Re: [PATCH v4 3/5] mm: Defer ZONE_DEVICE page initialization to the point where we init pgmap

[Alexander Duyck]

On 9/21/2018 12:50 PM, Pasha Tatashin wrote:
> On 9/20/18 6:29 PM, Alexander Duyck wrote:
> > The ZONE_DEVICE pages were being initialized in two locations. One was with
> > the memory_hotplug lock held and another was outside of that lock. The
> > problem with this is that it was nearly doubling the memory initialization
> > time. Instead of doing this twice, once while holding a global lock and
> > once without, I am opting to defer the initialization to the one outside of
> > the lock. This allows us to avoid serializing the overhead for memory init
> > and we can instead focus on per-node init times.
> >
> > One issue I encountered is that devm_memremap_pages and
> > hmm_devmmem_pages_create were initializing only the pgmap field the same
> > way. One wasn't initializing hmm_data, and the other was initializing it to
> > a poison value. Since this is something that is exposed to the driver in
> > the case of hmm I am opting for a third option and just initializing
> > hmm_data to 0 since this is going to be exposed to unknown third party
> > drivers.
> >
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
>
> > +void __ref memmap_init_zone_device(struct zone *zone,
> > +				   unsigned long start_pfn,
> > +				   unsigned long size,
> > +				   struct dev_pagemap *pgmap)
> > +{
> > +	unsigned long pfn, end_pfn = start_pfn + size;
> > +	struct pglist_data *pgdat = zone->zone_pgdat;
> > +	unsigned long zone_idx = zone_idx(zone);
> > +	unsigned long start = jiffies;
> > +	int nid = pgdat->node_id;
> > +
> > +	if (WARN_ON_ONCE(!pgmap || !is_dev_zone(zone)))
> > +		return;
> > +
> > +	/*
> > +	 * The call to memmap_init_zone should have already taken care
> > +	 * of the pages reserved for the memmap, so we can just jump to
> > +	 * the end of that region and start processing the device pages.
> > +	 */
> > +	if (pgmap->altmap_valid) {
> > +		struct vmem_altmap *altmap = &pgmap->altmap;
> > +
> > +		start_pfn = altmap->base_pfn + vmem_altmap_offset(altmap);
> > +		size = end_pfn - start_pfn;
> > +	}
> > +
> > +	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> > +		struct page *page = pfn_to_page(pfn);
> > +
> > +		__init_single_page(page, pfn, zone_idx, nid);
> > +
> > +		/*
> > +		 * Mark page reserved as it will need to wait for onlining
> > +		 * phase for it to be fully associated with a zone.
> > +		 *
> > +		 * We can use the non-atomic __set_bit operation for setting
> > +		 * the flag as we are still initializing the pages.
> > +		 */
> > +		__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;
>
> __init_single_page()
>    mm_zero_struct_page()
>
> Takes care of zeroing, no need to do another store here.

The problem is __init_singe_page also calls INIT_LIST_HEAD which I 
believe sets the prev pointer which overlaps with hmm_data.

> Looks good otherwise.
>
> Reviewed-by: Pavel Tatashin <pavel.tatashin@xxxxxxxxxxxxx>

Thanks for the review.