| Message ID | 20180820044851.414-1-david@fromorbit.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | xfs: feature flag rework | expand |
On Mon, Aug 20, 2018 at 6:49 AM Dave Chinner <david@fromorbit.com> wrote: > > Hi folks, > > This series reworks how the kernel code handles feature bits. It > seemed to me to be a bit crazy to repeatedly have to check multiple > values in the superblock to check whether a feature is supported. > e.g. to check if reflink is available, we first mask the version > number to extract it from the superblock field, then we check it > against the v5 value, then we load and check the feature bit from > the superblock. > > This could all just be a single flag check - a flag that is set at > mount time after doing all of the above, and so it's just a single > operation to check. > > I'm also tired of typing "xfs_sb_version_hasfoo" everytime a check > needs to be done. Not to mention having a different mechanism to > check mount option based features. > > And then there's the conflation of mount option feature flags and > runtime state in the one set of flags. The flags field is not > updated atomically, so there no guarantee that a change of a state > flag does race with anything else and we lose it. > > So this patch set moves all the feature flags into a new m_features > field in the struct xfs_mount. It doesn't matter if they are mount > options or superblock feature bits - they all end up in the one > place whether they are [almost entirely] read only. These are now > checked by "xfs_has_foo(mp)" functions (following the ext4 feature > check syntax), and for the very few features that can be dynamically > added or removed there are also xfs_feat_add/remove_foo(mp) helpers. > IOWs, both superblock and mount option features are checked the same > way. > > The state flags (clean mount, unmounting, shutdown, etc) are all > moved to a new m_state field, which is manipulated entirely by > atomic bitops to avoid update races. THere are a couple of wrappers > for the common checks - xfs_is_read_only(mp) and > xfs_is_shut_down(mp). The latter replaces the XFS_FORCED_SHUTDOWN() > macro. > > There's a bit of extra work around the attr2 feature bit - we've got > the superblock bit, and then "attr2" and "noattr2" mount options and > somewhat convoluted interactions. This patchset changes the > behaviour of all these now - the attr2 bit on disk and mount option > now mean the same thing, and noattr2 will turn off the superblock > bit if it is set and prevent attr2 inodes from being created. This > simplifies the mount time checking and clearing of these features. > > Also, it gives us a clean separation of "inode32 mount option" and > "inode32 allocation is active". The former is a feature bit (i.e. > SMALL_INUMS), and the latter is a state bit (32BITINODES) that is > set if the filesystem size requires the allocator to limit > allocation to 32 bit inode space. > > This also allows us to get rid of all the xfs_sb_version_hasfoo() > inline functions. THere is a single function to populate m_features > from the superblock bits, and the low level checks that are purely > superblock based (i.e. on disk conversion and verifiers) directly > check the superblock fields rather than use wrappers. > > Overall, this doesn't change the amount of code. If does reduce the > built binary size by about 3.2kB, mainly through replacing the > xfs_sb_version...() checks with a single m_features flag check. It > makes the code a bit simpler, easier to read, and less shouty and > separates runtime state from filesystem features. > > -Dave. > > I have tried to give this patchset some review. Generally, it looks ok, but I didn't run any tests and could only look for inconsistencies between added and removed code and similar low-level stuff. I'm not sure if it is enough to add my Reviewed-by. :) Cheers, Jan