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

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Date:   Tue, 22 Oct 2019 15:49:50 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        dan.j.williams@...el.com
Subject: Re: [PATCH 0/4] [RFC] Migrate Pages in lieu of discard

On Fri 18-10-19 07:54:20, Dave Hansen wrote:
> On 10/18/19 12:44 AM, Michal Hocko wrote:
> > How does this compare to
> > http://lkml.kernel.org/r/1560468577-101178-1-git-send-email-yang.shi@linux.alibaba.com
> 
> It's a _bit_ more tied to persistent memory and it appears a bit more
> tied to two tiers rather something arbitrarily deep.  They're pretty
> similar conceptually although there are quite a few differences.
> 
> For instance, what I posted has a static mapping for the migration path.
>  If node A is in reclaim, we always try to allocate pages on node B.
> There are no restrictions on what those nodes can be.  In Yang Shi's
> apporach, there's a dynamic search for a target migration node on each
> migration that follows the normal alloc fallback path.  This ends up
> making migration nodes special.

As we have discussed at LSFMM this year and there seemed to be a goog
consensus on that, the resulting implementation should be as pmem
neutral as possible. After all node migration mode sounds like a
reasonable feature even without pmem. So I would be more inclined to the
normal alloc fallback path rather than a very specific and static
migration fallback path. If that turns out impractical then sure let's
come up with something more specific but I think there is quite a long
route there because we do not really have much of an experience with
this so far.

> There are also some different choices that are pretty arbitrary.  For
> instance, when you allocation a migration target page, should you cause
> memory pressure on the target?

Those are details to really sort out and they require some
experimentation to.

> To be honest, though, I don't see anything fatally flawed with it.  It's
> probably a useful exercise to factor out the common bits from the two
> sets and see what we can agree on being absolutely necessary.

Makes sense. What would that be? Is there a real consensus on having the
new node_reclaim mode to be the configuration mechanism? Do we want to
support generic NUMA without any PMEM in place as well for starter?

Thanks!
-- 
Michal Hocko
SUSE Labs