Messages in this thread Patch in this message | | | Subject | Re: [PATCH] blk-throttle: Calculate allowed value only when the throttle is enabled | From | Yu Kuai <> | Date | Sat, 7 Oct 2023 09:23:58 +0800 |
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Hi,
在 2023/10/06 0:24, Oleg Nesterov 写道: > Hi Li, > > On 10/05, Li Nan wrote: >> >>> I don't think this change is sufficient to prevent kernel crash, as a >>> "clever" user could still set the bps_limit to U64_MAX - 1 (or another >>> large value), which probably would still result in the same crash. The >>> comment in mul_u64_u64_div_u64 suggests there's something we can do to >>> better handle the overflow case, but I'm not sure what it's referring >>> to. ("Will generate an #DE when the result doesn't fit u64, could fix >>> with an __ex_table[] entry when it becomes an issue.") Otherwise, we >> >> When (a * mul) overflows, a divide 0 error occurs in >> mul_u64_u64_div_u64(). Commit 3dc167ba5729 ("sched/cputime: Improve >> cputime_adjust()") changed func and said: "Will generate an #DE when the >> result doesn't fit u64, could fix with an __ex_table[] entry when it >> becomes an issue." But we are unsure of how to fix it. Could you please >> explain how to fix this issue. > > Not sure I understand the question... > > OK, we can change mul_u64_u64_div_u64() to trap the exception, say, > > static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div) > { > u64 q; > > asm ("mulq %2; 1: divq %3; 2:\n" > _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_DEFAULT|EX_FLAG_CLEAR_AX) > : "=a" (q) > : "a" (a), "rm" (mul), "rm" (div) > : "rdx"); > > return q; > } > > should (iiuc) return 0 if the result doesn't fit u64 or div == 0. > > But even if we forget that this is x86-specific, how can this help? > What should calculate_bytes_allowed() do/return in this case?
I believe, U64_MAX should be returned if result doesn't fit u64; > >>> probably need to remove the mul_u64_u64_div_u64 and check for >>> overflow/potential overflow ourselves? > > probably yes...
How about this?
diff --git a/block/blk-throttle.c b/block/blk-throttle.c index 1101fb6f6cc8..5482c316a103 100644 --- a/block/blk-throttle.c +++ b/block/blk-throttle.c @@ -723,6 +723,10 @@ static unsigned int calculate_io_allowed(u32 iops_limit,
static u64 calculate_bytes_allowed(u64 bps_limit, unsigned long jiffy_elapsed) { + if (jiffy_elapsed > HZ && + bps_limit > mul_u64_u64_div_u64(U64_MAX, (u64)HZ, (u64)jiffy_elapsed); + return U64_MAX; + return mul_u64_u64_div_u64(bps_limit, (u64)jiffy_elapsed, (u64)HZ); }
> Oleg. > > . >
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