mm/shmem, swap: rework swap entry and index calculation for large swapin

mm/shmem, swap: rework swap entry and index calculation for large swapin

Instead of calculating the swap entry differently in different swapin
paths, calculate it early before the swap cache lookup and use that for
the lookup and later swapin.  And after swapin have brought a folio,
simply round it down against the size of the folio.

This is simple and effective enough to verify the swap value.  A folio's
swap entry is always aligned by its size.  Any kind of parallel split or
race is acceptable because the final shmem_add_to_page_cache ensures that
all entries covered by the folio are correct, and thus there will be no
data corruption.

This also prevents false positive cache lookup.  If a shmem read request's
index points to the middle of a large swap entry, previously, shmem will
try the swap cache lookup using the large swap entry's starting value
(which is the first sub swap entry of this large entry).  This will lead
to false positive lookup results if only the first few swap entries are
cached but the actual requested swap entry pointed by the index is
uncached.  This is not a rare event, as swap readahead always tries to
cache order 0 folios when possible.

And this shouldn't cause any increased repeated faults.  Instead, no
matter how the shmem mapping is split in parallel, as long as the mapping
still contains the right entries, the swapin will succeed.

The final object size and stack usage are also reduced due to simplified
code:

./scripts/bloat-o-meter mm/shmem.o.old mm/shmem.o
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-145 (-145)
Function                                     old     new   delta
shmem_swapin_folio                          4056    3911    -145
Total: Before=33242, After=33097, chg -0.44%

Stack usage (Before vs After):
mm/shmem.c:2314:12:shmem_swapin_folio   264     static
mm/shmem.c:2314:12:shmem_swapin_folio   256     static

And while at it, round down the index too if swap entry is round down.
The index is used either for folio reallocation or confirming the mapping
content.  In either case, it should be aligned with the swap folio.

Link: https://lkml.kernel.org/r/20250728075306.12704-8-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-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: Dev Jain <dev.jain@arm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>