linux-kernel - Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap

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Date:   Wed, 10 Oct 2018 19:24:51 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Pasha Tatashin <pavel.tatashin@...rosoft.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        rppt@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        yi.z.zhang@...ux.intel.com
Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the
 point where we init pgmap

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?

> 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.

-- 
Michal Hocko
SUSE Labs