Re: [patch 1/2] [PATCH] mm: Save soft-dirty bits on swapped pages

[Minchan Kim]

Hello,

On Wed, Jul 31, 2013 at 12:41:55AM +0400, Cyrill Gorcunov wrote:
> Andy Lutomirski reported that in case if a page with _PAGE_SOFT_DIRTY
> bit set get swapped out, the bit is getting lost and no longer
> available when pte read back.
> 
> To resolve this we introduce _PTE_SWP_SOFT_DIRTY bit which is
> saved in pte entry for the page being swapped out. When such page
> is to be read back from a swap cache we check for bit presence
> and if it's there we clear it and restore the former _PAGE_SOFT_DIRTY
> bit back.
> 
> One of the problem was to find a place in pte entry where we can
> save the _PTE_SWP_SOFT_DIRTY bit while page is in swap. The
> _PAGE_PSE was chosen for that, it doesn't intersect with swap
> entry format stored in pte.
> 
> Reported-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Signed-off-by: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> 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>
> ---
>  arch/x86/include/asm/pgtable.h       |   15 +++++++++++++++
>  arch/x86/include/asm/pgtable_types.h |   13 +++++++++++++
>  fs/proc/task_mmu.c                   |   21 +++++++++++++++------
>  include/asm-generic/pgtable.h        |   15 +++++++++++++++
>  include/linux/swapops.h              |    2 ++
>  mm/memory.c                          |    2 ++
>  mm/rmap.c                            |    6 +++++-
>  mm/swapfile.c                        |   19 +++++++++++++++++--
>  8 files changed, 84 insertions(+), 9 deletions(-)
> 
> Index: linux-2.6.git/arch/x86/include/asm/pgtable.h
> ===================================================================
> --- linux-2.6.git.orig/arch/x86/include/asm/pgtable.h
> +++ linux-2.6.git/arch/x86/include/asm/pgtable.h
> @@ -314,6 +314,21 @@ static inline pmd_t pmd_mksoft_dirty(pmd
>  	return pmd_set_flags(pmd, _PAGE_SOFT_DIRTY);
>  }
>  
> +static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
> +{
> +	return pte_set_flags(pte, _PAGE_SWP_SOFT_DIRTY);
> +}
> +
> +static inline int pte_swp_soft_dirty(pte_t pte)
> +{
> +	return pte_flags(pte) & _PAGE_SWP_SOFT_DIRTY;
> +}
> +
> +static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
> +{
> +	return pte_clear_flags(pte, _PAGE_SWP_SOFT_DIRTY);
> +}
> +
>  /*
>   * Mask out unsupported bits in a present pgprot.  Non-present pgprots
>   * can use those bits for other purposes, so leave them be.
> Index: linux-2.6.git/arch/x86/include/asm/pgtable_types.h
> ===================================================================
> --- linux-2.6.git.orig/arch/x86/include/asm/pgtable_types.h
> +++ linux-2.6.git/arch/x86/include/asm/pgtable_types.h
> @@ -67,6 +67,19 @@
>  #define _PAGE_SOFT_DIRTY	(_AT(pteval_t, 0))
>  #endif
>  
> +/*
> + * Tracking soft dirty bit when a page goes to a swap is tricky.
> + * We need a bit which can be stored in pte _and_ not conflict
> + * with swap entry format. On x86 bits 6 and 7 are *not* involved
> + * into swap entry computation, but bit 6 is used for nonlinear
> + * file mapping, so we borrow bit 7 for soft dirty tracking.
> + */
> +#ifdef CONFIG_MEM_SOFT_DIRTY
> +#define _PAGE_SWP_SOFT_DIRTY	_PAGE_PSE
> +#else
> +#define _PAGE_SWP_SOFT_DIRTY	(_AT(pteval_t, 0))
> +#endif
> +
>  #if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
>  #define _PAGE_NX	(_AT(pteval_t, 1) << _PAGE_BIT_NX)
>  #else
> Index: linux-2.6.git/fs/proc/task_mmu.c
> ===================================================================
> --- linux-2.6.git.orig/fs/proc/task_mmu.c
> +++ linux-2.6.git/fs/proc/task_mmu.c
> @@ -730,8 +730,14 @@ static inline void clear_soft_dirty(stru
>  	 * of how soft-dirty works.
>  	 */
>  	pte_t ptent = *pte;
> -	ptent = pte_wrprotect(ptent);
> -	ptent = pte_clear_flags(ptent, _PAGE_SOFT_DIRTY);
> +
> +	if (pte_present(ptent)) {
> +		ptent = pte_wrprotect(ptent);
> +		ptent = pte_clear_flags(ptent, _PAGE_SOFT_DIRTY);
> +	} else if (is_swap_pte(ptent)) {
> +		ptent = pte_swp_clear_soft_dirty(ptent);
> +	}
> +
>  	set_pte_at(vma->vm_mm, addr, pte, ptent);
>  #endif
>  }
> @@ -752,14 +758,15 @@ static int clear_refs_pte_range(pmd_t *p
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>  	for (; addr != end; pte++, addr += PAGE_SIZE) {
>  		ptent = *pte;
> -		if (!pte_present(ptent))
> -			continue;
>  
>  		if (cp->type == CLEAR_REFS_SOFT_DIRTY) {
>  			clear_soft_dirty(vma, addr, pte);
>  			continue;
>  		}
>  
> +		if (!pte_present(ptent))
> +			continue;
> +
>  		page = vm_normal_page(vma, addr, ptent);
>  		if (!page)
>  			continue;
> @@ -930,8 +937,10 @@ static void pte_to_pagemap_entry(pagemap
>  		flags = PM_PRESENT;
>  		page = vm_normal_page(vma, addr, pte);
>  	} else if (is_swap_pte(pte)) {
> -		swp_entry_t entry = pte_to_swp_entry(pte);
> -
> +		swp_entry_t entry;
> +		if (pte_swp_soft_dirty(pte))
> +			flags2 |= __PM_SOFT_DIRTY;
> +		entry = pte_to_swp_entry(pte);
>  		frame = swp_type(entry) |
>  			(swp_offset(entry) << MAX_SWAPFILES_SHIFT);
>  		flags = PM_SWAP;
> Index: linux-2.6.git/include/asm-generic/pgtable.h
> ===================================================================
> --- linux-2.6.git.orig/include/asm-generic/pgtable.h
> +++ linux-2.6.git/include/asm-generic/pgtable.h
> @@ -417,6 +417,21 @@ static inline pmd_t pmd_mksoft_dirty(pmd
>  {
>  	return pmd;
>  }
> +
> +static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
> +{
> +	return pte;
> +}
> +
> +static inline int pte_swp_soft_dirty(pte_t pte)
> +{
> +	return 0;
> +}
> +
> +static inline pte_t pte_swp_clear_soft_dirty(pte_t pte)
> +{
> +	return pte;
> +}
>  #endif
>  
>  #ifndef __HAVE_PFNMAP_TRACKING
> Index: linux-2.6.git/include/linux/swapops.h
> ===================================================================
> --- linux-2.6.git.orig/include/linux/swapops.h
> +++ linux-2.6.git/include/linux/swapops.h
> @@ -67,6 +67,8 @@ static inline swp_entry_t pte_to_swp_ent
>  	swp_entry_t arch_entry;
>  
>  	BUG_ON(pte_file(pte));
> +	if (pte_swp_soft_dirty(pte))
> +		pte = pte_swp_clear_soft_dirty(pte);

Why do you remove soft-dirty flag whenever pte_to_swp_entry is called?
Isn't there any problem if we use mincore?

>  	arch_entry = __pte_to_swp_entry(pte);
>  	return swp_entry(__swp_type(arch_entry), __swp_offset(arch_entry));
>  }
> Index: linux-2.6.git/mm/memory.c
> ===================================================================
> --- linux-2.6.git.orig/mm/memory.c
> +++ linux-2.6.git/mm/memory.c
> @@ -3115,6 +3115,8 @@ static int do_swap_page(struct mm_struct
>  		exclusive = 1;
>  	}
>  	flush_icache_page(vma, page);
> +	if (pte_swp_soft_dirty(orig_pte))
> +		pte = pte_mksoft_dirty(pte);
>  	set_pte_at(mm, address, page_table, pte);
>  	if (page == swapcache)
>  		do_page_add_anon_rmap(page, vma, address, exclusive);
> Index: linux-2.6.git/mm/rmap.c
> ===================================================================
> --- linux-2.6.git.orig/mm/rmap.c
> +++ linux-2.6.git/mm/rmap.c
> @@ -1236,6 +1236,7 @@ int try_to_unmap_one(struct page *page,
>  			   swp_entry_to_pte(make_hwpoison_entry(page)));
>  	} else if (PageAnon(page)) {
>  		swp_entry_t entry = { .val = page_private(page) };
> +		pte_t swp_pte;
>  
>  		if (PageSwapCache(page)) {
>  			/*
> @@ -1264,7 +1265,10 @@ int try_to_unmap_one(struct page *page,
>  			BUG_ON(TTU_ACTION(flags) != TTU_MIGRATION);
>  			entry = make_migration_entry(page, pte_write(pteval));
>  		}
> -		set_pte_at(mm, address, pte, swp_entry_to_pte(entry));
> +		swp_pte = swp_entry_to_pte(entry);
> +		if (pte_soft_dirty(pteval))
> +			swp_pte = pte_swp_mksoft_dirty(swp_pte);
> +		set_pte_at(mm, address, pte, swp_pte);
>  		BUG_ON(pte_file(*pte));
>  	} else if (IS_ENABLED(CONFIG_MIGRATION) &&
>  		   (TTU_ACTION(flags) == TTU_MIGRATION)) {
> Index: linux-2.6.git/mm/swapfile.c
> ===================================================================
> --- linux-2.6.git.orig/mm/swapfile.c
> +++ linux-2.6.git/mm/swapfile.c
> @@ -866,6 +866,21 @@ unsigned int count_swap_pages(int type,
>  }
>  #endif /* CONFIG_HIBERNATION */
>  
> +static inline int maybe_same_pte(pte_t pte, pte_t swp_pte)

Nitpick.
If maybe_same_pte is used widely, it looks good to me
but it's used for only swapoff at the moment so I think pte_swap_same
would be better name.

> +{
> +#ifdef CONFIG_MEM_SOFT_DIRTY
> +	/*
> +	 * When pte keeps soft dirty bit the pte generated
> +	 * from swap entry does not has it, still it's same
> +	 * pte from logical point of view.
> +	 */
> +	pte_t swp_pte_dirty = pte_swp_mksoft_dirty(swp_pte);
> +	return pte_same(pte, swp_pte) || pte_same(pte, swp_pte_dirty);
> +#else
> +	return pte_same(pte, swp_pte);
> +#endif
> +}
> +
>  /*
>   * No need to decide whether this PTE shares the swap entry with others,
>   * just let do_wp_page work it out if a write is requested later - to
> @@ -892,7 +907,7 @@ static int unuse_pte(struct vm_area_stru
>  	}
>  
>  	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> -	if (unlikely(!pte_same(*pte, swp_entry_to_pte(entry)))) {
> +	if (unlikely(!maybe_same_pte(*pte, swp_entry_to_pte(entry)))) {
>  		mem_cgroup_cancel_charge_swapin(memcg);
>  		ret = 0;
>  		goto out;
> @@ -947,7 +962,7 @@ static int unuse_pte_range(struct vm_are
>  		 * swapoff spends a _lot_ of time in this loop!
>  		 * Test inline before going to call unuse_pte.
>  		 */
> -		if (unlikely(pte_same(*pte, swp_pte))) {
> +		if (unlikely(maybe_same_pte(*pte, swp_pte))) {
>  			pte_unmap(pte);
>  			ret = unuse_pte(vma, pmd, addr, entry, page);
>  			if (ret)
> 
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-- 
Kind regards,
Minchan Kim
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