Multi-size THP enables performance improvements by allocating large,
pte-mapped folios for anonymous memory.  However I've observed that on an
arm64 system running a parallel workload (e.g.  kernel compilation) across
many cores, under high memory pressure, the speed regresses.  This is due
to bottlenecking on the increased number of TLBIs added due to all the
extra folio splitting when the large folios are swapped out.

Therefore, solve this regression by adding support for swapping out mTHP
without needing to split the folio, just like is already done for
PMD-sized THP.  This change only applies when CONFIG_THP_SWAP is enabled,
and when the swap backing store is a non-rotating block device.  These are
the same constraints as for the existing PMD-sized THP swap-out support.

Note that no attempt is made to swap-in (m)THP here - this is still done
page-by-page, like for PMD-sized THP.  But swapping-out mTHP is a
prerequisite for swapping-in mTHP.

The main change here is to improve the swap entry allocator so that it can
allocate any power-of-2 number of contiguous entries between [1, (1 <<
PMD_ORDER)].  This is done by allocating a cluster for each distinct order
and allocating sequentially from it until the cluster is full.  This
ensures that we don't need to search the map and we get no fragmentation
due to alignment padding for different orders in the cluster.  If there is
no current cluster for a given order, we attempt to allocate a free
cluster from the list.  If there are no free clusters, we fail the
allocation and the caller can fall back to splitting the folio and
allocates individual entries (as per existing PMD-sized THP fallback).

The per-order current clusters are maintained per-cpu using the existing
infrastructure.  This is done to avoid interleving pages from different
tasks, which would prevent IO being batched.  This is already done for the
order-0 allocations so we follow the same pattern.

As is done for order-0 per-cpu clusters, the scanner now can steal order-0
entries from any per-cpu-per-order reserved cluster.  This ensures that
when the swap file is getting full, space doesn't get tied up in the
per-cpu reserves.

This change only modifies swap to be able to accept any order mTHP.  It
doesn't change the callers to elide doing the actual split.  That will be
done in separate changes.

Link: https://lkml.kernel.org/r/20240408183946.2991168-6-ryan.roberts@arm.comSigned-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Gao Xiang <xiang@kernel.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>