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    SubjectRe: [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

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