linux-kernel - [RFC PATCH 5/5] Revert "mm/migrate: __unmap_and

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Message-ID: <20230618065824.1365750-1-yosryahmed@google.com>
Date:   Sun, 18 Jun 2023 06:58:24 +0000
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Yu Zhao <yuzhao@...gle.com>,
        "Jan Alexander Steffens (heftig)" <heftig@...hlinux.org>,
        Steven Barrett <steven@...uorix.net>,
        Brian Geffon <bgeffon@...gle.com>,
        "T.J. Alumbaugh" <talumbau@...gle.com>,
        Gaosheng Cui <cuigaosheng1@...wei.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        David Howells <dhowells@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Greg Thelen <gthelen@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Yosry Ahmed <yosryahmed@...gle.com>
Subject: [RFC PATCH 5/5] Revert "mm/migrate: __unmap_and_move() push good
 newpage to LRU"

This reverts commit c3096e6782b733158bf34f6bbb4567808d4e0740.

That commit made sure we immediately add the new page to the LRU before
remove_migration_ptes() is called in migrate_move_folio() (used to be
__unmap_and_move() back then), such that the rmap walk will rebuild the
correct mlock_count for the page again. This was needed because the
mlock_count was lost when the page is isolated. This is no longer the
case since mlock_count no longer overlays page->lru.

Revert the commit (the code was foliated afterward the commit, so the
revert is updated as such).

Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
---
 mm/migrate.c | 24 +++++++++---------------
 1 file changed, 9 insertions(+), 15 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 01cac26a3127..68f693731865 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1279,19 +1279,6 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
 	if (unlikely(!is_lru))
 		goto out_unlock_both;
 
-	/*
-	 * When successful, push dst to LRU immediately: so that if it
-	 * turns out to be an mlocked page, remove_migration_ptes() will
-	 * automatically build up the correct dst->mlock_count for it.
-	 *
-	 * We would like to do something similar for the old page, when
-	 * unsuccessful, and other cases when a page has been temporarily
-	 * isolated from the unevictable LRU: but this case is the easiest.
-	 */
-	folio_add_lru(dst);
-	if (page_was_mapped)
-		lru_add_drain();
-
 	if (page_was_mapped)
 		remove_migration_ptes(src, dst, false);
 
@@ -1301,9 +1288,16 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
 	/*
 	 * If migration is successful, decrease refcount of dst,
 	 * which will not free the page because new page owner increased
-	 * refcounter.
+	 * refcounter. As well, if it is LRU folio, add the folio to LRU
+	 * list in here. Use the old state of the isolated source folio to
+	 * determine if we migrated a LRU folio. dst was already unlocked
+	 * and possibly modified by its owner - don't rely on the folio
+	 * state.
 	 */
-	folio_put(dst);
+	if (unlikely(!is_lru))
+		folio_put(dst);
+	else
+		folio_putback_lru(dst);
 
 	/*
 	 * A folio that has been migrated has all references removed
-- 
2.41.0.162.gfafddb0af9-goog