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b=GX/i+5+coUrveNcvaY0xrsZhuVmtLA73CcoyMgGHIOeUe/2ZqEWjBRanPSO0/jWo3 hZXs98sRQ5y6Z/l7LJLzomKm6ll1TUyBUHzWgkM7q14s2RJ6TOP8Q4EBEL8cb44sya 5oDv87oF7WZfwYblAmUApiym4/OeEpy2nrZwVTmJz97NhAF2m7f3AoEvMRKJf87btW APaptSK4lju2hZNVR2ZfI55SEZ+I7PAGQa4GUS0kiEAifPyUG0gfYefAUsPXNuG5S+ bwIjD6aaNbxBciAczMQ3Uz2wEIn01AmfPXvPT21LRmBoXnc/2Tkv530H41GKV3lo2n FdNYfWLdVvtbg== From: SJ Park To: Cc: SJ Park , stable@vger.kernel.org, Andrew Morton , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [RFC PATCH v1.1 1/5] mm/damon/core: avoid infinite kdamond_merge_regions() internal loop Date: Sun, 12 Jul 2026 10:03:23 -0700 Message-ID: <20260712170328.91144-2-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260712170328.91144-1-sj@kernel.org> References: <20260712170328.91144-1-sj@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: uhpnnotshxe56r57zob8sskhbo4y9xa5 X-Rspamd-Queue-Id: DB94980004 X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1783875819-841942 X-HE-Meta: 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 xPNC6liA U9UO/z6PKsfY/lqcwXpylIW+G6CisEdkdB2vW2HXXPj1MKOPi7F610JxVTCKe2TD++lIZehJCc8U6wlC3J7LsLRUuC71oDvDfTj2vUAgHPTOBzBxK8q/kUPwQ78zc3e8Mkc70mCFCv1cjHcnwPw1rdXvNnseKGA0ekgnK9z1f532YUi7xxICfYAjW76YLzG6p6yqQDFTidqmM8cS2sG5NbVRl8p1uoy4+zpyWllCMwJTtVWxCiT2LeR0gqD4bUkv7SQFoKwsOx9x/gRI/ZeS3qVaiIQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Due to online parameter update like events, the number of DAMON regions could be higher than the user-set upper limit. kdamond_merge_regions() repeats merge regions until the number meets the limit, while doubling the merge threshold up to the theoretical maximum threshold. It is tried only up to the theoretical maximum threshold because even the aggressive merging can fail from reducing the number of regions under the user-defined upper limit. For example, there could be many user-defined non-contiguous regions that cannot be merged. The threshold based loop break condition is evaluated by comparing the threshold for the next merging try against the theoretical maximum threshold. If max_thres is larger than UINT_MAX / 2, doubling the threshold could make it overflow, and bypass the loop break condition. In the case, if the number of regions cannot be reduced under the upper limit like explained above, the loop will run infinitely. Prevent the case by doing the break condition check before doubling the threshold. Also, prevent the threshold exceeding the maximum threshold, as it could overflow and apply the wrong merge threshold. This issue is unlikely to occur in real world, since having the max_thres higher than UINT_MAX / 2 require unrealistically large aggregation intervals compared to the sampling interval. Also, it requires an unrealistically large number of uncontiguous regions setup. Nonetheless, the consequence is bad and the fix is simple. The issue was discovered [1] by Sashiko. [1] https://lore.kernel.org/20260709145425.96247-1-sj@kernel.org Fixes: 310d6c15e910 ("mm/damon/core: merge regions aggressively when max_nr_regions is unmet") Cc: # 6.10.x Signed-off-by: SJ Park --- mm/damon/core.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/mm/damon/core.c b/mm/damon/core.c index 806a67d02a6e9..f3b6a46fdaabd 100644 --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -3369,15 +3369,20 @@ static void kdamond_merge_regions(struct damon_ctx *c, unsigned int threshold, max_thres = c->attrs.aggr_interval / (c->attrs.sample_interval ? c->attrs.sample_interval : 1); - do { + while (true) { nr_regions = 0; damon_for_each_target(t, c) { damon_merge_regions_of(t, threshold, sz_limit, c); nr_regions += damon_nr_regions(t); } - threshold = max(1, threshold * 2); - } while (nr_regions > c->attrs.max_nr_regions && - threshold / 2 < max_thres); + if (nr_regions <= c->attrs.max_nr_regions || + max_thres <= threshold) + break; + if (threshold < max_thres / 2) + threshold = max(1, threshold * 2); + else + threshold = max_thres; + } } #ifdef CONFIG_DAMON_DEBUG_SANITY -- 2.47.3