Carlos had a case where "find" seemed to start spinning
forever and never return.
This was on a filesystem with non-default multi-fsb (8k)
directory blocks, and a fragmented directory with extents
like this:
0:[0,133646,2,0]
1:[2,195888,1,0]
2:[3,195890,1,0]
3:[4,195892,1,0]
4:[5,195894,1,0]
5:[6,195896,1,0]
6:[7,195898,1,0]
7:[8,195900,1,0]
8:[9,195902,1,0]
9:[10,195908,1,0]
10:[11,195910,1,0]
11:[12,195912,1,0]
12:[13,195914,1,0]
...
i.e. the first extent is a contiguous 2-fsb dir block, but
after that it is fragmented into 1 block extents.
At the top of the readdir path, we allocate a mapping array
which (for this filesystem geometry) can hold 10 extents; see
the assignment to map_info->map_size. During readdir, we are
therefore able to map extents 0 through 9 above into the array
for readahead purposes. If we count by 2, we see that the last
mapped index (9) is the first block of a 2-fsb directory block.
At the end of xfs_dir2_leaf_readbuf() we have 2 loops to fill
more readahead; the outer loop assumes one full dir block is
processed each loop iteration, and an inner loop that ensures
that this is so by advancing to the next extent until a full
directory block is mapped.
The problem is that this inner loop may step past the last
extent in the mapping array as it tries to reach the end of
the directory block. This will read garbage for the extent
length, and as a result the loop control variable 'j' may
become corrupted and never fail the loop conditional.
The number of valid mappings we have in our array is stored
in map->map_valid, so stop this inner loop based on that limit.
There is an ASSERT at the top of the outer loop for this
same condition, but we never made it out of the inner loop,
so the ASSERT never fired.
Huge appreciation for Carlos for debugging and isolating
the problem.
Debugged-and-analyzed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Tested-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Linux kernel
============
This file was moved to Documentation/admin-guide/README.rst
Please notice that there are several guides for kernel developers and users.
These guides can be rendered in a number of formats, like HTML and PDF.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.
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.
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