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mm: zswap: drop cold writeback folios via swap dropbehind

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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>
This commit is contained in:
Alexandre Ghiti 2026-09-21 17:13:04 +02:00 committed by Andrew Morton
parent 7ccf5a3e5b
commit 4fce3f3c69
1 changed files with 23 additions and 10 deletions
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33
mm/zswap.c
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@ -1032,7 +1032,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
*/
if (IS_ERR(folio))
return PTR_ERR(folio);
folio_add_lru(folio);
folio_clear_active(folio);
/*
* folio is locked, and the swapcache is now secured against
@ -1046,12 +1047,12 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
tree = swap_zswap_tree(swpentry);
if (entry != xa_load(tree, offset)) {
ret = -ENOMEM;
goto out;
goto err;
}
if (!zswap_decompress(entry, folio)) {
ret = -EIO;
goto out;
goto err;
}
xa_erase(tree, offset);
@ -1065,18 +1066,30 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
/* folio is up to date */
folio_mark_uptodate(folio);
/* move it to the tail of the inactive list after end_writeback */
folio_set_reclaim(folio);
folio_set_dropbehind(folio);
/*
* Drop our reference before starting writeback so the swap cache holds
* the only one: the drop in folio_end_writeback() needs that for
* remove_mapping_set_shadow() to succeed, otherwise the folio is
* handed back to reclaim instead.
*
* Nothing can free the folio in the meantime: we hold the folio lock
* until writeback starts, PG_writeback then blocks swap cache removal,
* and folio_end_writeback() takes its own reference before clearing
* PG_writeback and donates it to the drop.
*/
folio_put(folio);
/* start writeback */
__swap_writeout(&ctx, folio);
swap_write_submit(&ctx);
out:
if (ret) {
swap_cache_del_folio(folio);
folio_unlock(folio);
}
return 0;
err:
swap_cache_del_folio(folio);
folio_unlock(folio);
folio_put(folio);
return ret;
}