Re: [PATCHv2] mm/huge_memory: do not add dropped split tail folios to LRU

[Matthew Wilcox]

On Fri, Jun 12, 2026 at 10:34:56AM +0800, zhaoyang.huang wrote:
> [2]
> *F: big folio before split
> *T: tail folio after split
> CPU0 (f2fs GC)              CPU1 (split_folio_to_order)          CPU2 (folio_isolate_lru)
> *F: pagecache refs = n
> *F: extra refs = split
> *F: PG_lru set, mapping != NULL
>                             split_folio_to_order(F)
>                               folio_ref_freeze(F, 1)

I'll stop you right there.  folio_ref_freeze() is called with
folio_cache_ref_count(folio) + 1.  That's defined as:

static unsigned int folio_cache_ref_count(const struct folio *folio)
{
        if (folio_test_anon(folio) && !folio_test_swapcache(folio))
                return 0;
        return folio_nr_pages(folio);
}

so you'd have to freeze it with 2^order + 1.  The 2^order is the number of
references the page cache takes on it and the 1 is the reference held by
the caller of split_folio_to_order().  If anybody else has a reference
to it at this point, the call to freeze will fail.