Two loose ends from the mount-by-name commit, both closed.
A slot is a device number, so supporting more than one snapshot is a
small array rather than a redesign: SCHFS_SNAP_DEV through
SCHFS_SNAP_DEV + SCHFS_SNAP_SLOTS - 1, each with its own volume and its
own scratch pages, allocated on first use. Four of them -- "one at a
time" is an odd thing to have to explain, and the cost is two pages per
slot. Not many, because each one is a device number and those are a
small fixed space.
The leak mattered more. umount removed the mount table entry and left the
slot marked in use, so a snapshot could be unmounted and then never
mounted again until reboot. sys_umount2 releases it now, keyed off the
device it already has in hand.
Verified in the running system: two snapshots mounted at once, both
readable, both unmounted, and a third mount afterwards succeeds and reads
correctly -- which is the case that failed before.
schfs-check passes; bootcheck 108 [ok].
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
`mount -t schfs @golden /mnt` mounts a snapshot. The source argument
finally means something: sys_mount ignored it outright, because there was
one volume and nothing to choose between. `@name` rather than a path
because a snapshot is not a device -- there is no node in /dev for it and
there should not be, since it lives inside the volume already mounted.
A second volume rather than a parameter threaded through all 23 schfs_k*
entry points. Only the read side can ever address it, so the write path
keeps talking to the live volume and needed no change; the four read
entry points grew _dev variants and pick the volume from the device the
inode already carries. One fs_ops table serves both, because every entry
takes an inode and dispatches on it -- a second table would be the same
pointers under a different name.
It needs its own device number: a snapshot's object ids are the *same
numbers* as the live tree's, so one device would make the two views alias
in the inode cache and hand a reader whichever was cached first.
Three things this turned up, none of them new but all of them reachable
for the first time:
- sys_mount refused with `dev == SCHFS_DEV`, meaning "do not mount SchFS
onto SchFS". A snapshot's mount point is necessarily on the live root,
so that refused the entire feature. The real condition is mounting a
device onto *itself*, which is `dev == fsdev`.
- createat allocates an inode and then panics if dirlink fails --
reasonable about a writable filesystem, a crash on a read-only one.
The refusal now happens before anything is allocated. Unlink and
rename already handled a refusal properly.
- a refused create reported ENOENT, so writing to a snapshot said "No
such file or directory" -- the sentence a shell prints for every
create failure. It reports EROFS now.
Verified in the running system against an image built with
--snapshot golden: the mount succeeds, ls lists the snapshot's tree,
/mnt2/etc/motd reads back through it, and a write is refused with
"Read-only file system".
One at a time, and no umount path for it yet: mount_del_by_root removes
the mount table entry, but ksnap_unmount is only called on the failure
paths. schfs-check passes; bootcheck 108 [ok].
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.
No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.
