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schfs: several snapshots at once, and unmount gives the slot back

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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>
This commit is contained in:
Nexoniarz 2026-08-10 21:30:28 +02:00
commit b34779111a
6 changed files with 75 additions and 47 deletions
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27
kernel/sys/sys_fs.c
View file

@ -1417,7 +1417,8 @@ addr_t sys_mount(void)
// the first second thing to mount. `@` 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 that is already mounted.
int snapmount = 0;
int snapmount = 0;
uint snapdev = 0;
if (source && source[0] == '@') {
const char *nm = source + 1;
unsigned nlen = 0;
@ -1429,24 +1430,24 @@ addr_t sys_mount(void)
// Distinct codes, because "resource busy" for every possible failure
// tells the caller nothing and cost an hour of guessing once already.
int r = schfs_ksnap_mount(nm, nlen);
if (r != 0) {
cprintf("schfs: snapshot mount '%s' failed (%d)\n", nm, r);
if (r < 0) {
switch (r) {
case -1: return -ENODEV; /* no volume mounted to look in */
case -2: return -EBUSY; /* a snapshot is already mounted */
case -2: return -EBUSY; /* every snapshot slot in use */
case -3: return -ENOMEM; /* no scratch blocks */
default: return -ENOENT; /* no snapshot by that name */
}
}
snapdev = (uint)r;
snapmount = 1;
}
if (!schfs_kmounted())
return -ENODEV;
uint64 root = snapmount ? schfs_ksnap_root() : schfs_kroot();
uint64 root = snapmount ? schfs_ksnap_root(snapdev) : schfs_kroot();
if (root == 0) {
if (snapmount)
schfs_ksnap_unmount();
schfs_ksnap_unmount(snapdev);
return -ENODEV;
}
@ -1468,12 +1469,12 @@ addr_t sys_mount(void)
// the only volume there could be. A snapshot is a second device, and its
// mount point is necessarily on the live root -- there is nowhere else to
// put it -- so the old spelling refused every snapshot mount.
uint fsdev = snapmount ? SCHFS_SNAP_DEV : SCHFS_DEV;
uint fsdev = snapmount ? snapdev : SCHFS_DEV;
if (dev == fsdev)
return -EBUSY;
if (mount_add(dev, inum, fsdev, root) != 0) {
if (snapmount)
schfs_ksnap_unmount();
schfs_ksnap_unmount(snapdev);
return -EBUSY;
}
return 0;
@ -1495,5 +1496,13 @@ addr_t sys_umount2(void)
iput(dp);
end_op();
return mount_del_by_root(dev, inum) == 0 ? 0 : -EINVAL;
if (mount_del_by_root(dev, inum) != 0)
return -EINVAL;
// Release the snapshot slot with the mount. Without this the table entry
// went away and the slot stayed taken, so a volume could be unmounted and
// never mounted again until reboot.
if (SCHFS_IS_SNAP_DEV(dev))
schfs_ksnap_unmount(dev);
return 0;
}