From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 87D6D3D5221 for ; Sat, 18 Jul 2026 17:15:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784394930; cv=none; b=YAGIlobcMQDiUFwvBbda67tt9kNIRoRpLhuDDozASBKijH4qBx0kzZz9K8RlLH+opgEo1oJ+VJQy6JL/EKlLPIp5ImcSrrpiLcnLeZnlsKi/T2Cldedq/MHbpF6OGr63B6P54J9I9GvK81HN6tkpj0f3V5G0Ath/Q33QijZdwyU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784394930; c=relaxed/simple; bh=7qKkZUpzK5Z17vc2xWgf9UBAFwZdrz8iD8ETBsAxrj4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=m6WVI2W61jtLbK90o5wGlURn1YbA9U9FowE2lcjCthOokGXKVlFpD42sCr6EzDl49Mmbpw0B9JloxPPt8iH9TRUcDfa+VWwkCb9LKklKKLZ/O4La8DJv+OCL7Taf+5+9iyOu1FpJtxK9l/FEoQpQj3s+zLhv3pxpf9XX3bdMH8w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=oRVcimv9; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="oRVcimv9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9C9591F000E9; Sat, 18 Jul 2026 17:15:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784394929; bh=7Dov8cYchOJ2vpcxiDhZSqVa2TTsqQDY/WWJEP3CD1Q=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=oRVcimv9SILbZLiZmYoKkAMXrKg4SAvg4kvIfFNM4CgUTBLAxYwA4FPKAXfGKde5a ciaFBP/Tjantbs5FEn+uI2HyA3GzjLGAtkd2gMNnlEm1fvd89whyIqJ4CsSsVz+RrZ nAil740R1zt7AIwwwDyLFLvTyY0vYIp4OWrVBL/rX6uSSb6RXVZwOGHWqIzKFKrLYL NojWMrSkQM49MNcZ176FBzaRhH63dsfMhyrsR89kZjNMKIOh0ZgyXFo1WjqOAIBnAt Rj/sXarAxSS2l/PsLKceHAm9eL0chXjkOXpmYCetPNApe9q1CKYKYF1gb2HoPG6ExN E3Rxpvtm4UivA== From: SJ Park To: sashiko-bot@kernel.org Cc: SJ Park , damon@lists.linux.dev Subject: Re: [PATCH] mm/damon/core: handle region split failure in apply_min_nr_regions() Date: Sat, 18 Jul 2026 10:15:21 -0700 Message-ID: <20260718171523.87547-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260718170058.66B1A1F000E9@smtp.kernel.org> References: Precedence: bulk X-Mailing-List: damon@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Sat, 18 Jul 2026 17:00:57 +0000 sashiko-bot@kernel.org wrote: > Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider: > > Pre-existing issues: > - [High] Incomplete bug fix in damon_apply_min_nr_regions() leads to O(N^2) allocator thrashing on allocation failure. > - [High] Silent failure of damon_split_region_at() bypasses DAMON quotas and bounds checks. > -- > > --- Patch [1]: [PATCH] mm/damon/core: handle region split failure in apply_min_nr_regions() --- > commit 51e7e9012c52dde83bac4be44f0bbfbdbe01572d > Author: SJ Park > Subject: mm/damon/core: handle region split failure in apply_min_nr_regions() > > damon_apply_min_nr_regions() repeatedly split each region until its size > becomes small enough to meet the user-defined low limit of the number of > regions. The loop assumes the split operation (damon_split_region_at()) > will always succeed and create the new region. > > Link: https://lore.kernel.org/20260717011834.120715-1-sj@kernel.org > > > diff --git a/mm/damon/core.c b/mm/damon/core.c > > index 20d267c615faf..3d829e0ad63b5 100644 > > --- a/mm/damon/core.c > > +++ b/mm/damon/core.c > > @@ -1913,6 +1913,9 @@ static unsigned long damon_apply_min_nr_regions(struct damon_ctx *ctx) > > damon_for_each_region_safe(r, next, t) { > > while (damon_sz_region(r) > max_region_sz) { > > damon_split_region_at(t, r, max_region_sz); > > + /* split might failed */ > > + if (r == damon_last_region(t)) > > + break; > > [Severity: High] > This isn't a bug introduced by this patch, but does this fix leave an > O(N^2) allocator thrashing loop when an allocation fails on a non-last > region? > > If a region fails to split due to memory pressure and is not the last > region, the inner loop unconditionally advances r = damon_next_region(r). > This moves to the next pre-existing region instead of a newly created > one, and the inner loop will attempt to split this next region, continuing > to the end of the list. > > Then, the outer damon_for_each_region_safe() loop advances to its > pre-saved next pointer and repeats the entire process. > > Could this severely thrash the page allocator with redundant > kmem_cache_alloc() calls and cause RCU stalls or lockups during an > OOM condition? The allocation is arguably too small to fail. And the while loop is supposed to run only occasionally. Most regions in most case would not exceed max_region_sz. But good call. I will separately work on making this more safe. > > [Severity: High] > This is also a pre-existing issue and was not introduced by this patch, but > since we are handling split failures here, could the silent failure of > damon_split_region_at() bypass DAMON quotas in other callers? > > Because damon_split_region_at() returns void, callers like > damos_apply_scheme() cannot detect if the split failed: > > mm/damon/core.c:damos_apply_scheme() { > ... > damon_split_region_at(t, r, sz); > } > if (damos_core_filter_out(c, t, r, s)) > return; > ... > sz_applied = c->ops.apply_scheme(c, t, r, s, > &sz_ops_filter_passed); > ... > damos_charge_quota(quota, sz, sz_applied); > } > > If the split fails, the region remains oversized and the scheme is > erroneously applied to the entire original region. > > The quota charging logic then only charges the intended small size (sz), > which permanently hides the limit bypass. > > Could this cause DAMON schemes to violate their strict resource quota > limits and apply actions to much larger memory areas than permitted? Good finding. Seems not urgent and not related to this patch. I will separately work on this. > > > r = damon_next_region(r); > > } > > } > > -- > Sashiko AI review ยท https://sashiko.dev/#/patchset/20260718163708.84113-1-sj@kernel.org?part=1 Thanks, SJ