The patch titled Subject: mm/damon: implement a function for max nr_accesses safe calculation has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-damon-implement-a-function-for-max-nr_accesses-safe-calculation.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: SeongJae Park sj@kernel.org Subject: mm/damon: implement a function for max nr_accesses safe calculation Date: Thu, 19 Oct 2023 19:49:20 +0000

Patch series "avoid divide-by-zero due to max_nr_accesses overflow".

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.

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.

This patch (of 4):

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. Implement a function that handles the corner case.

Note that this commit is not fixing the real issue since this is only introducing the safe function that will replaces the problematic divisions. The replacements will be made by followup commits, to make backporting on stable series easier.

Link: https://lkml.kernel.org/r/20231019194924.100347-1-sj@kernel.org Link: https://lkml.kernel.org/r/20231019194924.100347-2-sj@kernel.org Fixes: 198f0f4c58b9 ("mm/damon/vaddr,paddr: support pageout prioritization") Signed-off-by: SeongJae Park sj@kernel.org Cc: stable@vger.kernel.org [5.16+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/damon.h | 7 +++++++ 1 file changed, 7 insertions(+)

--- a/include/linux/damon.h~mm-damon-implement-a-function-for-max-nr_accesses-safe-calculation +++ a/include/linux/damon.h @@ -642,6 +642,13 @@ static inline bool damon_target_has_pid( return ctx->ops.id == DAMON_OPS_VADDR || ctx->ops.id == DAMON_OPS_FVADDR; }

+static inline unsigned int damon_max_nr_accesses(const struct damon_attrs *attrs) +{ + /* {aggr,sample}_interval are unsigned long, hence could overflow */ + return min(attrs->aggr_interval / attrs->sample_interval, + (unsigned long)UINT_MAX); +} +

int damon_start(struct damon_ctx **ctxs, int nr_ctxs, bool exclusive); int damon_stop(struct damon_ctx **ctxs, int nr_ctxs); _

Patches currently in -mm which might be from sj@kernel.org are

mm-damon-implement-a-function-for-max-nr_accesses-safe-calculation.patch mm-damon-core-avoid-divide-by-zero-during-monitoring-results-update.patch mm-damon-ops-common-avoid-divide-by-zero-during-region-hotness-calculation.patch mm-damon-lru_sort-avoid-divide-by-zero-in-hot-threshold-calculation.patch