linux-kernel - Re: [PATCH 0/5] avoid divide-by-zero due to max_nr

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Message-Id: <20231020171901.63994-1-sj@kernel.org>
Date:   Fri, 20 Oct 2023 17:19:01 +0000
From:   SeongJae Park <sj@...nel.org>
To:     SeongJae Park <sj@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, damon@...ts.linux.dev,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        acsjakub@...zon.de
Subject: Re: [PATCH 0/5] avoid divide-by-zero due to max_nr_accesses overflow

On Thu, 19 Oct 2023 19:49:19 +0000 SeongJae Park <sj@...nel.org> wrote:

> The maximum nr_accesses of given DAMON context can be calculated by
> dividing the aggregation interval by the sampling interval.  Some logics
> in DAMON uses the maximum nr_accesses as a divisor.  Hence, the value
> shouldn't be zero.  Such case is avoided since DAMON avoids setting the
> agregation interval as samller than the sampling interval.  However,
> since nr_accesses is unsigned int while the intervals are unsigned long,
> the maximum nr_accesses could be zero while casting.

Actually, the issue was reported by Jakub, and I didn't add 'Reported-by:' tags
for him.  I sure Andrew could add that on his own, but I want to minimize
Andrew's load, so will send v2 of this patchset.  Andrew, please let me know if
that doesn't help but only increasing your load.


Thanks,
SJ

> 
> Avoid the divide-by-zero by implementing a function that handles the
> corner case (first patch), and replaces the vulnerable direct max
> nr_accesses calculations (remaining patches).
> 
> Note that the patches for the replacements are divided for broken
> commits, to make backporting on required tres easier.  Especially, the
> last patch is for a patch that not yet merged into the mainline but in
> mm tree.
> 
> SeongJae Park (5):
>   mm/damon: implement a function for max nr_accesses safe calculation
>   mm/damon/core: avoid divide-by-zero during monitoring results update
>   mm/damon/ops-common: avoid divide-by-zero during region hotness
>     calculation
>   mm/damon/lru_sort: avoid divide-by-zero in hot threshold calculation
>   mm/damon/core: avoid divide-by-zero from pseudo-moving window length
>     calculation
> 
>  include/linux/damon.h |  7 +++++++
>  mm/damon/core.c       | 12 +++---------
>  mm/damon/lru_sort.c   |  4 +---
>  mm/damon/ops-common.c |  5 ++---
>  4 files changed, 13 insertions(+), 15 deletions(-)
> 
> 
> base-commit: e845524c56a529768a8793e96304db09134eafdf
> -- 
> 2.34.1
> 
>