linux-kernel - Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm

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Message-ID: <20180327144320.GI5652@dhcp22.suse.cz>
Date:   Tue, 27 Mar 2018 16:43:20 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mateusz Guzik <mguzik@...hat.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>, adobriyan@...il.com,
        willy@...radead.org, gorcunov@...nvz.org,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and
 env_start|end in mm_struct

On Tue 27-03-18 16:31:23, Mateusz Guzik wrote:
> On Tue, Mar 27, 2018 at 08:29:39AM +0200, Michal Hocko wrote:
> > On Tue 27-03-18 02:20:39, Yang Shi wrote:
> > [...]
> > The patch looks reasonable to me. Maybe it would be better to be more
> > explicit about the purpose of the patch. As others noticed, this alone
> > wouldn't solve the mmap_sem contention issues. I _think_ that if you
> > were more explicit about the mmap_sem abuse it would trigger less
> > questions.
> > 
> 
> >From what I gather even with other fixes the kernel will still end up
> grabbing the semaphore. In this case I don't see what's the upside of
> adding the spinlock for args. The downside is growth of mm_struct.

Because accessing the specific address in the address space can be later
changed to use a more fine-grained locking. There are people
experimenting with range locking. These mmap_sem abusers, on the other
hand, will require the full range lock without a good reason. So it is
really worth it to remove them and replace by a more fine grained
locking.

If the mm_struct grow is a real concern (I haven't checked that) then we
can use a set of hashed locks or something else.
-- 
Michal Hocko
SUSE Labs