mm/shmem, swap: improve cached mTHP handling and fix potential hang

The current swap-in code assumes that, when a swap entry in shmem mapping
is order 0, its cached folios (if present) must be order 0 too, which
turns out not always correct.

The problem is shmem_split_large_entry is called before verifying the
folio will eventually be swapped in, one possible race is:

CPU1 CPU2
shmem_swapin_folio
/* swap in of order > 0 swap entry S1 */
folio = swap_cache_get_folio
/* folio = NULL */
order = xa_get_order
/* order > 0 */
folio = shmem_swap_alloc_folio
/* mTHP alloc failure, folio = NULL */
<... Interrupted ...>
shmem_swapin_folio
/* S1 is swapped in */
shmem_writeout
/* S1 is swapped out, folio cached */
shmem_split_large_entry(..., S1)
/* S1 is split, but the folio covering it has order > 0 now */

Now any following swapin of S1 will hang: `xa_get_order` returns 0, and
folio lookup will return a folio with order > 0. The
`xa_get_order(&mapping->i_pages, index) != folio_order(folio)` will always
return false causing swap-in to return -EEXIST.

And this looks fragile. So fix this up by allowing seeing a larger folio
in swap cache, and check the whole shmem mapping range covered by the
swapin have the right swap value upon inserting the folio. And drop the
redundant tree walks before the insertion.

This will actually improve performance, as it avoids two redundant Xarray
tree walks in the hot path, and the only side effect is that in the
failure path, shmem may redundantly reallocate a few folios causing
temporary slight memory pressure.

And worth noting, it may seems the order and value check before inserting
might help reducing the lock contention, which is not true. The swap
cache layer ensures raced swapin will either see a swap cache folio or
failed to do a swapin (we have SWAP_HAS_CACHE bit even if swap cache is
bypassed), so holding the folio lock and checking the folio flag is
already good enough for avoiding the lock contention. The chance that a
folio passes the swap entry value check but the shmem mapping slot has
changed should be very low.

Link: https://lkml.kernel.org/r/20250728075306.12704-1-ryncsn@gmail.com
Link: https://lkml.kernel.org/r/20250728075306.12704-2-ryncsn@gmail.com
Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Kemeng Shi <shikemeng@huaweicloud.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: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>