Re: [PATCH v2] mm/mglru: fix and remove redundant unevictable folio handling

On Wed, 12 Aug 2026 20:22:39 +0800 Kairui Song via B4 Relay <[email protected]> wrote:

> From: Kairui Song <[email protected]>
> 
> sort_folio() has a shortcut for moving folios that are no longer
> evictable but are still sitting on a generation list.  However, this
> shortcut is buggy.  It does not follow the PG_lru usage convention,
> and it has a more serious issue.
> 
> Unevictable folios are not threaded on lists[LRU_UNEVICTABLE], so that
> folio->lru can be reused to hold folio->mlock_count (see the comment in
> lruvec_init()).  Hence lruvec_add_folio() skips the list_add() for them,
> and every other place that turns a folio unevictable initialises
> mlock_count explicitly: lru_add() sets it to 0, __mlock_folio() and
> __mlock_new_folio() set it to !!folio_test_mlocked(folio).
> sort_folio() sets nothing, and the lru_gen_del_folio() right above it
> may have already poisoned folio->lru via list_del(), so mlock_count
> ends up aliasing LIST_POISON2, which reads as 0x122, i.e. 290.  The
> result is user visible.  On munlock, __munlock_folio() decrements that
> bogus count, finds it still non-zero and bails out before clearing
> PG_mlocked, so the folio remains unevictable and the Mlocked
> accounting stays inflated until the folio is freed.
> 
> The shortcut also touches the LRU flags in the wrong order.  It calls
> lru_gen_del_folio() while PG_lru is still set, so a concurrent
> folio_test_clear_lru() (e.g. compaction, folio_isolate_lru()) can
> succeed on a folio that has already been taken off the generation list,
> which may lead to unexpected behavior.
> 
> So fix it by isolating them as common folios and letting the generic
> shrink path cull them. This matches the classical LRU behavior, and
> there should be no visible effect on the generic eviction or isolation
> behavior.
> 
> There is no performance concern either, such a folio goes through this
> once, and then it is off the generation lists for good.

Great, thanks.

> Fixes: ac35a4902374 ("mm: multi-gen LRU: minimal implementation")

The above description makes me think -stable wants this.

That ac35a4902374 is four years old makes me think that we can feed
this in for 7.3-rc1.

OK?