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

[Yi Zhang]

On 2018-10-10 at 11:18:49 -0700, Dan Williams wrote:
> On Wed, Oct 10, 2018 at 10:30 AM Michal Hocko <mhocko@xxxxxxxxxx> 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?
> >
> > > 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.
> >
> 
> Right, so we're in a situation where a hack is needed for KVM's
> current interpretation of the Reserved flag relative to dax mapped
> pages. I'm arguing to push that knowledge / handling as deep as
> possible into the core rather than hack the leaf implementations like
> KVM, i.e. disable the Reserved flag for all non-MEMORY_DEVICE_*
> ZONE_DEVICE types.
> 
> Here is the KVM thread about why they need a change:
> 
>     https://lkml.org/lkml/2018/9/7/552
> 
> ...and where I pushed back on a KVM-local hack:
> 
>     https://lkml.org/lkml/2018/9/19/154
Yeah, Thank Dan, I think I can going on with something like this:

@@ -5589,6 +5589,7 @@ void __ref memmap_init_zone_device(struct zone *zone,
 		struct page *page = pfn_to_page(pfn);
 
 		__init_single_page(page, pfn, zone_idx, nid);
+		/* Could we move this a little bit earlier as I can
+		 * direct use is_dax_page(page), or something else?
+		 */
+		page->pgmap = pgmap;
 
 		/*
 		 * Mark page reserved as it will need to wait for onlining
@@ -5597,14 +5598,14 @@ void __ref memmap_init_zone_device(struct zone *zone,
 		 * We can use the non-atomic __set_bit operation for setting
 		 * the flag as we are still initializing the pages.
 		 */
-		__SetPageReserved(page);
+		 if(!is_dax_page(page))
+			__SetPageReserved(page);
 
 		/*
 		 * ZONE_DEVICE pages union ->lru with a ->pgmap back
 		 * pointer and hmm_data.  It is a bug if a ZONE_DEVICE
 		 * page is ever freed or placed on a driver-private list.
 		 */
-		page->pgmap = pgmap;
 		page->hmm_data = 0;


After Alex's patch merged.