mirror of
https://github.com/torvalds/linux.git
synced 2024-09-21 07:23:06 +00:00
4e98cc905c
Certain types of filesystem metadata can only be checked by scanning every file in the entire filesystem. Specific examples of this include quota counts, file link counts, and reverse mappings of file extents. Directory and parent pointer reconstruction may also fall into this category. File scanning is much trickier than scanning AG metadata because we have to take inode locks in the same order as the rest of [VX]FS, we can't be holding buffer locks when we do that, and scanning the whole filesystem takes time. Earlier versions of the online repair patchset relied heavily on fsfreeze as a means to quiesce the filesystem so that we could take locks in the proper order without worrying about concurrent updates from other writers. Reviewers of those patches opined that freezing the entire fs to check and repair something was not sufficiently better than unmounting to run fsck offline. I don't agree with that 100%, but the message was clear: find a way to repair things that minimizes the quiet period where nobody can write to the filesystem. Generally, building btree indexes online can be split into two phases: a collection phase where we compute the records that will be put into the new btree; and a construction phase, where we construct the physical btree blocks and persist them. While it's simple to hold resource locks for the entirety of the two phases to ensure that the new index is consistent with the rest of the system, we don't need to hold resource locks during the collection phase if we have a means to receive live updates of other work going on elsewhere in the system. The goal of this patch, then, is to enable online fsck to learn about metadata updates going on in other threads while it constructs a shadow copy of the metadata records to verify or correct the real metadata. To minimize the overhead when online fsck isn't running, we use srcu notifiers because they prioritize fast access to the notifier call chain (particularly when the chain is empty) at a cost to configuring notifiers. Online fsck should be relatively infrequent, so this is acceptable. The intended usage model is fairly simple. Code that modifies a metadata structure of interest should declare a xfs_hook_chain structure in some well defined place, and call xfs_hook_call whenever an update happens. Online fsck code should define a struct notifier_block and use xfs_hook_add to attach the block to the chain, along with a function to be called. This function should synchronize with the fsck scanner to update whatever in-memory data the scanner is collecting. When finished, xfs_hook_del removes the notifier from the list and waits for them all to complete. Originally, I selected srcu notifiers over blocking notifiers to implement live hooks because they seemed to have fewer impacts to scalability. The per-call cost of srcu_notifier_call_chain is higher (19ns) than blocking_notifier_ (4ns) in the single threaded case, but blocking notifiers use an rwsem to stabilize the list. Cacheline bouncing for that rwsem is costly to runtime code when there are a lot of CPUs running regular filesystem operations. If there are no hooks installed, this is a total waste of CPU time. Therefore, I stuck with srcu notifiers, despite trading off single threaded performance for multithreaded performance. I also wasn't thrilled with the very high teardown time for srcu notifiers, since the caller has to wait for the next rcu grace period. This can take a long time if there are a lot of CPUs. Then I discovered the jump label implementation of static keys. Jump labels use kernel code patching to replace a branch with a nop sled when the key is disabled. IOWs, they can eliminate the overhead of _call_chain when there are no hooks enabled. This makes blocking notifiers competitive again -- scrub runs faster because teardown of the chain is a lot cheaper, and runtime code only pays the rwsem locking overhead when scrub is actually running. With jump labels enabled, calls to empty notifier chains are elided from the call sites when there are no hooks registered, which means that the overhead is 0.36ns when fsck is not running. This is perfect for most of the architectures that XFS is expected to run on (e.g. x86, powerpc, arm64, s390x, riscv). For architectures that don't support jump labels (e.g. m68k) the runtime overhead of checking the static key is an atomic counter read. This isn't great, but it's still cheaper than taking a shared rwsem. Signed-off-by: Darrick J. Wong <djwong@kernel.org> Reviewed-by: Christoph Hellwig <hch@lst.de> |
||
|---|---|---|
| .. | ||
| libxfs | ||
| scrub | ||
| Kconfig | ||
| Makefile | ||
| xfs_acl.c | ||
| xfs_acl.h | ||
| xfs_aops.c | ||
| xfs_aops.h | ||
| xfs_attr_inactive.c | ||
| xfs_attr_item.c | ||
| xfs_attr_item.h | ||
| xfs_attr_list.c | ||
| xfs_bio_io.c | ||
| xfs_bmap_item.c | ||
| xfs_bmap_item.h | ||
| xfs_bmap_util.c | ||
| xfs_bmap_util.h | ||
| xfs_buf_item_recover.c | ||
| xfs_buf_item.c | ||
| xfs_buf_item.h | ||
| xfs_buf.c | ||
| xfs_buf.h | ||
| xfs_dahash_test.c | ||
| xfs_dahash_test.h | ||
| xfs_dir2_readdir.c | ||
| xfs_discard.c | ||
| xfs_discard.h | ||
| xfs_dquot_item_recover.c | ||
| xfs_dquot_item.c | ||
| xfs_dquot_item.h | ||
| xfs_dquot.c | ||
| xfs_dquot.h | ||
| xfs_drain.c | ||
| xfs_drain.h | ||
| xfs_error.c | ||
| xfs_error.h | ||
| xfs_export.c | ||
| xfs_export.h | ||
| xfs_extent_busy.c | ||
| xfs_extent_busy.h | ||
| xfs_extfree_item.c | ||
| xfs_extfree_item.h | ||
| xfs_file.c | ||
| xfs_filestream.c | ||
| xfs_filestream.h | ||
| xfs_fsmap.c | ||
| xfs_fsmap.h | ||
| xfs_fsops.c | ||
| xfs_fsops.h | ||
| xfs_globals.c | ||
| xfs_health.c | ||
| xfs_hooks.c | ||
| xfs_hooks.h | ||
| xfs_icache.c | ||
| xfs_icache.h | ||
| xfs_icreate_item.c | ||
| xfs_icreate_item.h | ||
| xfs_inode_item_recover.c | ||
| xfs_inode_item.c | ||
| xfs_inode_item.h | ||
| xfs_inode.c | ||
| xfs_inode.h | ||
| xfs_ioctl.c | ||
| xfs_ioctl.h | ||
| xfs_ioctl32.c | ||
| xfs_ioctl32.h | ||
| xfs_iomap.c | ||
| xfs_iomap.h | ||
| xfs_iops.c | ||
| xfs_iops.h | ||
| xfs_itable.c | ||
| xfs_itable.h | ||
| xfs_iunlink_item.c | ||
| xfs_iunlink_item.h | ||
| xfs_iwalk.c | ||
| xfs_iwalk.h | ||
| xfs_linux.h | ||
| xfs_log_cil.c | ||
| xfs_log_priv.h | ||
| xfs_log_recover.c | ||
| xfs_log.c | ||
| xfs_log.h | ||
| xfs_message.c | ||
| xfs_message.h | ||
| xfs_mount.c | ||
| xfs_mount.h | ||
| xfs_mru_cache.c | ||
| xfs_mru_cache.h | ||
| xfs_notify_failure.c | ||
| xfs_pnfs.c | ||
| xfs_pnfs.h | ||
| xfs_pwork.c | ||
| xfs_pwork.h | ||
| xfs_qm_bhv.c | ||
| xfs_qm_syscalls.c | ||
| xfs_qm.c | ||
| xfs_qm.h | ||
| xfs_quota.h | ||
| xfs_quotaops.c | ||
| xfs_refcount_item.c | ||
| xfs_refcount_item.h | ||
| xfs_reflink.c | ||
| xfs_reflink.h | ||
| xfs_rmap_item.c | ||
| xfs_rmap_item.h | ||
| xfs_rtalloc.c | ||
| xfs_rtalloc.h | ||
| xfs_stats.c | ||
| xfs_stats.h | ||
| xfs_super.c | ||
| xfs_super.h | ||
| xfs_symlink.c | ||
| xfs_symlink.h | ||
| xfs_sysctl.c | ||
| xfs_sysctl.h | ||
| xfs_sysfs.c | ||
| xfs_sysfs.h | ||
| xfs_trace.c | ||
| xfs_trace.h | ||
| xfs_trans_ail.c | ||
| xfs_trans_buf.c | ||
| xfs_trans_dquot.c | ||
| xfs_trans_priv.h | ||
| xfs_trans.c | ||
| xfs_trans.h | ||
| xfs_xattr.c | ||
| xfs_xattr.h | ||
| xfs.h | ||