From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 BE88431AA85 for ; Mon, 15 Dec 2025 04:13:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765771987; cv=none; b=mswyPGwTx+GjuSjnqKuCBs/4MSr4jGY/YUgMRS+6KV2/0meSWon6l/PQN/4Wvgldl1g+kNlIBcyI1VvnTkCYyKaUgmCTvP4y1tqrnNHznFQNBwPw7/bpc2M1zs8EwT+3lJ4Ww9LoKNeki1Yowwu1XRwKK5RzzYQXH6u9foeRtZE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765771987; c=relaxed/simple; bh=HfxLhV8ZFitVhzUMoJQHt8CT0BjpVI6HXi7ygzufSn0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=IDIWfueA+MC7i+B1Ty6fSQqLvs5fBxDHA/i471WViuYbxkqkomakLAMzJb/8gd0AxxuukmlU6h2GNeb6G0Qyue7yC/eTAHoQsfmCM/FP9irhdiPO6Gf+vYZisSh3cAlBnieRS8ybxzMlPr0ciNWI55EMaoV+SzycUEemHFV/1Rs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=K6smD1x1; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="K6smD1x1" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1765771982; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Tfnd8QOpt3biOWyVmRNtULIkeysgwJ1eAPZzg5ISCgk=; b=K6smD1x1ClAa5FQB9sXDX3VRbFw25anKTUQYBqNVokAHrjWh0u6138lYrtKqLTJQR35oq1 mEOrZdq95uZeE/kiwYp40KWcMOtwttyt2tipyCwmj6Go8CF71wttprMsI3KOqwZubu7jAr LUWy2IY9m2ivwzocXJ4bXYV0p02uL6g= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-333-RTWczNdqM_6uP8R1vnF15w-1; Sun, 14 Dec 2025 23:13:00 -0500 X-MC-Unique: RTWczNdqM_6uP8R1vnF15w-1 X-Mimecast-MFC-AGG-ID: RTWczNdqM_6uP8R1vnF15w_1765771978 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B9CC91956050; Mon, 15 Dec 2025 04:12:56 +0000 (UTC) Received: from localhost (unknown [10.72.112.95]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 5E9A819560B4; Mon, 15 Dec 2025 04:12:54 +0000 (UTC) Date: Mon, 15 Dec 2025 12:12:45 +0800 From: Baoquan He To: Kairui Song Cc: linux-mm@kvack.org, Andrew Morton , Barry Song , Chris Li , Nhat Pham , Yosry Ahmed , David Hildenbrand , Johannes Weiner , Youngjun Park , Hugh Dickins , Baolin Wang , Ying Huang , Kemeng Shi , Lorenzo Stoakes , "Matthew Wilcox (Oracle)" , linux-kernel@vger.kernel.org, Kairui Song Subject: Re: [PATCH v4 10/19] mm, swap: consolidate cluster reclaim and usability check Message-ID: References: <20251205-swap-table-p2-v4-0-cb7e28a26a40@tencent.com> <20251205-swap-table-p2-v4-10-cb7e28a26a40@tencent.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251205-swap-table-p2-v4-10-cb7e28a26a40@tencent.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 On 12/05/25 at 03:29am, Kairui Song wrote: > From: Kairui Song > > Swap cluster cache reclaim requires releasing the lock, so the cluster > may become unusable after the reclaim. To prepare for checking swap > cache using the swap table directly, consolidate the swap cluster > reclaim and the check logic. > > We will want to avoid touching the cluster's data completely with the ~~~~~~~~ 'want to' means 'will'? > swap table, to avoid RCU overhead here. And by moving the cluster usable > check into the reclaim helper, it will also help avoid a redundant scan of > the slots if the cluster is no longer usable, and we will want to avoid ~~~~~~~~~~~~ this place too. > touching the cluster. > > Also, adjust it very slightly while at it: always scan the whole region > during reclaim, don't skip slots covered by a reclaimed folio. Because > the reclaim is lockless, it's possible that new cache lands at any time. > And for allocation, we want all caches to be reclaimed to avoid > fragmentation. Besides, if the scan offset is not aligned with the size > of the reclaimed folio, we might skip some existing cache and fail the > reclaim unexpectedly. > > There should be no observable behavior change. It might slightly improve > the fragmentation issue or performance. > > Signed-off-by: Kairui Song > --- > mm/swapfile.c | 45 +++++++++++++++++++++++++++++---------------- > 1 file changed, 29 insertions(+), 16 deletions(-) > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 5a766d4fcaa5..2703dfafc632 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -777,33 +777,51 @@ static int swap_cluster_setup_bad_slot(struct swap_cluster_info *cluster_info, > return 0; > } > > +/* > + * Reclaim drops the ci lock, so the cluster may become unusable (freed or > + * stolen by a lower order). @usable will be set to false if that happens. > + */ > static bool cluster_reclaim_range(struct swap_info_struct *si, > struct swap_cluster_info *ci, > - unsigned long start, unsigned long end) > + unsigned long start, unsigned int order, > + bool *usable) > { > + unsigned int nr_pages = 1 << order; > + unsigned long offset = start, end = start + nr_pages; > unsigned char *map = si->swap_map; > - unsigned long offset = start; > int nr_reclaim; > > spin_unlock(&ci->lock); > do { > switch (READ_ONCE(map[offset])) { > case 0: > - offset++; > break; > case SWAP_HAS_CACHE: > nr_reclaim = __try_to_reclaim_swap(si, offset, TTRS_ANYWAY); > - if (nr_reclaim > 0) > - offset += nr_reclaim; > - else > + if (nr_reclaim < 0) > goto out; > break; > default: > goto out; > } > - } while (offset < end); > + } while (++offset < end); ~~~~~ '++offset' is conflicting with nr_reclaim returned from __try_to_reclaim_swap(). can you explain? > out: > spin_lock(&ci->lock); > + > + /* > + * We just dropped ci->lock so cluster could be used by another > + * order or got freed, check if it's still usable or empty. > + */ > + if (!cluster_is_usable(ci, order)) { > + *usable = false; > + return false; > + } > + *usable = true; > + > + /* Fast path, no need to scan if the whole cluster is empty */ > + if (cluster_is_empty(ci)) > + return true; > + > /* > * Recheck the range no matter reclaim succeeded or not, the slot > * could have been be freed while we are not holding the lock. > @@ -900,9 +918,10 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si, > unsigned long start = ALIGN_DOWN(offset, SWAPFILE_CLUSTER); > unsigned long end = min(start + SWAPFILE_CLUSTER, si->max); > unsigned int nr_pages = 1 << order; > - bool need_reclaim, ret; > + bool need_reclaim, ret, usable; > > lockdep_assert_held(&ci->lock); > + VM_WARN_ON(!cluster_is_usable(ci, order)); > > if (end < nr_pages || ci->count + nr_pages > SWAPFILE_CLUSTER) > goto out; > @@ -912,14 +931,8 @@ static unsigned int alloc_swap_scan_cluster(struct swap_info_struct *si, > if (!cluster_scan_range(si, ci, offset, nr_pages, &need_reclaim)) > continue; > if (need_reclaim) { > - ret = cluster_reclaim_range(si, ci, offset, offset + nr_pages); > - /* > - * Reclaim drops ci->lock and cluster could be used > - * by another order. Not checking flag as off-list > - * cluster has no flag set, and change of list > - * won't cause fragmentation. > - */ > - if (!cluster_is_usable(ci, order)) > + ret = cluster_reclaim_range(si, ci, offset, order, &usable); > + if (!usable) > goto out; > if (cluster_is_empty(ci)) > offset = start; > > -- > 2.52.0 >