On Fri 08-04-22 13:45:24, Zhang Yi wrote: > On 2022/4/7 21:55, Jan Kara wrote: > > On Thu 07-04-22 16:14:24, Zhang Yi wrote: > >> 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. > > > > OK, I was thinking a bit more about this and the comment was actually a bit > > misleading AFAICT. So we have: > > > > EXT4_INDEX_EXTRA_TRANS_BLOCKS for addition of entry into the directory. > > +3 for inode, inode bitmap, group descriptor allocation > > EXT4_DATA_TRANS_BLOCKS for the data block allocation and modification. > > > > So things actually look OK, just the comment was wrong and in the old code > > the credits were overestimated (because we've allocated the data block in a > > separate transaction). > > > > Yes，I will update the comments in my v2 iteration. > > >> 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? > > > > Yes, -2 looks strange but at the same time I fail to see why > > EXT4_XATTR_TRANS_BLOCKS would need to be accounted for normal data > > operation and why we're reserving 6 blocks there. I'll raise it on today's > > ext4 call if someone remembers... > > > > I guess the 6 blocks were: > > 1. Ref count update on old xattr block > 2. new xattr block > 3. block bitmap update for new xattr block > 4. group descriptor for new xattr block > 5. block bitmap update for old xattr block > 6. group descriptor for old block > > The 5 and 6 looks like were overestimated in cases of 1) we just update the > old ref count to no zero, 2) we free the old xattr block and the credits has > already counted in the default revoke credits. Yes, your explanation of 6 blocks in EXT4_XATTR_TRANS_BLOCKS looks good. But I still wonder why we count with modification of xattrs for each data block write. EXT4_XATTR_TRANS_BLOCKS was added to EXT4_DATA_TRANS_BLOCKS back at the times when it was still ext3 and we have added xattr support to ext3. Looking at places where EXT4_DATA_TRANS_BLOCKS is used (mostly in fs/ext4/namei.c when adding entry into a directory), this was probably to account for a fact that when we create new inode, we may be cloning or otherwise modifying xattr block as well. So it seems that EXT4_DATA_TRANS_BLOCKS has somewhat misleading name (it should rather be called EXT4_INODE_CREATE_BLOCKS or something like that) but in principle we indeed need to count with xattr block modifications. Anyway, that's for a separate cleanup. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
