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 20A202236FC for ; Wed, 17 Dec 2025 11:16:12 +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=1765970175; cv=none; b=ojq96/Wzsps0lABYO7yGG3Coqtbw7EqG+eZi8OckMdXkVwT/DHQi2wXRbJx/nyhdUx8SZBRKL7Qrb/EfYkRALS7jakjLuUxUsu8oeKKIVN0P81nrw1KM/fc695VIDlCwb4g2Ms+JbF9EoOddJCgKuzd8XtSqpU2JgdYvSNqrf2Q= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765970175; c=relaxed/simple; bh=i5vphJ4urGe97Fj6ZHx6B9Q0BWJxkJAAaXPBbvr+pHY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=BnTQ9ZTpTqVGyW6fSe8x1WuWEVX6JQTT0aBXUCUJhVgXVDbKpp2Z/spu+VyW5aLA9tyxSGzTEKZoYI+aLs1M4uJuUkbJvOGbby15LVzPcb2czeZz+smp/YqJtqsNhP7kfJiMSlSQs7Uay8xVMkBRndDduN5A0XOgTFv/A7r0e7Y= 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=XZC9CJi1; 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="XZC9CJi1" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1765970172; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=fziopwXFv8kkjMBB9/Q3xq/TrYYxeYKMSvdF100voDc=; b=XZC9CJi1QDY1FCXZx01YsqKja0uEsy9Q67l88E4Fj+7EVtDzWmFhOomuiK0UvS4k4tVAdj 49xQX8uaoxpJmJA8XKtmjZQicxQN2Z6A4FX3vZ4Wo1A2qkHBQgazXFg0sPtOQet2Bj4DtR IpByR9zYwPIhHOdPn5/8swV4XRrQbnI= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-103-0RmcSPelOCKUZAeOLm2ZmQ-1; Wed, 17 Dec 2025 06:16:08 -0500 X-MC-Unique: 0RmcSPelOCKUZAeOLm2ZmQ-1 X-Mimecast-MFC-AGG-ID: 0RmcSPelOCKUZAeOLm2ZmQ_1765970166 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (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-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 987AD18005AE; Wed, 17 Dec 2025 11:16:05 +0000 (UTC) Received: from localhost (unknown [10.72.112.95]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 6FDA91955F24; Wed, 17 Dec 2025 11:16:03 +0000 (UTC) Date: Wed, 17 Dec 2025 19:15:57 +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 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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 On 12/15/25 at 12:38pm, Kairui Song wrote: > On Mon, Dec 15, 2025 at 12:13 PM Baoquan He wrote: > > > > 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'? > > Sorry about my english, I mean in the following commit, we need to > avoid accessing the cluster's table (ci->table) when the cluster is > empty, so the reclaim helper need to check cluster status before > accessing it. Got it, I could be wrong. Please ignore this nit pick unless any english native speaker raise concern on this. > > > > > > 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? > > What do you mean conflicting? If (nr_reclaim < 0), reclaim failed, > this loop ends. If (nr_reclaim == 0), the slot is likely concurrently > freed so the loop should just continue to iterate & reclaim to ensure > all slots are freed. If nr_reclaim > 0, the reclaim just freed a folio > of nr_reclaim pages. We can round up by nr_reclaim to skip the slots > that were occupied by the folio, but note here we are not locking the > ci so there could be new folios landing in that range. Just keep > iterating the reclaim seems still a good option and that makes the > code simpler, and in practice maybe faster as there are less branches > and calculations involved. I see now. The 'conflicting' may be not precise. I didn't understand this because __try_to_reclaim_swap() is called in several places, and all of them have the same situation about lock releasing and retaking on ci->lock around __try_to_reclaim_swap(). As you said, we may need refactor __try_to_reclaim_swap() and make change in all those places. > > I mentioned `always scan the whole region during reclaim, don't skip > slots covered by a reclaimed folio` in the commit message, I can add a > few more comments too. > > > > 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 > > > > > >