linux-kernel - [PATCH 1/3] mm: use_mm: fix for arches checking mm

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Message-Id: <20200223192520.20808-2-aarcange@redhat.com>
Date:   Sun, 23 Feb 2020 14:25:18 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Rafael Aquini <aquini@...hat.com>,
        Mark Salter <msalter@...hat.com>
Cc:     Jon Masters <jcm@...masters.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        Michal Hocko <mhocko@...nel.org>, QI Fuli <qi.fuli@...itsu.com>
Subject: [PATCH 1/3] mm: use_mm: fix for arches checking mm_users to optimize TLB flushes

alpha, ia64, mips, powerpc, sh, sparc are relying on a check on
mm->mm_users to know if they can skip some remote TLB flushes for
single threaded processes.

Most callers of use_mm() tend to invoke mmget_not_zero() or
get_task_mm() before use_mm() to ensure the mm will remain alive in
between use_mm() and unuse_mm().

Some callers however don't increase mm_users and they instead rely on
serialization in __mmput() to ensure the mm will remain alive in
between use_mm() and unuse_mm(). Not increasing mm_users during
use_mm() is however unsafe for aforementioned arch TLB flushes
optimizations. So either mmget()/mmput() should be added to the
problematic callers of use_mm()/unuse_mm() or we can embed them in
use_mm()/unuse_mm() which is more robust.

Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
---
 mm/mmu_context.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/mmu_context.c b/mm/mmu_context.c
index 3e612ae748e9..ced0e1218c0f 100644
--- a/mm/mmu_context.c
+++ b/mm/mmu_context.c
@@ -30,6 +30,7 @@ void use_mm(struct mm_struct *mm)
 		mmgrab(mm);
 		tsk->active_mm = mm;
 	}
+	mmget(mm);
 	tsk->mm = mm;
 	switch_mm(active_mm, mm, tsk);
 	task_unlock(tsk);
@@ -57,6 +58,7 @@ void unuse_mm(struct mm_struct *mm)
 	task_lock(tsk);
 	sync_mm_rss(mm);
 	tsk->mm = NULL;
+	mmput(mm);
 	/* active_mm is still 'mm' */
 	enter_lazy_tlb(mm, tsk);
 	task_unlock(tsk);