The branch can be fetched from my github: https://github.com/adam900710/btrfs-progs.git low_mem_fsck_v2.1
====== RFC v2.1 patchset is a readability enhancement update. ====== Original btrfsck checks extent tree in a very efficient method, by recording every checked extent in extent record tree to ensure every extent will be iterated for at most 2 times. However extent records are all stored in heap memory, and consider how large a btrfs file system can be, it can easily eat up all memory and cause OOM for TB-sized metadata. Instead of such heap memory usage, we introduce low memory usage fsck mode. In this mode, we will use btrfs_search_slot() only and avoid any heap memory allocation. The work flow is: 1) Iterate extent tree And check whether the referencer of every backref exists. 2) Iterate other trees And check whether the backref of every tree block/data exists in extent tree. So in theory, every extent is iterated twice. But since we don't have extent record, but use btrfs_search_slot() every time we check, it may cause extra IO. I assume the extra IO is reasonable and would make btrfsck able to handle super large fs. TODO features: 1) Fs tree check code Fs tree check also eats a lot of memory. Fixing extent tree check is not a final solution, fs check also needs such rework, or we can still eat up a lot of memory 2) Repair Repair should be the same as old btrfsck, but still need to determine the repair principle. Current repair sometimes uses backref to repair data extent, sometimes uses data extent to fix backref. We need a consistent principle, or we will screw things up. 3) Replace current fsck code We assume the low memory mode has less lines of code, and may be easier for review and expand. If low memory mode is stable enough, we will consider to replace current extent and chunk tree check codes to free a lot of lines. Changelog: v1.1: Fix a typo which makes keyed data backref check always fail v2: Don't return minus value with error bits. (Suggested by Josef) Follow kernel code styling. (Suggested by Josef) Enhance check on data backref count. (Slow down check speed though) v2.1: Fix an error report which could access uninitialized keys (Pointed out by David) Improve readability by rename multiple btrfs_key to names which represents its root or search destination. Like in check_chunk_item(), rename keys to "chunk_key","bg_key","devext_key" to avoid unintended above overwrite/access to uninitialized key. Slightly speedup block group used space accounting, by change its search start to correct block group number. checkpatch style fixes. Lu Fengqi (16): btrfs-progs: fsck: Introduce function to check tree block backref in extent tree btrfs-progs: fsck: Introduce function to check data backref in extent tree btrfs-progs: fsck: Introduce function to query tree block level btrfs-progs: fsck: Introduce function to check referencer of a backref btrfs-progs: fsck: Introduce function to check shared block ref btrfs-progs: fsck: Introduce function to check referencer for data backref btrfs-progs: fsck: Introduce function to check shared data backref btrfs-progs: fsck: Introduce function to check an extent btrfs-progs: fsck: Introduce function to check dev extent item btrfs-progs: fsck: Introduce function to check dev used space btrfs-progs: fsck: Introduce function to check block group item btrfs-progs: fsck: Introduce function to check chunk item btrfs-progs: fsck: Introduce hub function for later fsck btrfs-progs: fsck: Introduce function to speed up fs tree check btrfs-progs: fsck: Introduce traversal function for fsck btrfs-progs: fsck: Introduce low memory mode Documentation/btrfs-check.asciidoc | 6 + cmds-check.c | 1757 +++++++++++++++++++++++++++++++++--- ctree.h | 2 + extent-tree.c | 2 +- 4 files changed, 1622 insertions(+), 145 deletions(-) -- 2.8.2 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html