Date: Tue, 4 Apr 2017 11:36:53 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>,
        penguin-kernel@I-love.SAKURA.ne.jp, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, hpa@zytor.com, chris@chris-wilson.co.uk,
        hch@lst.de, mingo@elte.hu, jszhang@marvell.com, joelaf@google.com,
        joaodias@google.com, willy@infradead.org, tglx@linutronix.de,
        thellstrom@vmware.com, stable@vger.kernel.org
Subject: Re: [PATCH 1/4] mm/vmalloc: allow to call vfree() in atomic context
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 <20170330152229.f2108e718114ed77acae7405@linux-foundation.org>
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On Thu 30-03-17 15:22:29, Andrew Morton wrote:
> On Thu, 30 Mar 2017 13:27:16 +0300 Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
[...]
> > This can be fixed in vmgfx, but it would be better to make vfree()
> > non-sleeping again because we may have other bugs like this one.
> 
> I tend to disagree: adding yet another schedule_work() introduces
> additional overhead and adds some risk of ENOMEM errors which wouldn't
> occur with a synchronous free.

I do not think ENOMEM would be a problem. We are talking about lazy
handling already. Besides that the allocation path also does this lazy
free AFAICS.

> > __purge_vmap_area_lazy() is the only function in the vfree() path that
> > wants to be able to sleep. So it make sense to schedule
> > __purge_vmap_area_lazy() via schedule_work() so it runs only in sleepable
> > context.
> 
> vfree() already does
> 
> 	if (unlikely(in_interrupt()))
> 		__vfree_deferred(addr);
> 
> so it seems silly to introduce another defer-to-kernel-thread thing
> when we already have one.

But this only cares about the IRQ context and this patch aims at atomic
context in general. I agree it would have been better to reduce this
deferred behavior to only _atomic_ context but we not have a reliable
way to detect that on non-preemptive kernels AFAIR.
-- 
Michal Hocko
SUSE Labs