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From: Michal Hocko <mhocko@kernel.org>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: 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>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/2] mm: use_mm: fix for arches checking mm_users to optimize TLB flushes
Date: Wed, 19 Feb 2020 12:58:55 +0100	[thread overview]
Message-ID: <20200219115855.GR4151@dhcp22.suse.cz> (raw)
In-Reply-To: <20200218185618.GB14027@redhat.com>

On Tue 18-02-20 13:56:18, Andrea Arcangeli wrote:
> Hi Michal!
> 
> On Tue, Feb 18, 2020 at 12:31:03PM +0100, Michal Hocko wrote:
> > On Mon 03-02-20 15:17:44, Andrea Arcangeli wrote:
> > > 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.
> > 
> > I would prefer we do not do that because then the real owner of the mm
> > cannot really tear down the address space and the life time of it is
> > bound to a kernel thread doing the use_mm. This is undesirable I would
> > really prefer if the existing few users would use mmget only when they
> > really need to access mm.
> 
> If the existing few users that don't already do the explicit mmget
> will have to start doing it too, the end result will be exactly the
> same that you described in your "cons" (lieftime of the mm will still
> be up to who did mmget;use_mm and didn't call unuse_mm;mmput yet).

Well, they should use mmget only for moments when they access the
address space.

> One reason to prefer adding the mmget to the callers to forget it,
> would be to avoid an atomic op in use_mm (for those callers that
> didn't forget it), but if anybody is doing use_mm in a fast path that
> won't be very fast anyway so I didn't think this was worth the
> risk. If that microoptimization in a slow path is the reason we should
> add mmget to the callers that forgot it that would be fine with me
> although I think it's risky because if already happened once and it
> could happen again (and when it happens it only bits a few arches if
> used with a few drivers).

The primary concern really is that use_mm is usually not bound in time
and we do not want to pin the address space for such a long time without
any binding to a killable process.
-- 
Michal Hocko
SUSE Labs


  reply	other threads:[~2020-02-19 11:59 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 ` [PATCH 1/2] mm: use_mm: fix for arches checking mm_users to optimize " Andrea Arcangeli
2020-02-18 11:31   ` Michal Hocko
2020-02-18 18:56     ` Andrea Arcangeli
2020-02-19 11:58       ` Michal Hocko [this message]
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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