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One of the first steps of log recovery is to check for the special case of a zeroed log. If the first cycle in the log is zero or the tail portion of the log is zeroed, the head is set to the first instance of cycle 0. xlog_find_zeroed() includes a sanity check that enforces that the first cycle in the log must be 1 if the last cycle is 0. While this is true in most cases, the check is not totally valid because it doesn't consider the case where the filesystem crashed after a partial/out of order log buffer completion that wraps around the end of the physical log. For example, consider a filesystem that has completed most of the first cycle of the log, reaches the end of the physical log and splits the next single log buffer write into two in order to wrap around the end of the log. If these I/Os are reordered, the second (wrapped) I/O completes and the first happens to fail, the log is left in a state where the last cycle of the log is 0 and the first cycle is 2. This causes the xlog_find_zeroed() sanity check to fail and prevents the filesystem from mounting. This situation has been reproduced on particular systems via repeated runs of generic/475. This is an expected state that log recovery already knows how to deal with, however. Since the log is still partially zeroed, the head is detected correctly and points to a valid tail. The subsequent stale block detection clears blocks beyond the head up to the tail (within a maximum range), with the express purpose of clearing such out of order writes. As expected, this removes the out of order cycle 2 blocks at the physical start of the log. In other words, the only thing that prevents a clean mount and recovery of the filesystem in this scenario is the specific (last == 0 && first != 1) sanity check in xlog_find_zeroed(). Since the log head/tail are now independently validated via cycle, log record and CRC checks, this highly specific first cycle check is of dubious value. Remove it and rely on the higher level validation to determine whether log content is sane and recoverable. Signed-off-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Dave Chinner <dchinner@redhat.com> Signed-off-by: Dave Chinner <david@fromorbit.com> |
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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.
See Documentation/00-INDEX for a list of what is contained in each file.
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.