Hello, Dave. On Fri, Jul 17, 2015 at 09:21:59AM +1000, Dave Chinner wrote: > Tejun, is this all that needs to be done to support cgroup aware > writeback in a filesystem (i.e. wbc_init_bio, wbc_account_io in the > writepage path, superblock flag)? Yes, this is about it. It gets slightly more complex if bio's are bounced to another task as they would need to be associated with the current cgroup. The following is from Documentation/cgroups/blkio-controller.txt Filesystem support for cgroup writeback --------------------------------------- A filesystem can make writeback IOs cgroup-aware by updating address_space_operations->writepage[s]() to annotate bio's using the following two functions. * wbc_init_bio(@wbc, @bio) Should be called for each bio carrying writeback data and associates the bio with the inode's owner cgroup. Can be called anytime between bio allocation and submission. * wbc_account_io(@wbc, @page, @bytes) Should be called for each data segment being written out. While this function doesn't care exactly when it's called during the writeback session, it's the easiest and most natural to call it as data segments are added to a bio. With writeback bio's annotated, cgroup support can be enabled per super_block by setting MS_CGROUPWB in ->s_flags. This allows for selective disabling of cgroup writeback support which is helpful when certain filesystem features, e.g. journaled data mode, are incompatible. wbc_init_bio() binds the specified bio to its cgroup. Depending on the configuration, the bio may be executed at a lower priority and if the writeback session is holding shared resources, e.g. a journal entry, may lead to priority inversion. There is no one easy solution for the problem. Filesystems can try to work around specific problem cases by skipping wbc_init_bio() or using bio_associate_blkcg() directly. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
