[PATCH v2] mm: remove redundant lru_add_drain() prior to unmapping pages

[Jianfeng Wang <jianfeng.w.wang@oracle.com>]

When unmapping VMA pages, pages will be gathered in batch and released by
tlb_finish_mmu() if CONFIG_MMU_GATHER_NO_GATHER is not set. The function
tlb_finish_mmu() is responsible for calling free_pages_and_swap_cache(),
which calls lru_add_drain() to drain cached pages in folio_batch before
releasing gathered pages. Thus, it is redundant to call lru_add_drain()
before gathering pages, if CONFIG_MMU_GATHER_NO_GATHER is not set.

Remove lru_add_drain() prior to gathering and unmapping pages in
exit_mmap() and unmap_region() if CONFIG_MMU_GATHER_NO_GATHER is not set.

Note that the page unmapping process in oom_killer (e.g., in
__oom_reap_task_mm()) also uses tlb_finish_mmu() and does not have
redundant lru_add_drain(). So, this commit makes the code more consistent.

Signed-off-by: Jianfeng Wang <jianfeng.w.wang@oracle.com>
---
 mm/mmap.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/mmap.c b/mm/mmap.c
index 1971bfffcc03..da0308eef435 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2330,7 +2330,10 @@ static void unmap_region(struct mm_struct *mm, struct ma_state *mas,
 	struct mmu_gather tlb;
 	unsigned long mt_start = mas->index;
 
+	/* Defer lru_add_drain() to tlb_finish_mmu() for the ifndef case. */
+#ifdef CONFIG_MMU_GATHER_NO_GATHER
 	lru_add_drain();
+#endif
 	tlb_gather_mmu(&tlb, mm);
 	update_hiwater_rss(mm);
 	unmap_vmas(&tlb, mas, vma, start, end, tree_end, mm_wr_locked);
@@ -3300,7 +3303,10 @@ void exit_mmap(struct mm_struct *mm)
 		return;
 	}
 
+	/* Defer lru_add_drain() to tlb_finish_mmu() for the ifndef case. */
+#ifdef CONFIG_MMU_GATHER_NO_GATHER
 	lru_add_drain();
+#endif
 	flush_cache_mm(mm);
 	tlb_gather_mmu_fullmm(&tlb, mm);
 	/* update_hiwater_rss(mm) here? but nobody should be looking */
-- 
2.42.1