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From: Andrea Arcangeli <aarcange@redhat.com>
To: Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Jon Masters <jcm@jonmasters.org>,
	Rafael Aquini <aquini@redhat.com>,
	Mark Salter <msalter@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-arm-kernel@lists.infradead.org
Subject: [PATCH 1/2] mm: use_mm: fix for arches checking mm_users to optimize TLB flushes
Date: Mon,  3 Feb 2020 15:17:44 -0500	[thread overview]
Message-ID: <20200203201745.29986-2-aarcange@redhat.com> (raw)
In-Reply-To: <20200203201745.29986-1-aarcange@redhat.com>

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@redhat.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);



  reply	other threads:[~2020-02-03 20:17 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-03 20:17 [RFC] [PATCH 0/2] arm64: tlb: skip tlbi broadcast for single threaded TLB flushes Andrea Arcangeli
2020-02-03 20:17 ` Andrea Arcangeli [this message]
2020-02-18 11:31   ` [PATCH 1/2] mm: use_mm: fix for arches checking mm_users to optimize " Michal Hocko
2020-02-18 18:56     ` Andrea Arcangeli
2020-02-19 11:58       ` Michal Hocko
2020-02-19 20:04         ` Andrea Arcangeli
2020-02-03 20:17 ` [PATCH 2/2] arm64: tlb: skip tlbi broadcast for single threaded " Andrea Arcangeli
2020-02-10 17:51   ` Catalin Marinas
2020-02-10 20:14     ` Andrea Arcangeli
2020-02-11 14:00       ` Catalin Marinas
2020-02-12 12:57         ` Andrea Arcangeli
2020-02-12 14:13   ` qi.fuli
2020-02-12 15:26     ` Catalin Marinas

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