get_next_zpdesc() calls get_zspage() which unconditionally dereferences zpdesc->zspage without a NULL check. This causes a kernel oops when zpdesc->zspage has been set to NULL by reset_zpdesc() during a race between zspage destruction and page compaction/migration. The race window is documented in a TODO comment in zs_page_migrate(): "nothing prevents a zspage from getting destroyed while it is isolated for migration, as the page lock is temporarily dropped after zs_page_isolate() succeeded" The sequence is: 1. Compaction calls zs_page_isolate() on a zpdesc, then drops its page lock. 2. Concurrently, async_free_zspage() or free_zspage() destroys the zspage, calling reset_zpdesc() which sets zpdesc->zspage = NULL. 3. A subsequent zs_free() path calls trylock_zspage(), which iterates zpdescs via get_next_zpdesc(). get_zspage() dereferences the now- NULL backpointer, causing: BUG: kernel NULL pointer dereference, address: 0000000000000000 RIP: 0010:free_zspage+0x26/0x100 Call Trace: zs_free+0xf4/0x110 zswap_entry_free+0x7e/0x160 The migration side already has a NULL guard (zs_page_migrate line 1675: "if (!zpdesc->zspage) return 0;"), but get_next_zpdesc() lacks the same protection. Fix this by reading zpdesc->zspage directly in get_next_zpdesc() instead of going through get_zspage(), and returning NULL when the backpointer is NULL. This stops iteration safely — the caller treats it as the end of the page chain. Signed-off-by: Michael Fara --- mm/zsmalloc.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c --- a/mm/zsmalloc.c +++ b/mm/zsmalloc.c @@ -735,7 +735,19 @@ static struct zspage *get_zspage(struct zpdesc *zpdesc) static struct zpdesc *get_next_zpdesc(struct zpdesc *zpdesc) { - struct zspage *zspage = get_zspage(zpdesc); + struct zspage *zspage = zpdesc->zspage; + + /* + * If the backpointer is NULL, this zpdesc was already freed via + * reset_zpdesc() by a racing async_free_zspage() while isolated + * for compaction. See the TODO comment in zs_page_migrate(). + */ + if (unlikely(!zspage)) { + WARN_ON_ONCE(1); + return NULL; + } + + BUG_ON(zspage->magic != ZSPAGE_MAGIC); if (unlikely(ZsHugePage(zspage))) return NULL; -- 2.39.0