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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> 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 X-Rspamd-Server: rspam02 X-Stat-Signature: kafd6xu689iq7ar1eh3yek51141ohhnc X-Rspam-User: X-Rspamd-Queue-Id: 3034940002 X-HE-Tag: 1765771988-535145 X-HE-Meta: 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 oUldShJW h9OiQ4sW8endRHSHwgtSBe6HWaovlxr7mXnIXA0iw2Id234sGmkcjHq4NrdDFjMEry0Q9eVOf7/sZTyT2KsfqmLcrbhX+zYfF5s+377G3Mt06M6U= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 >