linux-kernel - Re: [RFC PATCH v1 0/6] use mm to manage NVDIMM (pmem) zone

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Message-ID: <20180510084119.GJ32366@dhcp22.suse.cz>
Date:   Thu, 10 May 2018 10:41:19 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Huaisheng Ye <yehs1@...ovo.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org, willy@...radead.org,
        vbabka@...e.cz, mgorman@...hsingularity.net,
        pasha.tatashin@...cle.com, alexander.levin@...izon.com,
        hannes@...xchg.org, penguin-kernel@...ove.SAKURA.ne.jp,
        colyli@...e.de, chengnt@...ovo.com, hehy1@...ovo.com,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org
Subject: Re: [RFC PATCH v1 0/6] use mm to manage NVDIMM (pmem) zone

I have only now noticed that you have posted this few days ago
http://lkml.kernel.org/r/1525704627-30114-1-git-send-email-yehs1@lenovo.com
There were some good questions asked there and I have many that are
common yet they are not covered in the cover letter. Please _always_
make sure to answer review comments before reposting. Otherwise some
important parts gets lost on the way.

On Thu 10-05-18 09:57:59, Michal Hocko wrote:
> On Tue 08-05-18 10:30:22, Huaisheng Ye wrote:
> > Traditionally, NVDIMMs are treated by mm(memory management) subsystem as
> > DEVICE zone, which is a virtual zone and both its start and end of pfn
> > are equal to 0, mm wouldn’t manage NVDIMM directly as DRAM, kernel uses
> > corresponding drivers, which locate at \drivers\nvdimm\ and
> > \drivers\acpi\nfit and fs, to realize NVDIMM memory alloc and free with
> > memory hot plug implementation.
> > 
> > With current kernel, many mm’s classical features like the buddy
> > system, swap mechanism and page cache couldn’t be supported to NVDIMM.
> > What we are doing is to expand kernel mm’s capacity to make it to handle
> > NVDIMM like DRAM. Furthermore we make mm could treat DRAM and NVDIMM
> > separately, that means mm can only put the critical pages to NVDIMM
> > zone, here we created a new zone type as NVM zone.
> 
> How do you define critical pages? Who is allowed to allocate from them?
> You do not seem to add _any_ user of GFP_NVM.
> 
> > That is to say for
> > traditional(or normal) pages which would be stored at DRAM scope like
> > Normal, DMA32 and DMA zones. But for the critical pages, which we hope
> > them could be recovered from power fail or system crash, we make them
> > to be persistent by storing them to NVM zone.
> 
> This brings more questions than it answers. First of all is this going
> to be any guarantee? Let's say I want GFP_NVM, can I get memory from
> other zones? In other words is such a request allowed to fallback to
> succeed? Are we allowed to reclaim memory from the new zone? What should
> happen on the OOM? How is the user expected to restore the previous
> content after reboot/crash?
> 
> I am sorry if these questions are answered in the respective patches but
> it would be great to have this in the cover letter to have a good
> overview of the whole design. From my quick glance over patches my
> previous concerns about an additional zone still hold, though.
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs