On 2022/4/11 15:55, Jan Kara wrote: > 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. > Indeed. Thanks, Yi.
