linux-kernel - Re: dm ioctl: Restore __GFP_HIGH in copy

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Message-ID: <20170523060534.GA12813@dhcp22.suse.cz>
Date:   Tue, 23 May 2017 08:05:35 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Mike Snitzer <snitzer@...hat.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Junaid Shahid <junaids@...gle.com>,
        Alasdair Kergon <agk@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        andreslc@...gle.com, gthelen@...gle.com, vbabka@...e.cz,
        linux-kernel@...r.kernel.org
Subject: Re: dm ioctl: Restore __GFP_HIGH in copy_params()

On Mon 22-05-17 13:35:41, David Rientjes wrote:
> On Mon, 22 May 2017, Mike Snitzer wrote:
[...]
> > While adding the __GFP_NOFAIL flag would serve to document expectations
> > I'm left unconvinced that the memory allocator will _not fail_ for an
> > order-0 page -- as Mikulas said most ioctls don't need more than 4K.
> 
> __GFP_NOFAIL would make no sense in kvmalloc() calls, ever, it would never 
> fallback to vmalloc :)

Sorry, I could have been more specific. You would have to opencode
kvmalloc obviously. It is documented to not support this flag for the
reasons you have mentioned above.

> I'm hoping this can get merged during the 4.12 window to fix the broken 
> commit d224e9381897.

I obviously disagree. Relying on memory reserves for _correctness_ is
clearly broken by design, full stop. But it is dm code and you are going
it is responsibility of the respective maintainers to support this code.
-- 
Michal Hocko
SUSE Labs