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

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Date:   Tue, 23 May 2017 12:44:18 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     David Rientjes <rientjes@...gle.com>,
        Mike Snitzer <snitzer@...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 Tue, 23 May 2017, Michal Hocko wrote:

> 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.

Block loop device is broken in the same way - it converts block requests 
to filesystem reads and writes and those FS reads and writes allocate 
memory.

Network block device needs an userspace daemon to perform I/O.

iSCSI also needs to allocate memory to perform I/O.

NFS and other networking filesystems are also broken in the same way (they 
need to receive a packet to acknowledge a write and packet reception needs 
to allocate memory).

So - what should these *broken* drivers do to reduce the possibility of 
the deadlock?

Mikulas

> -- 
> Michal Hocko
> SUSE Labs
>