From: Tao Cui Writing a multiple of 2^32 (e.g. 4294967296) to a legacy cgroup v1 throttle iops file (blkio.throttle.{read,write}_iops_device) silently truncates to 0: tg_set_conf() stores the sscanf-parsed u64 value into an unsigned int field with no clamping. The cgroup v2 path, tg_set_limit(), already clamps the same kind of value with min_t(u64, val, UINT_MAX), but the legacy path never did. Note that the "!v -> U64_MAX" mapping only catches an explicit zero and does not catch a value that truncates to zero. With iops stored as 0, tg_update_has_rules() sets has_rules_iops[] and the next IO reaches tg_within_iops_limit(), which computes jiffy_wait = max(jiffy_wait, HZ / iops_limit + 1); triggering a divide-by-zero oops. Fix it in tg_set_conf() by clamping the value to UINT_MAX, consistent with tg_set_limit(). This closes the truncation root cause: with 0 no longer reachable as a stored limit, the HZ / iops_limit divide is never hit. Signed-off-by: Tao Cui --- Changes in v4: - Drop the defensive "iops_limit == 0" check in tg_dispatch_iops_time(): with the tg_set_conf() clamp in place, 0 can never be stored as a limit, so the runtime check only guards an unreachable state. (Yu Kuai) - Drop the Fixes: tag: the unclamped write -- and the iops=0 behavior it can produce (calculate_io_allowed() returns 0, so no IO is issued) -- long predates the commit that added the HZ / iops_limit divide, so attributing it there was incorrect. (Yu Kuai) Changes in v3: - Drop the (u64) cast on UINT_MAX: the kernel's type-checked min() accepts two unsigned types of different width (both >= 4 bytes), so min(v, UINT_MAX) compiles clean. (David Laight) Changes in v2: - Use a "void *field" local for the config write so the assignment reads *(u64 *)field / *(unsigned int *)field instead of the (type *)((void *)tg + of_cft(of)->private) casts. - Use min(v, UINT_MAX) instead of min_t(u64, v, UINT_MAX). --- block/blk-throttle.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/block/blk-throttle.c b/block/blk-throttle.c index ffc3b70065d4..97ad7959d006 100644 --- a/block/blk-throttle.c +++ b/block/blk-throttle.c @@ -1383,10 +1383,12 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of, tg = blkg_to_tg(ctx.blkg); tg_update_carryover(tg); + void *field = (void *)tg + of_cft(of)->private; + if (is_u64) - *(u64 *)((void *)tg + of_cft(of)->private) = v; + *(u64 *)field = v; else - *(unsigned int *)((void *)tg + of_cft(of)->private) = v; + *(unsigned int *)field = min(v, UINT_MAX); tg_conf_updated(tg, false); ret = 0; -- 2.43.0