d1d832a0b5
When we log an inode, regardless of logging it completely or only that it exists, we always update it as logged (logged_trans and last_log_commit fields of the inode are updated). This is generally fine and avoids future attempts to log it from having to do repeated work that brings no value. However, if we write data to a file, then evict its inode after all the dealloc was flushed (and ordered extents completed), rename the file and fsync it, we end up not logging the new extents, since the rename may result in logging that the inode exists in case the parent directory was logged before. The following reproducer shows and explains how this can happen: $ mkfs.btrfs -f /dev/sdb $ mount /dev/sdb /mnt $ mkdir /mnt/dir $ touch /mnt/dir/foo $ touch /mnt/dir/bar # Do a direct IO write instead of a buffered write because with a # buffered write we would need to make sure dealloc gets flushed and # complete before we do the inode eviction later, and we can not do that # from user space with call to things such as sync(2) since that results # in a transaction commit as well. $ xfs_io -d -c "pwrite -S 0xd3 0 4K" /mnt/dir/bar # Keep the directory dir in use while we evict inodes. We want our file # bar's inode to be evicted but we don't want our directory's inode to # be evicted (if it were evicted too, we would not be able to reproduce # the issue since the first fsync below, of file foo, would result in a # transaction commit. $ ( cd /mnt/dir; while true; do :; done ) & $ pid=$! # Wait a bit to give time for the background process to chdir. $ sleep 0.1 # Evict all inodes, except the inode for the directory dir because it is # currently in use by our background process. $ echo 2 > /proc/sys/vm/drop_caches # fsync file foo, which ends up persisting information about the parent # directory because it is a new inode. $ xfs_io -c fsync /mnt/dir/foo # Rename bar, this results in logging that this inode exists (inode item, # names, xattrs) because the parent directory is in the log. $ mv /mnt/dir/bar /mnt/dir/baz # Now fsync baz, which ends up doing absolutely nothing because of the # rename operation which logged that the inode exists only. $ xfs_io -c fsync /mnt/dir/baz <power failure> $ mount /dev/sdb /mnt $ od -t x1 -A d /mnt/dir/baz |
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| Documentation | ||
| LICENSES | ||
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.