[PATCH 17/25] mm/fbatch: no lru_cache_disable() in __alloc_contig_migrate_range()

[Hugh Dickins]

Remove lru_cache_disable() from __alloc_contig_migrate_range(). It does
not now benefit from lru_add_drain_all() first; and gains little benefit
from invalidating buffer head LRUs first, since 5.0 commit 80409c65e2c6
("mm: migrate: make buffer_migrate_page_norefs() actually succeed").

This will be more controversial. lru_cache_disable()+lru_cache_enable()
were brought in for CMA page migration, see 5.13 commit d479960e44f2
("mm: disable LRU pagevec during the migration temporarily") through
8cc621d2f45d ("mm: fs: invalidate BH LRU during page migration") - I
guess the testing there must have been on a 4.19-based Android kernel,
without 5.0's buffer_migrate_page_norefs().

It's possible that invalidating BH LRUs perhaps 0 times, perhaps N
times, will average out worse than invalidating 1 time and stopping
everyone else; or that folio_test_clear_lru() failures manifest more
than before (note how folio migration has retries on raised refcount,
but isolate_migratepages_block() no retry on failed test_clear_lru).
But let's give this a try and look out for regressions.

Don't delete lru_cache_disable() yet: leaving stale folio pointers in
the per-cpu fbatches, with folio_try_get() yet to come on them, would be
bad for memory hotremoval: the lru_cache_disable() in offline_pages()
protects from that.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
 mm/page_alloc.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 083cbcb5bdde..96ab51b78a5f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7135,8 +7135,6 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
 		.reason = MR_CONTIG_RANGE,
 	};
 
-	lru_cache_disable();
-
 	while (pfn < end || !list_empty(&cc->migratepages)) {
 		if (fatal_signal_pending(current)) {
 			ret = -EINTR;
@@ -7170,7 +7168,6 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
 			break;
 	}
 
-	lru_cache_enable();
 	if (ret < 0) {
 		if (!(cc->gfp_mask & __GFP_NOWARN) && ret == -EBUSY)
 			alloc_contig_dump_pages(&cc->migratepages);
-- 
2.51.0