linux-kernel - Re: [PATCH] dm ioctl: Restore __GFP_HIGH in copy

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Message-ID: <20170522150321.GM8509@dhcp22.suse.cz>
Date:   Mon, 22 May 2017 17:03:22 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Mikulas Patocka <mpatocka@...hat.com>
Cc:     Junaid Shahid <junaids@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...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: [PATCH] dm ioctl: Restore __GFP_HIGH in copy_params()

On Mon 22-05-17 10:52:44, Mikulas Patocka wrote:
> 
> 
> On Mon, 22 May 2017, Michal Hocko wrote:
[...] 
> > I am not sure I understand. OOM killer is invoked for _all_ allocations
> > <= PAGE_ALLOC_COSTLY_ORDER that do not have __GFP_NORETRY as long as the
> > OOM killer is not disabled (oom_killer_disable) and that only happens
> > from the PM suspend path which makes sure that no userspace is active at
> > the time. AFAIU this is a userspace triggered path and so the later
> > shouldn't apply to it and GFP_KERNEL should be therefore sufficient.
> > Relying to a portion of memory reserves to prevent from deadlock seems
> > fundamentaly broken  to me.
> > 
> 
> The lvm2 was designed this way - it is broken, but there is not much that 
> can be done about it - fixing this would mean major rewrite. The only 
> thing we can do about it is to lower the deadlock probability with 
> __GFP_HIGH (or PF_MEMALLOC that was used some times ago).

But let me repeat. GFP_KERNEL allocation for order-0 page will not fail.
If you need non-failing semantic then just make it clear by adding
__GFP_NOFAIL rather than __GFP_HIGH. Memory reserves are a scarce
resource and there are users which might really need it from atomic
contexts.

Anyway, this is not the code I am maintaining so I will not argue more
and won't nack the patch. But is smells like a pure cargo cult, to be
honest.

If you really insist, though, I would just ask to have a more detailed
explanation why it is _believed_ the flag is needed because the vague
"Use __GFP_HIGH to avoid low memory issues when a device is suspended
and the ioctl is needed to resume it." doesn't really clarify much to be
honest.

Thanks!
-- 
Michal Hocko
SUSE Labs