mm/shmem: writeout free swap if swap_writeout() reactivates

mm/shmem: writeout free swap if swap_writeout() reactivates

If swap_writeout() returns AOP_WRITEPAGE_ACTIVATE (for example, because
zswap cannot compress and memcg disables writeback), there is no virtue in
keeping that folio in swap cache and holding the swap allocation:
shmem_writeout() switch it back to shmem page cache before returning.

Folio lock is held, and folio->memcg_data remains set throughout, so there
is no need to get into any memcg or memsw charge complications:
swap_free_nr() and delete_from_swap_cache() do as much as is needed (but
beware the race with shmem_free_swap() when inode truncated or evicted).

Doing the same for an anonymous folio is harder, since it will usually
have been unmapped, with references to the swap left in the page tables.
Adding a function to remap the folio would be fun, but not worthwhile
unless it has other uses, or an urgent bug with anon is demonstrated.

[hughd@google.com: use shmem_recalc_inode() rather than open coding, per Baolin]
Link: https://lkml.kernel.org/r/101a7d89-290c-545d-8a6d-b1174ed8b1e5@google.com
Link: https://lkml.kernel.org/r/5c911f7a-af7a-5029-1dd4-2e00b66d565c@google.com
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: David Rientjes <rientjes@google.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <21cnbao@gmail.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <ryncsn@gmail.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>