linux-kernel - [RFC PATCH] x86/fault: move might

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Message-Id: <20211119065831.31406-1-songmuchun@bytedance.com>
Date:   Fri, 19 Nov 2021 14:58:31 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        duanxiongchun@...edance.com, zhengqi.arch@...edance.com,
        Muchun Song <songmuchun@...edance.com>
Subject: [RFC PATCH] x86/fault: move might_sleep() out of mmap read lock

The mmap lock is supposed to be a contended lock sometimes, scheduling
to other task with holding mmap read lock does not seems to be a wise
choice. So move it to the front of mmap_read_trylock(). Although
mmap_read_lock() implies a might_sleep(), I think redundant check is
not a problem since this task is about to sleep and it is not a hot
path.

Signed-off-by: Muchun Song <songmuchun@...edance.com>
---
 arch/x86/mm/fault.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 4bfed53e210e..22fd1dfafa3d 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1323,6 +1323,8 @@ void do_user_addr_fault(struct pt_regs *regs,
 	}
 #endif
 
+	might_sleep();
+
 	/*
 	 * Kernel-mode access to the user address space should only occur
 	 * on well-defined single instructions listed in the exception
@@ -1346,13 +1348,6 @@ void do_user_addr_fault(struct pt_regs *regs,
 		}
 retry:
 		mmap_read_lock(mm);
-	} else {
-		/*
-		 * The above down_read_trylock() might have succeeded in
-		 * which case we'll have missed the might_sleep() from
-		 * down_read():
-		 */
-		might_sleep();
 	}
 
 	vma = find_vma(mm, address);
-- 
2.11.0