Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Sender: "yuzhao via sendgmr" <yuzhao@yuzhao.bld.corp.google.com>
Date:   Fri, 26 Feb 2021 02:17:17 -0700
In-Reply-To: <20210226091718.2927291-1-yuzhao@google.com>
Message-Id: <20210226091718.2927291-3-yuzhao@google.com>
Mime-Version: 1.0
References: <20210224084807.2179942-1-yuzhao@google.com> <20210226091718.2927291-1-yuzhao@google.com>
Subject: [PATCH v2 2/3] mm: use PF_NO_TAIL for PG_lru
From:   Yu Zhao <yuzhao@google.com>
To:     akpm@linux-foundation.org, alex.shi@linux.alibaba.com,
        vbabka@suse.cz, willy@infradead.org
Cc:     guro@fb.com, hannes@cmpxchg.org, hughd@google.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        mhocko@kernel.org, vdavydov.dev@gmail.com,
        Yu Zhao <yuzhao@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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@google.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