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 BFDE13947AD for ; Wed, 4 Mar 2026 09:21:08 +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=1772616068; cv=none; b=refwM5evA5ABWW7PmfhGB0to7NPi7abg84TcMKKf4dn6m3W7LWYy7Cjjt6W5+fhaqiiXbYNTmn2BGKDwgIHLcQ6c9WlV41YCWBgaZKwwOT91gonn52z3/x7HORRnNbK7LIcs1egNgHDB0616i+G727whbCJo70ZVDbGPAJd3+5c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772616068; c=relaxed/simple; bh=lYiqGVVRlGcEQbjg4Ysb7oHD+BPMIPaVIjThrPNbaIU=; h=Message-ID:Date:MIME-Version:Subject:To:References:From: In-Reply-To:Content-Type; b=t4PjKxwqs6xyZ0kOqMOYj4KEnyMuDN19DI5U+Plb7z5rNWnJa8kWqugFEPjNZq78yvZNnCMEJ6f18bN5OEDW+4ad45Ym69uplWIb7pgQhNAONoxW/0TNeZ1f5spS4luy13BRlFVs4etOJ/H8AuDjslbuGYXj7b+d6pELTGf/U3M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=u2VatNr2; 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="u2VatNr2" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 31F06C19423; Wed, 4 Mar 2026 09:21:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1772616068; bh=lYiqGVVRlGcEQbjg4Ysb7oHD+BPMIPaVIjThrPNbaIU=; h=Date:Subject:To:References:From:In-Reply-To:From; b=u2VatNr2EyMWrBy3hoSLl/D2A+kvAR8wZSxNY2qXDTdlTd3dvidpn+rN3eYsHUtpR ALLPBvaKYTbSwq1D1RjmFr/ldnUUSv99KDkVoH7J1xXwgANEQqbm3Sx+9LxnZYB5GG VbwhfwI39lqK31XvQqCYK51vgVvQMVcdmEOtqMXWPXnJiwe6rrYRsPdOP2Wj4JoOZ4 hgm4k/FPAdCsMjIqEm8EGN6oMpnuby416Sjz5wuTg06sPmc90CxbZZeAfl8upPkFBR XMLiG7scwlqGu3oqgNbH3Up4eLzyKTSJ6HNAl2TGKgBwdwiKNSH9yFTSaAEmQdFBPM Pyl/dHSpkHA8Q== Message-ID: Date: Wed, 4 Mar 2026 10:21:02 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv2] mm: bail out when the PMD has been set in bloom filter To: "zhaoyang.huang" , Andrew Morton , Axel Rasmussen , Yuanchu Xie , Wei Xu , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Lorenzo Stoakes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Zhaoyang Huang , steve.kang@unisoc.com References: <20260304031538.1258114-1-zhaoyang.huang@unisoc.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/4/26 04:15, zhaoyang.huang wrote: > From: Zhaoyang Huang > > Part of bloom filter utilization in MGLRU are listed below, in which we > can see that the step '3' will prevent the hot PMD to be carried to new > gen since the new arrived rmap_walk clears the page's young flag. > This commit would like to suggest to query the PMD in bloom filter > before starting the rmap walk to improve this. In terms of the cost, > test_bloom_filter only consume 20~30 instructions in modern processors(25 > instructions in ARM64). This is hard to follow, for example because * You forward-reference things * You talk about "this commit". A commit does not suggest anything, people to. * It's unclear what the real benefit of the patch is or which problem it tries to solve. You could start with a template like the following, where you clearly express what is currently problematic, and why, and how you are intending to optimize: "Currently, lru_gen_look_around() does not properly consider the bloom filter when processing a PTE. This is suboptimal, because TODO ... To improve performance, let's consult the bloom filter for the PMD we are processing, to just skip processing all PTEs in the PMD table. " In general, for performance optimizations that are not completely obvious, we prefer actual proof that it is worth the additional code. If it's a correctness issue, we should spell that out. I'm still not sure what it is :) > > 1. rmap_walk set suitable PMD in filters[max_seq] while all page's turn > to be non-young status > * rmap_walk->lru_gen_look_around->update_bloom_filter(max_seq) > 2. young pages gathering on the PMD if it is a hot VM area > 3. newly arrived rmap_walk in the same PMD clears page's young which > are set in step 2 > 4. walk_mm test the PMD again during aging, which will bring the > suitable PMD to filters[max_seq+1] > * walk_mm->walk_pmd_range->test_bloom_fitler->update_bloom_filter( > walk->seq + 1) > > Tested-by: syzbot@syzkaller.appspotmail.com That doesn't really count as "Tested". It just verified that the problem in v1 no longer results in a splat, but it didn't really test if the patch does the right thing. > Signed-off-by: Zhaoyang Huang > --- > v2: fix null-ptr-ref of mm_state and update commit message > --- > --- > mm/vmscan.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 10f1e7d716ca..5558a24d1564 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -4234,6 +4234,10 @@ bool lru_gen_look_around(struct page_vma_mapped_walk *pvmw) > /* avoid taking the LRU lock under the PTL when possible */ > walk = current->reclaim_state ? current->reclaim_state->mm_walk : NULL; > > + /* may the pmd has been set in bloom filter */ That comment is not really helpful given that the code does exactly that. > + if (mm_state && test_bloom_filter(mm_state, max_seq, pvmw->pmd)) > + return true; -- Cheers, David