Re: [PATCH 2/3] mm/migrate: see hole as invalid source page

[Jerome Glisse]

On Tue, Aug 06, 2019 at 04:00:10PM +0800, Pingfan Liu wrote:
> MIGRATE_PFN_MIGRATE marks a valid pfn, further more, suitable to migrate.
> As for hole, there is no valid pfn, not to mention migration.
> 
> Before this patch, hole has already relied on the following code to be
> filtered out. Hence it is more reasonable to see hole as invalid source
> page.
> migrate_vma_prepare()
> {
> 		struct page *page = migrate_pfn_to_page(migrate->src[i]);
> 
> 		if (!page || (migrate->src[i] & MIGRATE_PFN_MIGRATE))
> 		     \_ this condition
> }

NAK you break the API, MIGRATE_PFN_MIGRATE is use for 2 things,
first it allow the collection code to mark entry that can be
migrated, then it use by driver to allow driver to skip migration
for some entry (for whatever reason the driver might have), we
still need to keep the entry and not clear it so that we can
cleanup thing (ie remove migration pte entry).

> 
> Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> To: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  mm/migrate.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c2ec614..832483f 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2136,10 +2136,9 @@ static int migrate_vma_collect_hole(unsigned long start,
>  	unsigned long addr;
>  
>  	for (addr = start & PAGE_MASK; addr < end; addr += PAGE_SIZE) {
> -		migrate->src[migrate->npages] = MIGRATE_PFN_MIGRATE;
> +		migrate->src[migrate->npages] = 0;
>  		migrate->dst[migrate->npages] = 0;
>  		migrate->npages++;
> -		migrate->cpages++;
>  	}
>  
>  	return 0;
> @@ -2228,8 +2227,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  		pfn = pte_pfn(pte);
>  
>  		if (pte_none(pte)) {
> -			mpfn = MIGRATE_PFN_MIGRATE;
> -			migrate->cpages++;
> +			mpfn = 0;
>  			goto next;
>  		}
>  
> -- 
> 2.7.5
>