From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 57B9D1DE2D8; Fri, 10 Apr 2026 23:55:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775865337; cv=none; b=TISsi8MGXrxlKndMj5VdjYRugKgdb6tNkxdFIV1li2GE2iVEIQX4dToi/YWybyMRJQIiRm2Iwxlc36SRw+phGZkwF61ddtLGSxgf0n0JQ+KFIC9Es1OSve1P8A8M2/lsm2LFlzEyRdo7F1QESdgO9Lo8CkPIYc0XF7tjJ/5kLTU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775865337; c=relaxed/simple; bh=zw2XYHDnlTw0qahEiBS32vzXn4QNwRFUrjs/0wSN/yk=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=gXOqsJSb8eCkmP4+p4Dt7teZVEvmbK0R4KFA/NiCISvVvjUBLYtsVL/uGtM2hlmu2Da+m8aj9t7WKW0axb2j1rPVxdQkCaduSvH12KE5QZsvvyHmyaRtwxMQebftcisNfpqnGdQ2j789FpmNrRt9q4Bs/6Eq9jvSXJ+cOjTKLGM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=QiCNVA3p; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="QiCNVA3p" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DAB6BC19421; Fri, 10 Apr 2026 23:55:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775865337; bh=zw2XYHDnlTw0qahEiBS32vzXn4QNwRFUrjs/0wSN/yk=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=QiCNVA3po6Yxr8/tG452qobIJP2c2RhkzOCWFUUS/Y4TeXCnBhnZZPHPefmndSb7G o6RZWXMZ2GWPsXRcHxLSvswNhNFaaPx8tO/+oBzqeNDv8wZ7PtndWSP8sSsOnVzq/f DABSpZFfGpj3AuJ6IjSQPBFJ1nx5whO8CpL/4CWc5qSryGLdDIwZTalwl2zNuBzF/7 yrhOSbsvYzXeH3nNI2j9itsT9wK2Sww0GKBFBDLbs3e8qL9zR9rlhc1g+XKLxwFaVK L/c69jZGObXUmGOHRD8xVvmse2MPfOvxN6AoEhdxkzRLpuZ1/8LcIJGou4r6Ndc6HD aeFqXDAxPu2dw== From: SeongJae Park To: SeongJae Park Cc: damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: (sashiko review) [RFC PATCH v5 02/11] mm/damon/core: merge quota-sliced regions back Date: Fri, 10 Apr 2026 16:55:26 -0700 Message-ID: <20260410235527.90900-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260410142034.83798-3-sj@kernel.org> References: Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit TL; DR: sashiko found an issue. I will address that in the next revision. Forwarding sashiko review in a reply format with my inline comments below, for details of my view and doing discussions via mails if needed. > # review url: https://sashiko.dev/#/patchset/20260410142034.83798-3-sj@kernel.org > # start of sashiko.dev inline review > commit ae9f581463db42127fa24541161baf201709d5f9 > Author: SeongJae Park > > mm/damon/core: merge quota-sliced regions back > > This commit merges quota-sliced regions back together to reduce overhead. > It avoids merging regions if their age is non-zero, meaning no scheme > action was applied to them. > > > diff --git a/mm/damon/core.c b/mm/damon/core.c > > index 8dbc365bb8e97..97b375c720251 100644 > > --- a/mm/damon/core.c > > +++ b/mm/damon/core.c > > [ ... ] > > > +static void damos_apply_target(struct damon_ctx *c, struct damon_target *t) > > +{ > > + struct damon_region *r, *orig_region = NULL; > > + unsigned long orig_end_addr; > > + > > + damon_for_each_region(r, t) { > > + /* > > + * damon_do_apply_schemes() could split the region for the > > + * quota. Keeping the new slices is an overhead. Merge back > > + * the slices into the original region if there is no reason to > > + * keep those. > > + */ > > + if (!orig_region || orig_end_addr <= r->ar.start) { > > + orig_region = r; > > + orig_end_addr = r->ar.end; > > + } > > + damon_do_apply_schemes(c, t, r); > > + if (r == orig_region) > > + continue; > > + /* > > + * If no scheme was applied to the sliced region, the age of > > + * the slice ain't be reset. Don't merge that back. > > + * Otherwise, the monitored information of the region is lost. > > + */ > > + if (r->age) { > > + orig_region = NULL; > > + continue; > > + } > > + orig_region->ar.end = r->ar.end; > > + damon_destroy_region(r, t); > > + r = orig_region; > > + } > > +} > > Does this logic safely handle cases where orig_region and r have differing > ages? > > If a scheme applies to slice r (resetting r->age to 0), but was skipped > for orig_region (leaving orig_region->age > 0), the check if (r->age) > will be false. The slice r will then be merged into orig_region, and > the reset age of r will be lost because it structurally inherits the > non-zero age of orig_region. > > Could this cause age-based rate-limiting to be bypassed, allowing schemes > to erroneously apply to the same memory regions repeatedly in subsequent > intervals? > > Additionally, if both orig_region and r have the same non-zero age > (e.g., both were skipped), if (r->age) evaluates to true, setting > orig_region = NULL and skipping the merge entirely. > > Should the merge condition verify that the contiguous slices share identical > state before merging, perhaps by checking if (orig_region->age != r->age)? Good finding. I was thinking the scheme will anyway be applied, once the quota-split is happened. But I was wrong, since DAMOS core filter is applied after the split operation. Also, age is not reset if the action is DAMOS_STAT. I will address this in the next revision, like below. ''' --- a/mm/damon/core.c +++ b/mm/damon/core.c @@ -2439,35 +2439,32 @@ static void damon_do_apply_schemes(struct damon_ctx *c, static void damos_apply_target(struct damon_ctx *c, struct damon_target *t) { - struct damon_region *r, *orig_region = NULL; - unsigned long orig_end_addr; + struct damon_region *r; damon_for_each_region(r, t) { + struct damon_region *prev_r; + + damon_do_apply_schemes(c, t, r); /* - * damon_do_apply_schemes() could split the region for the + * damon_do_apply_scheems() could split the region for the * quota. Keeping the new slices is an overhead. Merge back - * the slices into the original region if there is no reason to - * keep those. + * the slices into the previous region if it doesn't lose any + * information. */ - if (!orig_region || orig_end_addr <= r->ar.start) { - orig_region = r; - orig_end_addr = r->ar.end; - } - damon_do_apply_schemes(c, t, r); - if (r == orig_region) + if (damon_first_region(t) == r) continue; - /* - * If no scheme was applied to the sliced region, the age of - * the slice ain't be reset. Don't merge that back. - * Otherwise, the monitored information of the region is lost. - */ - if (r->age) { - orig_region = NULL; + prev_r = damon_prev_region(r); + if (prev_r->ar.end != r->ar.start) continue; - } - orig_region->ar.end = r->ar.end; + if (prev_r->age != r->age) + continue; + if (prev_r->last_nr_accesses != r->last_nr_accesses) + continue; + if (prev_r->nr_accesses != r->nr_accesses) + continue; + prev_r->ar.end = r->ar.end; damon_destroy_region(r, t); - r = orig_region; + r = prev_r; } } ''' > > > # end of sashiko.dev inline review > # review url: https://sashiko.dev/#/patchset/20260410142034.83798-3-sj@kernel.org Thanks, SJ # hkml [1] generated a draft of this mail. You can regenerate # this using below command: # # hkml patch sashiko_dev --for_forwarding \ # 20260410142034.83798-3-sj@kernel.org # # [1] https://github.com/sjp38/hackermail