mm/shmem, swap: avoid redundant Xarray lookup during swapin

mm/shmem, swap: avoid redundant Xarray lookup during swapin

Patch series "mm/shmem, swap: bugfix and improvement of mTHP swap in", v6.

The current THP swapin path have several problems.  It may potentially
hang, may cause redundant faults due to false positive swap cache lookup,
and it issues redundant Xarray walks.  !CONFIG_TRANSPARENT_HUGEPAGE builds
may also contain unnecessary THP checks.

This series fixes all of the mentioned issues, the code should be more
robust and prepared for the swap table series.  Now 4 walks is reduced to
3 (get order & confirm, confirm, insert folio),
!CONFIG_TRANSPARENT_HUGEPAGE build overhead is also minimized, and comes
with a sanity check now.

The performance is slightly better after this series, sequential swap in
of 24G data from ZRAM, using transparent_hugepage_tmpfs=always (24 samples
each):

Before:         avg: 10.66s, stddev: 0.04
After patch 1:  avg: 10.58s, stddev: 0.04
After patch 2:  avg: 10.65s, stddev: 0.05
After patch 3:  avg: 10.65s, stddev: 0.04
After patch 4:  avg: 10.67s, stddev: 0.04
After patch 5:  avg: 9.79s,  stddev: 0.04
After patch 6:  avg: 9.79s,  stddev: 0.05
After patch 7:  avg: 9.78s,  stddev: 0.05
After patch 8:  avg: 9.79s,  stddev: 0.04

Several patches improve the performance by a little, which is about ~8%
faster in total.

Build kernel test showed very slightly improvement, testing with make -j48
with defconfig in a 768M memcg also using ZRAM as swap, and
transparent_hugepage_tmpfs=always (6 test runs):

Before:         avg: 3334.66s, stddev: 43.76
After patch 1:  avg: 3349.77s, stddev: 18.55
After patch 2:  avg: 3325.01s, stddev: 42.96
After patch 3:  avg: 3354.58s, stddev: 14.62
After patch 4:  avg: 3336.24s, stddev: 32.15
After patch 5:  avg: 3325.13s, stddev: 22.14
After patch 6:  avg: 3285.03s, stddev: 38.95
After patch 7:  avg: 3287.32s, stddev: 26.37
After patch 8:  avg: 3295.87s, stddev: 46.24

This patch (of 7):

Currently shmem calls xa_get_order to get the swap radix entry order,
requiring a full tree walk.  This can be easily combined with the swap
entry value checking (shmem_confirm_swap) to avoid the duplicated lookup
and abort early if the entry is gone already.  Which should improve the
performance.

Link: https://lkml.kernel.org/r/20250728075306.12704-1-ryncsn@gmail.com
Link: https://lkml.kernel.org/r/20250728075306.12704-3-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>