[RFC PATCH v2 2/2] mm: improve large folio reuse for LRU-cached folios

[Barry Song (Xiaomi)]

Large folios may now reside in the per-CPU LRU cache. Before
attempting to reuse them, drain the local LRU cache, which
can still be beneficial in cases where the folios are likely
to remain in this CPU's LRU cache:

int main(int argc, char *argv[])
{
    int i;
    while (1) {
        volatile int *p = mmap(0, SIZE, PROT_READ | PROT_WRITE,
                               MAP_PRIVATE | MAP_ANONYMOUS,
                               -1, 0);
        for (int i = 0; i < SIZE / sizeof(int); i++)
            p[i] = i;
        madvise((void *)p, SIZE, MADV_PAGEOUT);
        if (!fork())
            _exit(0);
        for (int i = 0; i < SIZE / sizeof(int); i++)
            p[i] = i;
        munmap((void *)p, SIZE);
    }
    return 0;
}

Signed-off-by: Barry Song (Xiaomi) <baohua@xxxxxxxxxx>
---
 mm/memory.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 5689b7cff76c..1d08ed5ba99b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4136,6 +4136,19 @@ static bool __wp_can_reuse_large_anon_folio(struct folio *folio,
 		folio_unlock(folio);
 	}
 
+	if (folio_may_be_lru_cached(folio) && !folio_test_lru(folio)) {
+		if (folio_ref_count(folio) > folio_large_mapcount(folio) + 1)
+			return false;
+		/*
+		 * A global LRU drain is too expensive, but a local drain
+		 * can still be beneficial. For example, large folios that
+		 * have just been swapped in and fall back from
+		 * do_swap_page() to do_wp_page() are likely still in this
+		 * CPU's LRU cache.
+		 */
+		lru_add_drain();
+	}
+
 	if (folio_large_mapcount(folio) != folio_ref_count(folio))
 		return false;
 
-- 
2.39.3 (Apple Git-146)