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    SubjectRe: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap
    On Wed 10-10-18 10:39:01, Alexander Duyck wrote:
    >
    >
    > On 10/10/2018 10:24 AM, Michal Hocko wrote:
    > > On Wed 10-10-18 09:39:08, Alexander Duyck wrote:
    > > > On 10/10/2018 2:58 AM, Michal Hocko wrote:
    > > > > On Tue 09-10-18 13:26:41, Alexander Duyck wrote:
    > > > > [...]
    > > > > > I would think with that being the case we still probably need the call to
    > > > > > __SetPageReserved to set the bit with the expectation that it will not be
    > > > > > cleared for device-pages since the pages are not onlined. Removing the call
    > > > > > to __SetPageReserved would probably introduce a number of regressions as
    > > > > > there are multiple spots that use the reserved bit to determine if a page
    > > > > > can be swapped out to disk, mapped as system memory, or migrated.
    > > > >
    > > > > PageReserved is meant to tell any potential pfn walkers that might get
    > > > > to this struct page to back off and not touch it. Even though
    > > > > ZONE_DEVICE doesn't online pages in traditional sense it makes those
    > > > > pages available for further use so the page reserved bit should be
    > > > > cleared.
    > > >
    > > > So from what I can tell that isn't necessarily the case. Specifically if the
    > > > pagemap type is MEMORY_DEVICE_PRIVATE or MEMORY_DEVICE_PUBLIC both are
    > > > special cases where the memory may not be accessible to the CPU or cannot be
    > > > pinned in order to allow for eviction.
    > >
    > > Could you give me an example please?
    >
    > Honestly I am getting a bit beyond my depth here so maybe Dan could explain
    > better. I am basing the above comment on Dan's earlier comment in this
    > thread combined with the comment that explains the "memory_type" field for
    > the pgmap:
    > https://elixir.bootlin.com/linux/v4.19-rc7/source/include/linux/memremap.h#L28
    >
    > > > The specific case that Dan and Yi are referring to is for the type
    > > > MEMORY_DEVICE_FS_DAX. For that type I could probably look at not setting the
    > > > reserved bit. Part of me wants to say that we should wait and clear the bit
    > > > later, but that would end up just adding time back to initialization. At
    > > > this point I would consider the change more of a follow-up optimization
    > > > rather than a fix though since this is tailoring things specifically for DAX
    > > > versus the other ZONE_DEVICE types.
    > >
    > > I thought I have already made it clear that these zone device hacks are
    > > not acceptable to the generic hotplug code. If the current reserve bit
    > > handling is not correct then give us a specific reason for that and we
    > > can start thinking about the proper fix.
    >
    > I might have misunderstood your earlier comment then. I thought you were
    > saying that we shouldn't bother with setting the reserved bit. Now it sounds
    > like you were thinking more along the lines of what I was here in my comment
    > where I thought the bit should be cleared later in some code specifically
    > related to DAX when it is exposing it for use to userspace or KVM.

    I was referring to my earlier comment that if you need to do something
    about struct page initialization (move_pfn_range_to_zone) outside of the
    lock (with the appropriate ground work that is needed) rather than
    pulling more zone device hacks into the generic hotplug code [1]

    [1] http://lkml.kernel.org/r/20180926075540.GD6278@dhcp22.suse.cz

    --
    Michal Hocko
    SUSE Labs

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