Re: [patch 1/3] [PATCH] mm: migration -- Do not loose soft dirty bitif page is in migration state

[Naoya Horiguchi]

On Tue, Oct 08, 2013 at 01:00:20PM +0400, Cyrill Gorcunov wrote:
> If page migration is turne on in the config and the page is migrating,
> we may loose soft dirty bit. If fork and mprotect if called on migrating
> pages (once migration is complete) pages do not obtain soft dirty bit
> in correspond pte entries. Fix it adding appropriate test on swap
> entries.
> 
> Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Matt Mackall <mpm@xxxxxxxxxxx>
> Cc: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx>
> Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> ---
>  mm/memory.c   |    2 ++
>  mm/migrate.c  |    2 ++
>  mm/mprotect.c |    7 +++++--
>  3 files changed, 9 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6.git/mm/memory.c
> ===================================================================
> --- linux-2.6.git.orig/mm/memory.c
> +++ linux-2.6.git/mm/memory.c
> @@ -837,6 +837,8 @@ copy_one_pte(struct mm_struct *dst_mm, s
>  					 */
>  					make_migration_entry_read(&entry);
>  					pte = swp_entry_to_pte(entry);
> +					if (pte_swp_soft_dirty(*src_pte))
> +						pte = pte_swp_mksoft_dirty(pte);
>  					set_pte_at(src_mm, addr, src_pte, pte);
>  				}
>  			}

When we convert pte to swap_entry, we convert soft-dirty bit in
pte_to_swp_entry(). So I think that it's better to convert it back
in swp_entry_to_pte() when we do swap_entry-to-pte conversion.

Thanks,
Naoya Horiguchi

> Index: linux-2.6.git/mm/migrate.c
> ===================================================================
> --- linux-2.6.git.orig/mm/migrate.c
> +++ linux-2.6.git/mm/migrate.c
> @@ -161,6 +161,8 @@ static int remove_migration_pte(struct p
>  
>  	get_page(new);
>  	pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
> +	if (pte_swp_soft_dirty(*ptep))
> +		pte = pte_mksoft_dirty(pte);
>  	if (is_write_migration_entry(entry))
>  		pte = pte_mkwrite(pte);
>  #ifdef CONFIG_HUGETLB_PAGE
> Index: linux-2.6.git/mm/mprotect.c
> ===================================================================
> --- linux-2.6.git.orig/mm/mprotect.c
> +++ linux-2.6.git/mm/mprotect.c
> @@ -94,13 +94,16 @@ static unsigned long change_pte_range(st
>  			swp_entry_t entry = pte_to_swp_entry(oldpte);
>  
>  			if (is_write_migration_entry(entry)) {
> +				pte_t newpte;
>  				/*
>  				 * A protection check is difficult so
>  				 * just be safe and disable write
>  				 */
>  				make_migration_entry_read(&entry);
> -				set_pte_at(mm, addr, pte,
> -					swp_entry_to_pte(entry));
> +				newpte = swp_entry_to_pte(entry);
> +				if (pte_swp_soft_dirty(oldpte))
> +					newpte = pte_swp_mksoft_dirty(newpte);
> +				set_pte_at(mm, addr, pte, newpte);
>  			}
>  			pages++;
>  		}
> 
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