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 E7CF820E338; Fri, 18 Oct 2024 18:33:13 +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=1729276395; cv=none; b=pr8AJOCw41Q1Sg6WwbF1k5QZPpwRhan8bkJ88MOjXhuZIBLo/HF7LFkuQGRrwey+1RW5gaTf3y2fhdt4QvUwurfXOzE0d48vDWGUoa45Ulti14wd/RSzd//H0vcAsWvnhmg3ZCTEDiEDCkZcDKLdBU/G0cZXILnDw4XBk16n/IE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1729276395; c=relaxed/simple; bh=JTS7l00ywra+1THYcJZQsN2BffW5M/t8l3w1Xebv1OM=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=tGfpRr2WxPh60NL+Qa6xknT1CqeED5vXfQ7nhI13fmhnjCw1ExaLo5iJVfHhEh7dqySZciMUbCCNdwSHiAwyXsl1ngpo3Fygd8WZTtqeMeaJ+Tt6+4qhA/VsIQv7ySCzRM71NVIM9v5opV/mbGeCEKh5yTXe65daLHawjsYBKc4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=McAj2UUm; 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="McAj2UUm" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4AA92C4CEC3; Fri, 18 Oct 2024 18:33:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1729276393; bh=JTS7l00ywra+1THYcJZQsN2BffW5M/t8l3w1Xebv1OM=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=McAj2UUmemN7Ro4awQUL87JqeBbXfmKjq2cM8Fwd7q1LREln2bEm3wx7q57aMals4 GxZhlCzr+MgJQQ90mW2t+ztjMqF9QqARyIhzrbpoIGsbN4kIG+BBCeallm2Db5QJRL zmJmxFEddNqSjQcSpdiJKr4jhjE/d+1GZdPK818Vud4dIInqnGRM5k3xzgz3CAAzdp MzEKZFo6JkFyDFwpoAJRIy5rZ+ytsNCIBXLajDmjvD3M5dBr/epwJ8srxEShQg34BI /xGkU54J96atDcVKRSxml40m57fWnVKGty2Q85cfj6sQRLZtpuxtGGvPv3rXZJNDW1 WKcJbdpvbolqw== From: SeongJae Park To: Zheng Yejian Cc: SeongJae Park , akpm@linux-foundation.org, sieberf@amazon.com, shakeel.butt@linux.dev, foersleo@amazon.de, damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/damon/vaddr: Fix issue in damon_va_evenly_split_region() Date: Fri, 18 Oct 2024 11:33:10 -0700 Message-Id: <20241018183310.115850-1-sj@kernel.org> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20241018035304.1050135-1-zhengyejian@huaweicloud.com> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Hi Zheng, Thank you for sharing this nice finding and fix! I have a few comments below. On Fri, 18 Oct 2024 11:53:04 +0800 Zheng Yejian wrote: > According to the logic of damon_va_evenly_split_region(), currently at > least following split cases would not meet the expectation: > > Suppose DAMON_MIN_REGION=0x1000, > Case1: Split [0x0, 0x1100) into 1 pieces, then the result would be > acutually [0x0, 0x1000), but NOT the expected [0x0, 0x1100) !!! Nice finding! However, as long as DAMON_MIN_REGION is respected, [0x0, 0x1100] region could not be created. So, the problematic case cannot happen in real? Please let me know if I'm missing something. And, why would someone call the function with nr_pieces 1? > Case2: Split [0x0, 0x3000) into 2 pieces, then the result would be > acutually 3 regions: > [0x0, 0x1000), [0x1000, 0x2000), [0x2000, 0x3000) > but NOT the expected 2 regions: > [0x0, 0x1000), [0x1000, 0x3000) !!! Nice finding! > > The root cause is that when calculating size of each split piece in > damon_va_evenly_split_region(): > > `sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, DAMON_MIN_REGION);` > > both the dividing and the ALIGN_DOWN may cause loss of precision, > then each time split one piece of size 'sz_piece' from origin 'start' to > 'end' would cause: > 1. For the above Case1, the 'end' value of the split 1 piece is > aligned but not updated!!! > 2. For the above Case2, more pieces are split out than expected!!! > > To fix it, in this patch: > - As for the expect to split 1 piece, just return 0; As mentioned above, I think this is not needed, since the problematic case is unreal. > - Count for each piece split and make sure no more than 'nr_pieces'; > - Add above two cases into damon_test_split_evenly(). Thank you for adding tests! > > BTW, currently when running kunit test, DAMON_MIN_REGION is redefined > as 1, then above ALIGN_DOWN cases may not be test, since every int > value is ALIGN-ed to 1. > > After this patch, damon-operations test passed: > > # ./tools/testing/kunit/kunit.py run damon-operations > [...] > ============== damon-operations (6 subtests) =============== > [PASSED] damon_test_three_regions_in_vmas > [PASSED] damon_test_apply_three_regions1 > [PASSED] damon_test_apply_three_regions2 > [PASSED] damon_test_apply_three_regions3 > [PASSED] damon_test_apply_three_regions4 > [PASSED] damon_test_split_evenly > ================ [PASSED] damon-operations ================= > > Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual memory address spaces") > Signed-off-by: Zheng Yejian > --- > mm/damon/tests/vaddr-kunit.h | 2 ++ > mm/damon/vaddr.c | 13 +++++++++---- > 2 files changed, 11 insertions(+), 4 deletions(-) > > diff --git a/mm/damon/tests/vaddr-kunit.h b/mm/damon/tests/vaddr-kunit.h > index a339d117150f..b9a03e4e29e5 100644 > --- a/mm/damon/tests/vaddr-kunit.h > +++ b/mm/damon/tests/vaddr-kunit.h > @@ -300,6 +300,8 @@ static void damon_test_split_evenly(struct kunit *test) > damon_test_split_evenly_fail(test, 0, 100, 0); > damon_test_split_evenly_succ(test, 0, 100, 10); > damon_test_split_evenly_succ(test, 5, 59, 5); > + damon_test_split_evenly_succ(test, 4, 6, 1); If my above assumption (the first problem is unreal) is not wrong, maybe this test is not needed? > + damon_test_split_evenly_succ(test, 0, 3, 2); Nice. > damon_test_split_evenly_fail(test, 5, 6, 2); > } > > diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c > index 08cfd22b5249..1f3cebd20829 100644 > --- a/mm/damon/vaddr.c > +++ b/mm/damon/vaddr.c > @@ -67,10 +67,14 @@ static int damon_va_evenly_split_region(struct damon_target *t, > unsigned long sz_orig, sz_piece, orig_end; > struct damon_region *n = NULL, *next; > unsigned long start; > + int i; Purpose of this variable is counting the number of splitted regions, and comparing it against 'nr_pieces', right? Because nr_pieces is 'unsigned int', let's make this 'unsigned int' type, too. > > if (!r || !nr_pieces) > return -EINVAL; > > + if (nr_pieces == 1) > + return 0; > + As mentioned above, I don't think this is not needed. > orig_end = r->ar.end; > sz_orig = damon_sz_region(r); > sz_piece = ALIGN_DOWN(sz_orig / nr_pieces, DAMON_MIN_REGION); > @@ -79,9 +83,11 @@ static int damon_va_evenly_split_region(struct damon_target *t, > return -EINVAL; > > r->ar.end = r->ar.start + sz_piece; > + /* origin region will be updated as the first one after splitting */ I don't think this comment is easy to understand. Let's just remove it. > + i = 1; > + n = r; Why we need this? for 'nr_pieces == 1' case? If so, I don't think we need to take care about the case for the above mentioned reason. Please let me know if I'm missing something. > next = damon_next_region(r); > - for (start = r->ar.end; start + sz_piece <= orig_end; > - start += sz_piece) { > + for (start = r->ar.end; i < nr_pieces; start += sz_piece, i++) { > n = damon_new_region(start, start + sz_piece); > if (!n) > return -ENOMEM; > @@ -89,8 +95,7 @@ static int damon_va_evenly_split_region(struct damon_target *t, > r = n; > } > /* complement last region for possible rounding error */ > - if (n) > - n->ar.end = orig_end; > + n->ar.end = orig_end; Maybe this change is related with the above 'n = r' line? But, I don't think we need that, as commented there. > > return 0; > } > -- > 2.25.1 Thanks, SJ