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schfs: snapshots

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A snapshot is a tree root that is kept. The B-tree is copy-on-write, so
the blocks a committed root reaches are already immutable -- nothing
overwrites a block a committed tree can reach -- and retaining one costs
a record: (SNAP_OBJECTID, KEY_SNAP, id) -> struct schfs_snap.

The record names the root as it was *before* the record was added.
Inserting it rewrites the path under copy-on-write and produces a new
root, so a snapshot that included its own creation would name a block
nobody had written yet.

What makes it survive is the derivation. There is no free-space map on
disk; it is rebuilt at mount by walking the tree, and that walk now
covers every snapshot root as well as the live one. Without it a block
reachable only from a snapshot looks free, gets handed out, and
overwrites the data the snapshot exists to keep -- silently, surfacing
only when the snapshot is read. A snapshot's own tree may hold older
snapshot records; those are deliberately not followed, or deleting a
snapshot could be undone by an older one's memory of it.

Runtime frees are suspended while a snapshot exists. Knowing which freed
blocks a snapshot still needs requires per-extent reference counting,
which is a design and not a flag; until that exists the safe answer is to
free nothing and let the next mount's derivation reclaim what is
genuinely unreachable. Allocation is unaffected and stays correct -- it
works from the in-use bitmap the derivation filled. The running
free-blocks *counter* does drift from the truth while a snapshot is
present, which is cosmetic and is reported by the test rather than
asserted away.

Tested in schfs_objtest: after a snapshot, the live tree shows new
contents while the snapshot still shows the old ones; a file deleted from
the live tree is still readable through the snapshot; deleting the
snapshot removes the record and the space comes back at the next mount.

schfs-check passes including the round-trip; bootcheck 108 [ok].

Not yet: snapshots are not reachable by name from userspace -- there is
no mount-a-snapshot path, so this is the format and the mechanism, not
the feature a user sees.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
This commit is contained in:
Nexoniarz 2026-08-10 19:10:24 +02:00
commit 6e2e9e8d05
6 changed files with 333 additions and 5 deletions
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108
kernel/fs/schfs/space.c
View file

@ -207,6 +207,23 @@ schfs_space_alloc(struct schfs_space *s)
void
schfs_space_free(struct schfs_space *s, uint64_t blk)
{
// A snapshot exists, so this block may still be reachable from it.
//
// Copy-on-write frees the old block whenever one is rewritten, and that is
// exactly the block a snapshot is holding. Freeing it would hand it out
// again and overwrite the data the snapshot exists to keep -- silently,
// with the damage only visible later when the snapshot is read.
//
// Knowing *which* freed blocks a snapshot still needs requires reference
// counting per extent, which is a real design and not a flag. Until that
// exists, the safe answer is to free nothing while a snapshot is present:
// the free map is derived at mount anyway, so everything genuinely
// unreachable is reclaimed on the next mount rather than lost. Conservative
// in the only direction that is safe -- it can under-report free space, and
// can never hand out a block that is still in use.
if (s->flags & SCHFS_SPACE_RETAIN_ALL)
return;
if (blk < SCHFS_FIRST_BLOCK || blk >= s->nblocks)
return;
if (!bit_test(s->inuse, blk))
@ -261,6 +278,42 @@ mark_leaf_extents(struct schfs_space *s, const void *blk)
}
}
// Snapshot roots found while walking, so they can be walked in turn.
//
// Collected rather than descended into immediately: the walk keeps one block
// buffer and a path, and re-entering it from inside itself would need a second
// of each.
#define SNAP_ROOTS_MAX 64
struct snap_roots {
int n;
int overflow;
uint64_t nr[SNAP_ROOTS_MAX];
uint32_t crc[SNAP_ROOTS_MAX];
};
static void
collect_snap_roots(struct snap_roots *sr, const void *blk)
{
const struct schfs_item *it = SCHFS_ITEMS(blk);
int n = SCHFS_HDR(blk)->nritems;
for (int i = 0; i < n; i++) {
if (it[i].key.type != SCHFS_KEY_SNAP)
continue;
if (it[i].size != sizeof(struct schfs_snap))
continue;
if ((int)it[i].offset + (int)it[i].size > SCHFS_BLKSIZE)
continue;
const struct schfs_snap *sn =
(const struct schfs_snap *)((const char *)blk + it[i].offset);
if (sr->n >= SNAP_ROOTS_MAX) { sr->overflow = 1; continue; }
sr->nr[sr->n] = sn->root;
sr->crc[sr->n] = sn->root_crc;
sr->n++;
}
}
// Walk the whole tree, marking every block it reaches.
//
// Iterative, with one block buffer and a path of block numbers, for the same
@ -268,8 +321,11 @@ mark_leaf_extents(struct schfs_space *s, const void *blk)
// the kernel stack is one block in total. Climbing re-reads the parent, so an
// internal node is read once per child — mount-time cost that the buffer cache
// largely absorbs, in exchange for bounded memory.
int
schfs_space_scan(struct schfs_vol *v)
// Walk one root, marking everything it reaches. Snapshot records encountered
// on the way are collected into `sr` when it is non-null.
static int
scan_root(struct schfs_vol *v, uint64_t root, uint32_t root_crc,
struct snap_roots *sr)
{
struct schfs_space *s = &v->space;
void *blk = v->ctx.a;
@ -281,10 +337,10 @@ schfs_space_scan(struct schfs_vol *v)
} path[SCHFS_MAX_LEVEL + 1];
int d = 0;
path[0].nr = v->tree.root;
path[0].crc = v->tree.root_crc;
path[0].nr = root;
path[0].crc = root_crc;
path[0].idx = 0;
schfs_space_mark(s, v->tree.root, 1);
schfs_space_mark(s, root, 1);
for (;;) {
if (schfs_node_read(&v->dev, path[d].nr, path[d].crc, blk) != 0)
@ -292,6 +348,8 @@ schfs_space_scan(struct schfs_vol *v)
if (SCHFS_HDR(blk)->level == 0) {
mark_leaf_extents(s, blk);
if (sr)
collect_snap_roots(sr, blk);
d--;
} else if (path[d].idx < SCHFS_HDR(blk)->nritems) {
struct schfs_kptr *kp = &SCHFS_KPTRS(blk)[path[d].idx];
@ -312,3 +370,43 @@ schfs_space_scan(struct schfs_vol *v)
return 0;
}
}
// Walk the live tree, then every snapshot's tree.
//
// The snapshots have to be walked or the derivation is actively dangerous: a
// block reachable only from a snapshot would look free, be handed out, and
// overwrite the very data the snapshot exists to keep. Nothing would complain
// at the time -- the corruption surfaces later, when the snapshot is read.
//
// The live tree is walked first because that is where the snapshot records
// live; a snapshot's own tree may contain older snapshot records, which are
// deliberately not followed. They name roots the live tree no longer keeps,
// and a snapshot that was deleted should not be resurrected by an older
// snapshot's memory of it.
int
schfs_space_scan(struct schfs_vol *v)
{
struct snap_roots sr;
sr.n = 0;
sr.overflow = 0;
int r = scan_root(v, v->tree.root, v->tree.root_crc, &sr);
if (r != 0)
return r;
if (sr.n > 0)
v->space.flags |= SCHFS_SPACE_RETAIN_ALL;
for (int i = 0; i < sr.n; i++) {
r = scan_root(v, sr.nr[i], sr.crc[i], 0);
if (r != 0)
return r;
}
// Refusing is the only safe answer. Carrying on would mean deriving a free
// map that considers a live snapshot's blocks available.
if (sr.overflow)
return -2;
return 0;
}