This patch changes the prefetch API so that file systems must now
provide a set of block numbers, rather than a set of buffers. The
result is a leaner and more well-defined API; linear computation of
the range of blocks to prefetch; duplicates no longer interfering
with the prefetch process; guaranteed inclusion of the block needed
next into the prefetch range; and, limits and policy decisions better
established by libminixfs now actually being moved into libminixfs.
Change-Id: I7e44daf2d2d164bc5e2f1473ad717f3ff0f0a77f
- The lmfs_get_block*(3) API calls may now return an error. The idea
is to encourage a next generation of file system services to do a
better job at dealing with block read errors than the MFS-derived
implementations do. These existing file systems have been changed
to panic immediately upon getting a block read error, in order to
let unchecked errors cause corruption. Note that libbdev already
retries failing I/O operations a few times first.
- The libminixfs block device I/O module (bio.c) now deals properly
with end-of-file conditions on block devices. Since a device or
partition size may not be a multiple of the root file system's block
size, support for partial block retrival has been added, with a new
internal lmfs_get_partial_block(3) call. A new test program,
test85, tests the new handling of EOF conditions when reading,
writing, and memory-mapping a block device.
Change-Id: I05e35b6b8851488328a2679da635ebba0c6d08ce
This patch employs one solution to resolve two independent but related
issues. Both issues are the result of one fundamental aspect of the
way VM's memory mapping works: VM uses its cache to map in blocks for
memory-mapped file regions, and for blocks already in the VM cache, VM
does not go to the file system before mapping them in. To preserve
consistency between the FS and VM caches, VM relies on being informed
about all updates to file contents through the block cache. The two
issues are both the result of VM not being properly informed about
such updates:
1. Once a file system provides libminixfs with an inode association
(inode number + inode offset) for a disk block, this association
is not broken until a new inode association is provided for it.
If a block is freed and reallocated as a metadata (non-inode)
block, its old association is maintained, and may be supplied to
VM's secondary cache. Due to reuse of inodes, it is possible
that the same inode association becomes valid for an actual file
block again. In that case, when that new file is memory-mapped,
under certain circumstances, VM may end up using the metadata
block to satisfy a page fault on the file, due to the stale inode
association. The result is a corrupted memory mapping, with the
application seeing data other than the current file contents
mapped in at the file block.
2. When a hole is created in a file, the underlying block is freed
from the device, but VM is not informed of this update, and thus,
if VM's cache contains the block with its previous inode
association, this block will remain there. As a result, if an
application subsequently memory-maps the file, VM will map in the
old block at the position of the hole, rather than an all-zeroes
block. Thus, again, the result is a corrupted memory mapping.
This patch resolves both issues by making the file system inform the
minixfs library about blocks being freed, so that libminixfs can
break the inode association for that block, both in its own cache and
in the VM cache. Since libminixfs does not know whether VM has the
block in its cache or not, it makes a call to VM for each block being
freed. Thus, this change introduces more calls to VM, but it solves
the correctness issues at hand; optimizations may be introduced
later. On the upside, all freed blocks are now marked as clean,
which should result in fewer blocks being written back to the device,
and the blocks are removed from the caches entirely, which should
result in slightly better cache usage.
This patch is necessary but not sufficient to resolve the situation
with respect to memory mapping of file holes in general. Therefore,
this patch extends test 74 with a (rather particular but effective)
test for the first issue, but not yet with a test for the second one.
This fixes#90.
Change-Id: Iad8b134d2f88a884f15d3fc303e463280749c467
