mm: swap: introduce swap_free_nr() for batched swap_free()

Patch series "large folios swap-in: handle refault cases first", v5.

This patchset is extracted from the large folio swapin series[1],
primarily addressing the handling of scenarios involving large folios in
the swap cache.  Currently, it is particularly focused on addressing the
refaulting of mTHP, which is still undergoing reclamation.  This approach
aims to streamline code review and expedite the integration of this
segment into the MM tree.

It relies on Ryan's swap-out series[2], leveraging the helper function
swap_pte_batch() introduced by that series.

Presently, do_swap_page only encounters a large folio in the swap cache
before the large folio is released by vmscan.  However, the code should
remain equally useful once we support large folio swap-in via
swapin_readahead().  This approach can effectively reduce page faults and
eliminate most redundant checks and early exits for MTE restoration in
recent MTE patchset[3].

The large folio swap-in for SWP_SYNCHRONOUS_IO and swapin_readahead() will
be split into separate patch sets and sent at a later time.

[1] https://lore.kernel.org/linux-mm/20240304081348.197341-1-21cnbao@gmail.com/
[2] https://lore.kernel.org/linux-mm/20240408183946.2991168-1-ryan.roberts@arm.com/
[3] https://lore.kernel.org/linux-mm/20240322114136.61386-1-21cnbao@gmail.com/ 


This patch (of 6):

While swapping in a large folio, we need to free swaps related to the
whole folio.  To avoid frequently acquiring and releasing swap locks, it
is better to introduce an API for batched free.  Furthermore, this new
function, swap_free_nr(), is designed to efficiently handle various
scenarios for releasing a specified number, nr, of swap entries.

Link: https://lkml.kernel.org/r/20240529082824.150954-1-21cnbao@gmail.com
Link: https://lkml.kernel.org/r/20240529082824.150954-2-21cnbao@gmail.com
Signed-off-by: Chuanhua Han <hanchuanhua@oppo.com>
Co-developed-by: Barry Song <v-songbaohua@oppo.com>
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: Chris Li <chrisl@kernel.org>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Gao Xiang <xiang@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Khalid Aziz <khalid.aziz@oracle.com>
Cc: Len Brown <len.brown@intel.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 47 insertions, 0 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index f1e559e216bd..92a045d34a97 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1356,6 +1356,53 @@ void swap_free(swp_entry_t entry)
 		__swap_entry_free(p, entry);
 }
 
+static void cluster_swap_free_nr(struct swap_info_struct *sis,
+		unsigned long offset, int nr_pages)
+{
+	struct swap_cluster_info *ci;
+	DECLARE_BITMAP(to_free, BITS_PER_LONG) = { 0 };
+	int i, nr;
+
+	ci = lock_cluster_or_swap_info(sis, offset);
+	while (nr_pages) {
+		nr = min(BITS_PER_LONG, nr_pages);
+		for (i = 0; i < nr; i++) {
+			if (!__swap_entry_free_locked(sis, offset + i, 1))
+				bitmap_set(to_free, i, 1);
+		}
+		if (!bitmap_empty(to_free, BITS_PER_LONG)) {
+			unlock_cluster_or_swap_info(sis, ci);
+			for_each_set_bit(i, to_free, BITS_PER_LONG)
+				free_swap_slot(swp_entry(sis->type, offset + i));
+			if (nr == nr_pages)
+				return;
+			bitmap_clear(to_free, 0, BITS_PER_LONG);
+			ci = lock_cluster_or_swap_info(sis, offset);
+		}
+		offset += nr;
+		nr_pages -= nr;
+	}
+	unlock_cluster_or_swap_info(sis, ci);
+}
+
+void swap_free_nr(swp_entry_t entry, int nr_pages)
+{
+	int nr;
+	struct swap_info_struct *sis;
+	unsigned long offset = swp_offset(entry);
+
+	sis = _swap_info_get(entry);
+	if (!sis)
+		return;
+
+	while (nr_pages) {
+		nr = min_t(int, nr_pages, SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER);
+		cluster_swap_free_nr(sis, offset, nr);
+		offset += nr;
+		nr_pages -= nr;
+	}
+}
+
 /*
  * Called after dropping swapcache to decrease refcnt to swap entries.
  */