Re: [PATCH] blk-throttle: fix divide-by-zero on legacy iops limit of 0
Tao Cui <[email protected]>
| Newsgroups | org.kernel.vger.cgroups,org.kernel.vger.linux-block,org.kernel.vger.linux-kernel |
|---|---|
| Message-ID | <[email protected]> |
在 2026/7/14 19:37, David Laight 写道: > On Tue, 14 Jul 2026 18:35:52 +0800 > Tao Cui <[email protected]> wrote: > >> From: Tao Cui <[email protected]> >> >> 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 two places: >> >> * tg_set_conf(): clamp the value to UINT_MAX, consistent with >> tg_set_limit(). This closes the truncation root cause (and the >> general silent truncation for any value above UINT_MAX). >> >> * tg_dispatch_iops_time(): treat iops_limit == 0 as unlimited so the >> divide in tg_within_iops_limit() is never reached, defending >> against any future path that could produce a zero limit. >> >> Signed-off-by: Tao Cui <[email protected]> >> --- >> block/blk-throttle.c | 10 ++++++++-- >> 1 file changed, 8 insertions(+), 2 deletions(-) >> >> diff --git a/block/blk-throttle.c b/block/blk-throttle.c >> index ffc3b70065d4..3f3c1374f4b2 100644 >> --- a/block/blk-throttle.c >> +++ b/block/blk-throttle.c >> @@ -883,7 +883,12 @@ static unsigned long tg_dispatch_iops_time(struct throtl_grp *tg, struct bio *bi >> u32 iops_limit = tg_iops_limit(tg, rw); >> unsigned long iops_wait; >> >> - if (iops_limit == UINT_MAX || tg->flags & THROTL_TG_CANCELING) >> + /* >> + * iops_limit == 0 is not a valid limit. Treat it as unlimited so we >> + * never reach the HZ / iops_limit divide in tg_within_iops_limit(). >> + */ >> + if (iops_limit == UINT_MAX || iops_limit == 0 || >> + tg->flags & THROTL_TG_CANCELING) >> return 0; >> >> tg_update_slice(tg, rw); >> @@ -1386,7 +1391,8 @@ static ssize_t tg_set_conf(struct kernfs_open_file *of, >> if (is_u64) >> *(u64 *)((void *)tg + of_cft(of)->private) = v; >> else >> - *(unsigned int *)((void *)tg + of_cft(of)->private) = v; >> + *(unsigned int *)((void *)tg + of_cft(of)->private) = >> + min_t(u64, v, UINT_MAX); > > The LHS casts look horrid - there has to be a nicer way to do that. > > And you don't need min_t() a plain min() will be fine. > Hi David, Both done in v2 — introduced a void *field local so the writes read *(u64 *)field / *(unsigned int *)field, and switched to min(v, (u64)UINT_MAX). Thanks, Tao > David > > > >> >> tg_conf_updated(tg, false); >> ret = 0; >