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Kirk McKusick
35a301555b
The link count for a UFS/FFS inode is stored in a signed 16-bit integer. Thus the maximum link count has been 32767. This limit has been recently hit by the poudriere build system when doing a ports build as it needs one directory per port and the number of ports recently passed 32767. A long-term solution would be to use one of the spare 32-bit fields in the inode to store the link count. However, the UFS1 format does not have a spare and adding the spare in UFS2 would make it hard to make it compatible when running on older kernels that use the original link count field. So this patch uses the much simpler approach of changing the existing link count field from a signed 16-bit value to an unsigned 16-bit value. It has the fewest lines of code changes. The only thing that changes is the type in the dinode and inode structures and the definition of UFS_LINK_MAX. It has the added benefit that it works with both UFS1 and UFS2. It allows easy backward compatibility. Indeed it is backward compatibility that is the primary reason to go with this approach. If a filesystem with the new organization is mounted on an older kernel, it still needs to work. Thus if we move the new link count to a new field, we still need to maintain the old link count as best as possible even when running on a kernel that knows about the larger link counts. And we would have to carry this overhead for the indefinite future. If we have a new link-count field, we will have to add a new filesystem flag to indicate that we are running with larger link counts. We will also need to add of one of the new-feature flags to say that we have larger link counts. Older kernels clear the new-feature flags that they do not know about, so when a filesystem is used on an older kernel and then moved back to a newer one, the newer one will know that the new link counts have not been maintained and that it will be necessary to run a full fsck on the filesystem to correct the link counts before it can be mounted. With this change, older kernels will generally work with the bigger counts. While it will not itself allow the link count to exceed 32767, it will have no problem working with inodes that have a link count greater than 32767. Since it tests that i_nlink <= UFS_LINK_MAX, counts that are bigger than 32767 will appear negative, so will still pass the test. Of course, if they ever drop below 32767, they will no longer be able to exceed 32767. The one issue is if the link count ever exceeds 65535 then it will wrap to zero and the older kernel will be none the wiser. But this corner case is likely to be very rare since these kernels and the applications running on them do not expect to be able to get link counts over 32767. And over time, the use of new filesystems on older kernels will become rarer and rarer. Reported-by: Mark Millard running poudriere on the ports tree Reviewed-by: kib, olce.freebsd_certner.fr Tested-by: Peter Holm, Mark Millard MFC-after: 2 weeks Differential Revision: https://reviews.freebsd.org/D42767
FreeBSD Kernel Source:
This directory contains the source files and build glue that make up the FreeBSD kernel and its modules, including both original and contributed software.
Kernel configuration files are located in the conf/ subdirectory of each
architecture. GENERIC is the configuration used in release builds. NOTES
contains documentation of all possible entries. LINT is a compile-only
configuration used to maximize build coverage and detect regressions.
Documentation:
Source code documentation is maintained in a set of man pages, under section 9.
These pages are located in share/man/man9, from the
top-level of the src tree. Consult intro(9)
for an overview of existing pages.
Some additional high-level documentation of the kernel is maintained in the Architecture Handbook.
Source Roadmap:
| Directory | Description |
|---|---|
| amd64 | AMD64 (64-bit x86) architecture support |
| arm | 32-bit ARM architecture support |
| arm64 | 64-bit ARM (AArch64) architecture support |
| cam | Common Access Method storage subsystem - cam(4) and ctl(4) |
| cddl | CDDL-licensed optional sources such as DTrace |
| conf | kernel build glue |
| compat | Linux compatibility layer, FreeBSD 32-bit compatibility |
| contrib | 3rd-party imported software such as OpenZFS |
| crypto | crypto drivers |
| ddb | interactive kernel debugger - ddb(4) |
| fs | most filesystems, excluding UFS, NFS, and ZFS |
| dev | device drivers and other arch independent code |
| gdb | kernel remote GDB stub - gdb(4) |
| geom | GEOM framework - geom(4) |
| i386 | i386 (32-bit x86) architecture support |
| kern | main part of the kernel |
| libkern | libc-like and other support functions for kernel use |
| modules | kernel module infrastructure |
| net | core networking code |
| net80211 | wireless networking (IEEE 802.11) - net80211(4) |
| netgraph | graph-based networking subsystem - netgraph(4) |
| netinet | IPv4 protocol implementation - inet(4) |
| netinet6 | IPv6 protocol implementation - inet6(4) |
| netipsec | IPsec protocol implementation - ipsec(4) |
| netpfil | packet filters - ipfw(4), pf(4), and ipfilter(4) |
| opencrypto | OpenCrypto framework - crypto(7) |
| powerpc | PowerPC/POWER (32 and 64-bit) architecture support |
| riscv | 64-bit RISC-V architecture support |
| security | security facilities - audit(4) and mac(4) |
| sys | kernel headers |
| tests | kernel unit tests |
| ufs | Unix File System - ffs(7) |
| vm | virtual memory system |
| x86 | code shared by AMD64 and i386 architectures |