linux-kernel - Re: [PATCH] blk-throttle: Calculate allowed value only when the throttle is enabled

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Message-ID: <CACGdZY+OOr4Q5ajM0za2babr34YztE7zjRyPXHgh_A64zvoBOw@mail.gmail.com>
Date:   Fri, 13 Oct 2023 14:51:22 -0700
From:   Khazhy Kumykov <khazhy@...omium.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Yu Kuai <yukuai1@...weicloud.com>,
        Li Nan <linan666@...weicloud.com>, tj@...nel.org,
        josef@...icpanda.com, axboe@...nel.dk, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, houtao1@...wei.com, yangerkun@...wei.com,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] blk-throttle: Calculate allowed value only when the
 throttle is enabled

Looking at the generic mul_u64_u64_div_u64 impl, it doesn't handle
overflow of the final result either, as far as I can tell. So while on
x86 we get a DE, on non-x86 we just get the wrong result.

(Aside: after 8d6bbaada2e0 ("blk-throttle: prevent overflow while
calculating wait time"), setting a very-high bps_limit would probably
also cause this crash, no?)

Would it be possible to have a "check_mul_u64_u64_div_u64_overflow()",
where if the result doesn't fit in u64, we indicate (and let the
caller choose what to do? Here we should just return U64_MAX)?

Absent that, maybe we can take inspiration from the generic
mul_u64_u64_div_u64? (Forgive the paste)

 static u64 calculate_bytes_allowed(u64 bps_limit, unsigned long jiffy_elapsed)
 {
+       /* Final result probably won't fit in u64 */
+       if (ilog2(bps_limit) + ilog2(jiffy_elapsed) - ilog2(HZ) > 62)
+               return U64_MAX;
        return mul_u64_u64_div_u64(bps_limit, (u64)jiffy_elapsed, (u64)HZ);
 }