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SubjectRe: [patch 1/2] [PATCH] mm: Save soft-dirty bits on swapped pages
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@amacapital.net>
> Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
> Cc: Pavel Emelyanov <xemul@parallels.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Matt Mackall <mpm@selenic.com>
> Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
> Cc: Stephen Rothwell <sfr@canb.auug.org.au>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> ---
> 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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