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

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Date:   Mon, 22 May 2017 11:37:25 +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 Fri 19-05-17 19:43:23, Mikulas Patocka wrote:
> 
> 
> On Fri, 19 May 2017, Michal Hocko wrote:
> 
> > On Thu 18-05-17 19:50:46, Junaid Shahid wrote:
> > > (Adding back the correct linux-mm email address and also adding linux-kernel.)
> > > 
> > > On Thursday, May 18, 2017 01:41:33 PM David Rientjes wrote:
> > [...]
> > > > Let's ask Mikulas, who changed this from PF_MEMALLOC to __GFP_HIGH, 
> > > > assuming there was a reason to do it in the first place in two different 
> > > > ways.
> > 
> > Hmm, the old PF_MEMALLOC used to have the following comment
> >         /*
> >          * Trying to avoid low memory issues when a device is
> >          * suspended. 
> >          */
> > 
> > I am not really sure what that means but __GFP_HIGH certainly have a
> > different semantic than PF_MEMALLOC. The later grants the full access to
> > the memory reserves while the prior on partial access. If this is _really_
> > needed then it deserves a comment explaining why.
> > -- 
> > Michal Hocko
> > SUSE Labs
> 
> Sometimes, I/O to a device mapper device is blocked until the userspace 
> daemon dmeventd does some action (for example, when dm-mirror leg fails, 
> dmeventd needs to mark the leg as failed in the lvm metadata and then 
> reload the device).
> 
> The dmeventd daemon mlocks itself in memory so that it doesn't generate 
> any I/O. But it must be able to call ioctls. __GFP_HIGH is there so that 
> the ioctls issued by dmeventd have higher chance of succeeding if some I/O 
> is blocked, waiting for dmeventd action. It reduces the possibility of 
> low-memory-deadlock, though it doesn't eliminate it entirely.

So what happens if the memory reserves are depleted. Do we deadlock? Why
is OOM killer insufficient to allow the further progress?
-- 
Michal Hocko
SUSE Labs