing something? [1]. https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=2df2c24aa6d2cd56777570d96494b921568b4405 Thanks, Yi.
