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

[Dan Williams]

On Tue, Oct 9, 2018 at 3:21 AM Yi Zhang <yi.z.zhang@xxxxxxxxxxxxxxx> wrote:
>
> On 2018-09-25 at 13:21:24 -0700, 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.
> >
> > Reviewed-by: Pavel Tatashin <pavel.tatashin@xxxxxxxxxxxxx>
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
> > ---
> >
> > v4: Moved moved memmap_init_zone_device to below memmmap_init_zone to avoid
> >     merge conflicts with other changes in the kernel.
> > v5: No change
> >
> >  include/linux/mm.h |    2 +
> >  kernel/memremap.c  |   24 +++++---------
> >  mm/hmm.c           |   12 ++++---
> >  mm/page_alloc.c    |   92 ++++++++++++++++++++++++++++++++++++++++++++++++++--
> >  4 files changed, 107 insertions(+), 23 deletions(-)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 06d7d7576f8d..7312fb78ef31 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -848,6 +848,8 @@ static inline bool is_zone_device_page(const struct page *page)
> >  {
> >       return page_zonenum(page) == ZONE_DEVICE;
> >  }
> > +extern void memmap_init_zone_device(struct zone *, unsigned long,
> > +                                 unsigned long, struct dev_pagemap *);
> >  #else
> >  static inline bool is_zone_device_page(const struct page *page)
> >  {
> > diff --git a/kernel/memremap.c b/kernel/memremap.c
> > index 5b8600d39931..d0c32e473f82 100644
> > --- a/kernel/memremap.c
> > +++ b/kernel/memremap.c
> > @@ -175,10 +175,10 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap)
> >       struct vmem_altmap *altmap = pgmap->altmap_valid ?
> >                       &pgmap->altmap : NULL;
> >       struct resource *res = &pgmap->res;
> > -     unsigned long pfn, pgoff, order;
> > +     struct dev_pagemap *conflict_pgmap;
> >       pgprot_t pgprot = PAGE_KERNEL;
> > +     unsigned long pgoff, order;
> >       int error, nid, is_ram;
> > -     struct dev_pagemap *conflict_pgmap;
> >
> >       align_start = res->start & ~(SECTION_SIZE - 1);
> >       align_size = ALIGN(res->start + resource_size(res), SECTION_SIZE)
> > @@ -256,19 +256,13 @@ void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap)
> >       if (error)
> >               goto err_add_memory;
> >
> > -     for_each_device_pfn(pfn, pgmap) {
> > -             struct page *page = pfn_to_page(pfn);
> > -
> > -             /*
> > -              * ZONE_DEVICE pages union ->lru with a ->pgmap back
> > -              * pointer.  It is a bug if a ZONE_DEVICE page is ever
> > -              * freed or placed on a driver-private list.  Seed the
> > -              * storage with LIST_POISON* values.
> > -              */
> > -             list_del(&page->lru);
> > -             page->pgmap = pgmap;
> > -             percpu_ref_get(pgmap->ref);
> > -     }
> > +     /*
> > +      * Initialization of the pages has been deferred until now in order
> > +      * to allow us to do the work while not holding the hotplug lock.
> > +      */
> > +     memmap_init_zone_device(&NODE_DATA(nid)->node_zones[ZONE_DEVICE],
> > +                             align_start >> PAGE_SHIFT,
> > +                             align_size >> PAGE_SHIFT, pgmap);
> >
> >       devm_add_action(dev, devm_memremap_pages_release, pgmap);
> >
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index c968e49f7a0c..774d684fa2b4 100644
> > --- a/mm/hmm.c
> > +++ b/mm/hmm.c
> > @@ -1024,7 +1024,6 @@ static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
> >       resource_size_t key, align_start, align_size, align_end;
> >       struct device *device = devmem->device;
> >       int ret, nid, is_ram;
> > -     unsigned long pfn;
> >
> >       align_start = devmem->resource->start & ~(PA_SECTION_SIZE - 1);
> >       align_size = ALIGN(devmem->resource->start +
> > @@ -1109,11 +1108,14 @@ static int hmm_devmem_pages_create(struct hmm_devmem *devmem)
> >                               align_size >> PAGE_SHIFT, NULL);
> >       mem_hotplug_done();
> >
> > -     for (pfn = devmem->pfn_first; pfn < devmem->pfn_last; pfn++) {
> > -             struct page *page = pfn_to_page(pfn);
> > +     /*
> > +      * Initialization of the pages has been deferred until now in order
> > +      * to allow us to do the work while not holding the hotplug lock.
> > +      */
> > +     memmap_init_zone_device(&NODE_DATA(nid)->node_zones[ZONE_DEVICE],
> > +                             align_start >> PAGE_SHIFT,
> > +                             align_size >> PAGE_SHIFT, &devmem->pagemap);
> >
> > -             page->pgmap = &devmem->pagemap;
> > -     }
> >       return 0;
> >
> >  error_add_memory:
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 926ad3083b28..7ec0997ded39 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5489,12 +5489,23 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
> >       if (highest_memmap_pfn < end_pfn - 1)
> >               highest_memmap_pfn = end_pfn - 1;
> >
> > +#ifdef CONFIG_ZONE_DEVICE
> >       /*
> >        * Honor reservation requested by the driver for this ZONE_DEVICE
> > -      * memory
> > +      * memory. We limit the total number of pages to initialize to just
> > +      * those that might contain the memory mapping. We will defer the
> > +      * ZONE_DEVICE page initialization until after we have released
> > +      * the hotplug lock.
> >        */
> > -     if (altmap && start_pfn == altmap->base_pfn)
> > -             start_pfn += altmap->reserve;
> > +     if (zone == ZONE_DEVICE) {
> > +             if (!altmap)
> > +                     return;
> > +
> > +             if (start_pfn == altmap->base_pfn)
> > +                     start_pfn += altmap->reserve;
> > +             end_pfn = altmap->base_pfn + vmem_altmap_offset(altmap);
> > +     }
> > +#endif
> >
> >       for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> >               /*
> > @@ -5538,6 +5549,81 @@ void __meminit memmap_init_zone(unsigned long size, int nid, unsigned long zone,
> >       }
> >  }
> >
> > +#ifdef CONFIG_ZONE_DEVICE
> > +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);
>
> So we need to hold the page reserved flag while memory onlining.
> But after onlined, Do we neeed to clear the reserved flag in the DEV/FS
> DAX memory type?
> @Dan, What will going on with this?
>

That comment is incorrect, device-pages are never onlined. So I think
we can just skip that call to __SetPageReserved() unless the memory
range is MEMORY_DEVICE_{PRIVATE,PUBLIC}.