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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 16 Oct 2019 20:45:33 -0700
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Subject: Re: [PATCH 0/4] [RFC] Migrate Pages in lieu of discard
To:     Dave Hansen <dave.hansen@linux.intel.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Jonathan Adams <jwadams@google.com>
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On Wed, Oct 16, 2019 at 3:49 PM Dave Hansen <dave.hansen@linux.intel.com> wrote:
>
> We're starting to see systems with more and more kinds of memory such
> as Intel's implementation of persistent memory.
>
> Let's say you have a system with some DRAM and some persistent memory.
> Today, once DRAM fills up, reclaim will start and some of the DRAM
> contents will be thrown out.  Allocations will, at some point, start
> falling over to the slower persistent memory.
>
> That has two nasty properties.  First, the newer allocations can end
> up in the slower persistent memory.  Second, reclaimed data in DRAM
> are just discarded even if there are gobs of space in persistent
> memory that could be used.
>
> This set implements a solution to these problems.  At the end of the
> reclaim process in shrink_page_list() just before the last page
> refcount is dropped, the page is migrated to persistent memory instead
> of being dropped.
>
> While I've talked about a DRAM/PMEM pairing, this approach would
> function in any environment where memory tiers exist.
>
> This is not perfect.  It "strands" pages in slower memory and never
> brings them back to fast DRAM.  Other things need to be built to
> promote hot pages back to DRAM.
>
> This is part of a larger patch set.  If you want to apply these or
> play with them, I'd suggest using the tree from here.  It includes
> autonuma-based hot page promotion back to DRAM:
>
>         http://lkml.kernel.org/r/c3d6de4d-f7c3-b505-2e64-8ee5f70b2118@intel.com
>
> This is also all based on an upstream mechanism that allows
> persistent memory to be onlined and used as if it were volatile:
>
>         http://lkml.kernel.org/r/20190124231441.37A4A305@viggo.jf.intel.com

The memory cgroup part of the story is missing here. Since PMEM is
treated as slow DRAM, shouldn't its usage be accounted to the
corresponding memcg's memory/memsw counters and the migration should
not happen for memcg limit reclaim? Otherwise some jobs can hog the
whole PMEM.

Also what happens when PMEM is full? Can the memory migrated to PMEM
be reclaimed (or discarded)?

Shakeel