linux-kernel - Re: [PATCH 0/4] [RFC] Migrate Pages in lieu of discard

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Message-ID: <197ba38f-0443-b3a7-7ce4-544bf97c58dd@intel.com>
Date:   Fri, 18 Oct 2019 08:10:34 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Suleiman Souhlal <suleiman@...gle.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, dan.j.williams@...el.com,
        Shakeel Butt <shakeelb@...gle.com>,
        Jonathan Adams <jwadams@...gle.com>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 0/4] [RFC] Migrate Pages in lieu of discard

On 10/18/19 1:11 AM, Suleiman Souhlal wrote:
> Another issue we ran into, that I think might also apply to this patch
> series, is that because kernel memory can't be allocated on persistent
> memory, it's possible for all of DRAM to get filled by user memory and
> have kernel allocations fail even though there is still a lot of free
> persistent memory. This is easy to trigger, just start an application
> that is bigger than DRAM.

Why doesn't this happen on everyone's laptops where DRAM is contended
between userspace and kernel allocations?  Does the OOM killer trigger
fast enough to save us?

> To mitigate that, we introduced a new watermark for DRAM zones above
> which user memory can't be allocated, to leave some space for kernel
> allocations.

I'd be curious why the existing users of ZONE_MOVABLE don't have to do
this?  Are there just no users of ZONE_MOVABLE?