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Re: [HMM 06/16] mm/migrate: add new boolean copy flag to migratepage() callback

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On Thu, Mar 16, 2017 at 12:05:25PM -0400, J?r?me Glisse wrote:
> Allow migration without copy in case destination page already have
> source page content. This is usefull for new dma capable migration
> where use device dma engine to copy pages.
> 
> This feature need carefull audit of filesystem code to make sure
> that no one can write to the source page while it is unmapped and
> locked. It should be safe for most filesystem but as precaution
> return error until support for device migration is added to them.
> 
> Signed-off-by: Jérôme Glisse <jglisse@redhat.com>

I really dislike the amount of boilerplace code this creates and the fact
that additional headers are needed for that boilerplate. As it's only of
relevance to DMA capable migration, why not simply infer from that if it's
an option instead of updating all supporters of migration?

If that is unsuitable, create a new migreate_mode for a no-copy
migration. You'll need to alter some sites that check the migrate_mode
and it *may* be easier to convert migrate_mode to a bitmask but overall
it would be less boilerplate and confined to just the migration code.

> diff --git a/mm/migrate.c b/mm/migrate.c
> index 9a0897a..cb911ce 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -596,18 +596,10 @@ static void copy_huge_page(struct page *dst, struct page *src)
>  	}
>  }
>  
> -/*
> - * Copy the page to its new location
> - */
> -void migrate_page_copy(struct page *newpage, struct page *page)
> +static void migrate_page_states(struct page *newpage, struct page *page)
>  {
>  	int cpupid;
>  
> -	if (PageHuge(page) || PageTransHuge(page))
> -		copy_huge_page(newpage, page);
> -	else
> -		copy_highpage(newpage, page);
> -
>  	if (PageError(page))
>  		SetPageError(newpage);
>  	if (PageReferenced(page))

> @@ -661,6 +653,19 @@ void migrate_page_copy(struct page *newpage, struct page *page)
>  
>  	mem_cgroup_migrate(page, newpage);
>  }
> +
> +/*
> + * Copy the page to its new location
> + */
> +void migrate_page_copy(struct page *newpage, struct page *page)
> +{
> +	if (PageHuge(page) || PageTransHuge(page))
> +		copy_huge_page(newpage, page);
> +	else
> +		copy_highpage(newpage, page);
> +
> +	migrate_page_states(newpage, page);
> +}
>  EXPORT_SYMBOL(migrate_page_copy);
>  
>  /************************************************************
> @@ -674,8 +679,8 @@ EXPORT_SYMBOL(migrate_page_copy);
>   * Pages are locked upon entry and exit.
>   */
>  int migrate_page(struct address_space *mapping,
> -		struct page *newpage, struct page *page,
> -		enum migrate_mode mode)
> +		 struct page *newpage, struct page *page,
> +		 enum migrate_mode mode, bool copy)
>  {
>  	int rc;
>  
> @@ -686,7 +691,11 @@ int migrate_page(struct address_space *mapping,
>  	if (rc != MIGRATEPAGE_SUCCESS)
>  		return rc;
>  
> -	migrate_page_copy(newpage, page);
> +	if (copy)
> +		migrate_page_copy(newpage, page);
> +	else
> +		migrate_page_states(newpage, page);
> +
>  	return MIGRATEPAGE_SUCCESS;
>  }
>  EXPORT_SYMBOL(migrate_page);

Other than some reshuffling, this is the place where the new copy
parameters it used and it has the mode parameter. At worst you end up
creating a helper to check two potential migrate modes to have either
ASYNC, SYNC or SYNC_LIGHT semantics. I expect you want SYNC symantics.

This patch is huge relative to the small thing it acatually requires.

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[HMM 00/16] HMM (Heterogeneous Memory Management) v18 Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
  [HMM 14/16] mm/migrate: allow migrate_vma() to alloc new page on empty entry Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
  [HMM 10/16] mm/hmm/mirror: mirror process address space on device with HMM helpers Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
    Re: [HMM 10/16] mm/hmm/mirror: mirror process address space on  device with HMM helpers Mel Gorman <mgorman@techsingularity.net> - 2017-03-19 21:10 +0100
  [HMM 15/16] mm/hmm/devmem: device memory hotplug using ZONE_DEVICE Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
  [HMM 09/16] mm/hmm: heterogeneous memory management (HMM for short) Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
    Re: [HMM 09/16] mm/hmm: heterogeneous memory management (HMM for  short) Mel Gorman <mgorman@techsingularity.net> - 2017-03-19 21:10 +0100
  [HMM 11/16] mm/hmm/mirror: helper to snapshot CPU page table v2 Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
    Re: [HMM 11/16] mm/hmm/mirror: helper to snapshot CPU page table v2 Mel Gorman <mgorman@techsingularity.net> - 2017-03-19 21:20 +0100
  [HMM 03/16] mm/ZONE_DEVICE/free-page: callback when page is freed v3 Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
    Re: [HMM 03/16] mm/ZONE_DEVICE/free-page: callback when page is  freed v3 Mel Gorman <mgorman@techsingularity.net> - 2017-03-19 21:10 +0100
  [HMM 04/16] mm/ZONE_DEVICE/unaddressable: add support for un-addressable device memory v3 Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
    Re: [HMM 04/16] mm/ZONE_DEVICE/unaddressable: add support for  un-addressable device memory v3 Mel Gorman <mgorman@techsingularity.net> - 2017-03-19 21:10 +0100
  [HMM 16/16] mm/hmm/devmem: dummy HMM device for ZONE_DEVICE memory v2 Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
    Re: [HMM 16/16] mm/hmm/devmem: dummy HMM device for ZONE_DEVICE  memory v2 Bob Liu <liubo95@huawei.com> - 2017-03-17 08:10 +0100
      Re: [HMM 16/16] mm/hmm/devmem: dummy HMM device for ZONE_DEVICE  memory v2 Jerome Glisse <jglisse@redhat.com> - 2017-03-17 18:00 +0100
  [HMM 01/16] mm/memory/hotplug: convert device bool to int to allow for more flags v3 Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
    Re: [HMM 01/16] mm/memory/hotplug: convert device bool to int to  allow for more flags v3 Mel Gorman <mgorman@techsingularity.net> - 2017-03-19 21:10 +0100
  [HMM 08/16] mm/migrate: migrate_vma() unmap page from vma while collecting pages Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
  [HMM 13/16] mm/hmm/migrate: support un-addressable ZONE_DEVICE page in migration Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:20 +0100
  [HMM 05/16] mm/ZONE_DEVICE/x86: add support for un-addressable device memory Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:30 +0100
  [HMM 02/16] mm/put_page: move ref decrement to put_zone_device_page() Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:30 +0100
    Re: [HMM 02/16] mm/put_page: move ref decrement to  put_zone_device_page() Mel Gorman <mgorman@techsingularity.net> - 2017-03-19 21:10 +0100
  [HMM 06/16] mm/migrate: add new boolean copy flag to migratepage() callback Jérôme Glisse <jglisse@redhat.com> - 2017-03-16 16:30 +0100
    Re: [HMM 06/16] mm/migrate: add new boolean copy flag to  migratepage() callback Mel Gorman <mgorman@techsingularity.net> - 2017-03-19 21:10 +0100
  Re: [HMM 00/16] HMM (Heterogeneous Memory Management) v18 Andrew Morton <akpm@linux-foundation.org> - 2017-03-16 21:50 +0100
    Re: [HMM 00/16] HMM (Heterogeneous Memory Management) v18 Jerome Glisse <jglisse@redhat.com> - 2017-03-17 01:00 +0100
      Re: [HMM 00/16] HMM (Heterogeneous Memory Management) v18 Bob Liu <liubo95@huawei.com> - 2017-03-17 09:30 +0100
        Re: [HMM 00/16] HMM (Heterogeneous Memory Management) v18 Jerome Glisse <jglisse@redhat.com> - 2017-03-17 17:00 +0100
      Re: [HMM 00/16] HMM (Heterogeneous Memory Management) v18 Bob Liu <liubo95@huawei.com> - 2017-03-17 09:50 +0100
        Re: [HMM 00/16] HMM (Heterogeneous Memory Management) v18 Jerome Glisse <jglisse@redhat.com> - 2017-03-17 17:20 +0100
  Re: [HMM 00/16] HMM (Heterogeneous Memory Management) v18 Mel Gorman <mgorman@techsingularity.net> - 2017-03-19 21:40 +0100

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