mm: zswap: drop cold writeback folios via swap dropbehind

zswap writeback decompresses an entry into a fresh swap cache folio and
writes it back. The folio is cold by construction, yet it is left on the
LRU for reclaim to find and free later, wasting a reclaim scan and keeping
cold memory resident longer than necessary.

Allocate the folio off the LRU and mark it PG_dropbehind so the swap
dropbehind path frees it from the swap cache once writeback completes.

__swap_cache_alloc_folio() evaluates a refault on the new folio, and
workingset_refault() sets PG_active when it looks recent. Until now
folio_add_lru() consumed that flag and __page_cache_release() cleared it
once the folio left the LRU. This folio never reaches the LRU, so nothing
would clear PG_active and the folio would be freed with a
PAGE_FLAGS_CHECK_AT_FREE flag set, tripping bad_page() under
CONFIG_DEBUG_VM. Clear it after allocation.

That is a workaround: the refault should not be evaluated on a writeback
buffer at all. A fix for that is on the mailing list [1].

Link: https://lore.kernel.org/20260921151306.625134-4-alex@ghiti.fr
Link: https://lore.kernel.org/linux-mm/20260911092012.92399-1-alex@ghiti.fr/ [1]
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Suggested-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Kunwu Chan <kunwu.chan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Barry Song <baohua@kernel.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Chis Li <chrisl@kernel.org>
Cc: Christian Brauner (Amutable) <brauner@kernel.org>
Cc: "David Hildenbrand (arm)" <david@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Tal Zussman <tz2294@columbia.edu>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Wei Xu <weixugc@google.com>
Cc: Yosry Ahmed <yosry@kernel.org>
Cc: Youngjun Park <youngjun.park@lge.com>
Cc: Yuanchu Xie <yuanchu@google.com>

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