On Wed, Sep 6, 2017 at 10:22 PM, Vishwanath Pai <v...@akamai.com> wrote: > On 09/06/2017 03:57 PM, Arnd Bergmann wrote: >> 64-bit division is expensive on 32-bit architectures, and >> requires a special function call to avoid a link error like: >> >> net/netfilter/xt_hashlimit.o: In function `hashlimit_mt_common': >> xt_hashlimit.c:(.text+0x1328): undefined reference to `__aeabi_uldivmod' >> >> In the case of hashlimit_mt_common, we don't actually need a >> 64-bit operation, we can simply rewrite the function slightly >> to make that clear to the compiler. >> >> Fixes: bea74641e378 ("netfilter: xt_hashlimit: add rate match mode") >> Signed-off-by: Arnd Bergmann <a...@arndb.de> >> --- >> net/netfilter/xt_hashlimit.c | 5 ++++- >> 1 file changed, 4 insertions(+), 1 deletion(-) >> >> diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c >> index 10d48234f5f4..50b53d86eef5 100644 >> --- a/net/netfilter/xt_hashlimit.c >> +++ b/net/netfilter/xt_hashlimit.c >> @@ -531,7 +531,10 @@ static u64 user2rate_bytes(u64 user) >> { >> u64 r; >> >> - r = user ? 0xFFFFFFFFULL / user : 0xFFFFFFFFULL; >> + if (user > 0xFFFFFFFFULL) >> + return 0; >> + >> + r = user ? 0xFFFFFFFFULL / (u32)user : 0xFFFFFFFFULL; >> r = (r - 1) << 4; >> return r; >> } >> > > I have submitted another patch to fix this: > https://patchwork.ozlabs.org/patch/809881/ > > We have seen this problem before, I was careful not to introduce this > again in the new patch but clearly I overlooked this particular line :( > > In the other cases we fixed it by replacing division with div64_u64().
div64_u64() seems needlessly expensive here since the dividend is known to be a 32-bit number. I guess the function is not called frequently though, so it doesn't matter much. Arnd