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

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Message-ID: <2254cfe1-5cd3-eedc-1f24-8e011dcf3575@microsoft.com>
Date:   Fri, 21 Sep 2018 19:50:18 +0000
From:   Pasha Tatashin <Pavel.Tatashin@...rosoft.com>
To:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
CC:     "mhocko@...e.com" <mhocko@...e.com>,
        "dave.jiang@...el.com" <dave.jiang@...el.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "jglisse@...hat.com" <jglisse@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "logang@...tatee.com" <logang@...tatee.com>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v4 3/5] mm: Defer ZONE_DEVICE page initialization to the
 point where we init pgmap



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@...ux.intel.com>

> +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.


Looks good otherwise.

Reviewed-by: Pavel Tatashin <pavel.tatashin@...rosoft.com>