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server: limit to one merge request when fs_root height <= 2

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When the fs_root is too short for subtrees, get_parent returns the
entire root for every request.  Multiple concurrent merges would each
independently CoW and modify the same root tree.  Processing their
completions would replace the root with each result, only keeping the
last and orphaning blocks allocated by earlier completions.

Limit to one outstanding request when fs_root.height <= 2 to prevent
this.

Signed-off-by: Auke Kok <auke.kok@versity.com>
This commit is contained in:
Auke Kok
2026-03-12 09:39:35 -07:00
parent a5e746d185
commit efc19c3d75
1 changed files with 16 additions and 0 deletions
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16
kmod/src/server.c
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@@ -2816,6 +2816,22 @@ restart:
goto out;
}
/*
* When the tree is too short for subtrees, get_parent
* returns the entire root for every request. Multiple
* concurrent merges would each independently CoW and
* modify the same root tree. Processing their
* completions would replace the root with each result,
* only keeping the last and orphaning blocks allocated
* by earlier completions. Limit to one outstanding
* request to avoid this.
*/
if (super->fs_root.height <= 2 &&
le64_to_cpu(stat.nr_requests) > 0) {
ret = -ENOENT;
goto out;
}
/* find the next logged item in the next range */
ret = next_least_log_item(sb, &super->logs_root, le64_to_cpu(stat.seq),
&rng.start, &rng.end, &next_key);