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Subject: Re: [PATCH v5 4/4] mm: Defer ZONE_DEVICE page initialization to the
 point where we init pgmap
To:     Dan Williams <dan.j.williams@intel.com>,
        Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-nvdimm <linux-nvdimm@lists.01.org>,
        Pasha Tatashin <pavel.tatashin@microsoft.com>,
        Michal Hocko <mhocko@suse.com>,
        Dave Hansen <dave.hansen@intel.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        rppt@linux.vnet.ibm.com, Ingo Molnar <mingo@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
References: <20180925200551.3576.18755.stgit@localhost.localdomain>
 <20180925202053.3576.66039.stgit@localhost.localdomain>
 <20181009170051.GA40606@tiger-server>
 <CAPcyv4g99_rJJSn0kWv5YO0Mzj90q1LH1wC3XrjCh1=x6mo7BQ@mail.gmail.com>
From:   Alexander Duyck <alexander.h.duyck@linux.intel.com>
Message-ID: <25092df0-b7b4-d456-8409-9c004cb6e422@linux.intel.com>
Date:   Tue, 9 Oct 2018 13:26:41 -0700
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On 10/9/2018 11:04 AM, Dan Williams wrote:
> On Tue, Oct 9, 2018 at 3:21 AM Yi Zhang <yi.z.zhang@linux.intel.com> 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@microsoft.com>
>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
>>> ---
>>>
>>> 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}.
> 

When pages are "onlined" via __free_pages_boot_core they clear the 
reserved bit, that is the reason for the comment. The reserved bit is 
meant to indicate that the page cannot be swapped out or moved based on 
the description of the bit.

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.

Thanks.

- Alex