linux-kernel - [PATCH v2 2/3] mm: use PF_NO_TAIL for PG

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Message-Id: <20210226091718.2927291-3-yuzhao@google.com>
Date:   Fri, 26 Feb 2021 02:17:17 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     akpm@...ux-foundation.org, alex.shi@...ux.alibaba.com,
        vbabka@...e.cz, willy@...radead.org
Cc:     guro@...com, hannes@...xchg.org, hughd@...gle.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mhocko@...nel.org, vdavydov.dev@...il.com,
        Yu Zhao <yuzhao@...gle.com>
Subject: [PATCH v2 2/3] mm: use PF_NO_TAIL for PG_lru

Trying to set or clear PG_lru on tail pages has been considered buggy.
Enforce this rule by changing the policy for PG_lru from PF_HEAD to
PF_NO_TAIL. This means setting or clearing PG_lru on tail pages won't
be "corrected" by compound_page(). Such "correction" isn't helpful --
even if a piece of buggy code has gotten away with
compound_page(tail)->flags, it will run into trouble with lru list
addition and deletion because they use tail->lru rather than
compound_page(tail)->lru.

bloat-o-meter result:
  add/remove: 0/0 grow/shrink: 0/11 up/down: 0/-535 (-535)

Signed-off-by: Yu Zhao <yuzhao@...gle.com>
---
 include/linux/page-flags.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 1995208a3763..c9626e54df8d 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -333,8 +333,8 @@ PAGEFLAG(Referenced, referenced, PF_HEAD)
 	__SETPAGEFLAG(Referenced, referenced, PF_HEAD)
 PAGEFLAG(Dirty, dirty, PF_HEAD) TESTSCFLAG(Dirty, dirty, PF_HEAD)
 	__CLEARPAGEFLAG(Dirty, dirty, PF_HEAD)
-PAGEFLAG(LRU, lru, PF_HEAD) __CLEARPAGEFLAG(LRU, lru, PF_HEAD)
-	TESTCLEARFLAG(LRU, lru, PF_HEAD)
+PAGEFLAG(LRU, lru, PF_NO_TAIL) __CLEARPAGEFLAG(LRU, lru, PF_NO_TAIL)
+	TESTCLEARFLAG(LRU, lru, PF_NO_TAIL)
 PAGEFLAG(Active, active, PF_HEAD) __CLEARPAGEFLAG(Active, active, PF_HEAD)
 	TESTCLEARFLAG(Active, active, PF_HEAD)
 PAGEFLAG(Workingset, workingset, PF_HEAD)
-- 
2.30.1.766.gb4fecdf3b7-goog