linux-kernel - Re: [RFC PATCH 2/3] mm/memory_hotplug: Create __shrink_pages and move it to offline

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Date:   Tue, 7 Aug 2018 11:19:57 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     osalvador@...hadventures.net, akpm@...ux-foundation.org,
        mhocko@...e.com, dan.j.williams@...el.com,
        pasha.tatashin@...cle.com, yasu.isimatu@...il.com,
        logang@...tatee.com, dave.jiang@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Oscar Salvador <osalvador@...e.de>
Subject: Re: [RFC PATCH 2/3] mm/memory_hotplug: Create __shrink_pages and
 move it to offline_pages

On Tue, Aug 07, 2018 at 04:54:57PM +0200, David Hildenbrand wrote:
> On 07.08.2018 15:52, Jerome Glisse wrote:
> > On Tue, Aug 07, 2018 at 03:37:56PM +0200, osalvador@...hadventures.net wrote:
> >> From: Oscar Salvador <osalvador@...e.de>
> > 
> > [...]
> > 
> >> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> >> index 9bd629944c91..e33555651e46 100644
> >> --- a/mm/memory_hotplug.c
> >> +++ b/mm/memory_hotplug.c
> > 
> > [...]
> > 
> >>  /**
> >>   * __remove_pages() - remove sections of pages from a zone
> >> - * @zone: zone from which pages need to be removed
> >> + * @nid: node which pages belong to
> >>   * @phys_start_pfn: starting pageframe (must be aligned to start of a section)
> >>   * @nr_pages: number of pages to remove (must be multiple of section size)
> >>   * @altmap: alternative device page map or %NULL if default memmap is used
> >> @@ -548,7 +557,7 @@ static int __remove_section(struct zone *zone, struct mem_section *ms,
> >>   * sure that pages are marked reserved and zones are adjust properly by
> >>   * calling offline_pages().
> >>   */
> >> -int __remove_pages(struct zone *zone, unsigned long phys_start_pfn,
> >> +int __remove_pages(int nid, unsigned long phys_start_pfn,
> >>  		 unsigned long nr_pages, struct vmem_altmap *altmap)
> >>  {
> >>  	unsigned long i;
> >> @@ -556,10 +565,9 @@ int __remove_pages(struct zone *zone, unsigned long phys_start_pfn,
> >>  	int sections_to_remove, ret = 0;
> >>  
> >>  	/* In the ZONE_DEVICE case device driver owns the memory region */
> >> -	if (is_dev_zone(zone)) {
> >> -		if (altmap)
> >> -			map_offset = vmem_altmap_offset(altmap);
> >> -	} else {
> >> +	if (altmap)
> >> +		map_offset = vmem_altmap_offset(altmap);
> >> +	else {
> > 
> > This will break ZONE_DEVICE at least for HMM. While i think that
> > altmap -> ZONE_DEVICE (ie altmap imply ZONE_DEVICE) the reverse
> > is not true ie ZONE_DEVICE does not necessarily imply altmap. So
> > with the above changes you change the expected behavior. You do
> > need the zone to know if it is a ZONE_DEVICE. You could also lookup
> > one of the struct page but my understanding is that this is what
> > you want to avoid in the first place.
> 
> I wonder if we could instead forward from the callers whether we are
> dealing with ZONE_DEVICE memory (is_device ...), at least that seems
> feasible in hmm code. Not having looked at details yet.
> 

Yes i believe this is doable, this add one more argument, to me it
looked like passing down the zone was good idea, i think with the
struct zone you can even remove the altmap argument.

Is there a reason why you do not want to pass down the struct zone ?

Cheers,
Jérôme