From: Martin Schwidefsky From: Hubertus Franke From: Himanshu Raj The volatile state for page cache and swap cache pages requires that the host system needs to be able to determine if a volatile page is dirty before removing it. This excludes almost all platforms from using the scheme. What is needed is a way to distinguish between pages that are purely read-only and pages that might get written to. This allows platforms with per-pte dirty bits to use the scheme and platforms with per-page dirty bits a small optimization. Whenever a writable pte is created a check is added that allows to move the page into the correct state. This needs to be done before the writable pte is established. To avoid unnecessary state transitions and the need for a counter, a new page flag PG_writable is added. Only the creation of the first writable pte will do a page state change. Even if all the writable ptes pointing to a page are removed again, the page stays in the safe state until all read-only users of the page have unmapped it as well. Only then is the PG_writable bit reset. The state a page needs to have if a writable pte is present depends on the platform. A platform with per-pte dirty bits wants to move the page into stable state, a platform with per-page dirty bits like s390 can decide to move the page into a special state that requires the host system to check the dirty bit before discarding a page. Signed-off-by: Martin Schwidefsky --- fs/exec.c | 1 include/linux/page-flags.h | 6 ++++ include/linux/page-states.h | 27 +++++++++++++++++++- mm/memory.c | 5 +++ mm/mprotect.c | 2 + mm/page-states.c | 58 ++++++++++++++++++++++++++++++++++++++++++-- mm/page_alloc.c | 3 +- mm/rmap.c | 1 8 files changed, 99 insertions(+), 4 deletions(-) Index: linux-2.6/fs/exec.c =================================================================== --- linux-2.6.orig/fs/exec.c +++ linux-2.6/fs/exec.c @@ -51,6 +51,7 @@ #include #include #include +#include #include #include Index: linux-2.6/include/linux/page-flags.h =================================================================== --- linux-2.6.orig/include/linux/page-flags.h +++ linux-2.6/include/linux/page-flags.h @@ -109,6 +109,7 @@ #endif #define PG_discarded 20 /* Page discarded by the hypervisor. */ +#define PG_writable 21 /* Page is mapped writable. */ /* * Manipulation of page state flags @@ -309,6 +310,11 @@ static inline void __ClearPageTail(struc #define TestSetPageDiscarded(page) 0 #endif +#define PageWritable(page) test_bit(PG_writable, &(page)->flags) +#define TestSetPageWritable(page) \ + test_and_set_bit(PG_writable, &(page)->flags) +#define ClearPageWritable(page) clear_bit(PG_writable, &(page)->flags) + struct page; /* forward declaration */ extern void cancel_dirty_page(struct page *page, unsigned int account_size); Index: linux-2.6/include/linux/page-states.h =================================================================== --- linux-2.6.orig/include/linux/page-states.h +++ linux-2.6/include/linux/page-states.h @@ -57,6 +57,9 @@ extern void page_discard(struct page *pa extern int __page_make_stable(struct page *page); extern void __page_make_volatile(struct page *page, int offset); extern void __pagevec_make_volatile(struct pagevec *pvec); +extern void __page_check_writable(struct page *page, pte_t pte, + unsigned int offset); +extern void __page_reset_writable(struct page *page); /* * Extended guest page hinting functions defined by using the @@ -78,6 +81,12 @@ extern void __pagevec_make_volatile(stru * from the LRU list and the radix tree of its mapping. * page_discard uses page_unmap_all to remove all page table * entries for a page. + * - page_check_writable: + * Checks if the page states needs to be adapted because a new + * writable page table entry refering to the page is established. + * - page_reset_writable: + * Resets the page state after the last writable page table entry + * refering to the page has been removed. */ static inline int page_make_stable(struct page *page) @@ -97,12 +106,26 @@ static inline void pagevec_make_volatile __pagevec_make_volatile(pvec); } +static inline void page_check_writable(struct page *page, pte_t pte, + unsigned int offset) +{ + if (page_host_discards() && pte_write(pte) && + !test_bit(PG_writable, &page->flags)) + __page_check_writable(page, pte, offset); +} + +static inline void page_reset_writable(struct page *page) +{ + if (page_host_discards() && test_bit(PG_writable, &page->flags)) + __page_reset_writable(page); +} + #else #define page_host_discards() (0) #define page_set_unused(_page,_order) do { } while (0) #define page_set_stable(_page,_order) do { } while (0) -#define page_set_volatile(_page) do { } while (0) +#define page_set_volatile(_page,_writable) do { } while (0) #define page_set_stable_if_present(_page) (1) #define page_discarded(_page) (0) #define page_volatile(_page) (0) @@ -117,6 +140,8 @@ static inline void pagevec_make_volatile #define page_make_volatile(_page, offset) do { } while (0) #define pagevec_make_volatile(_pagevec) do { } while (0) #define page_discard(_page) do { } while (0) +#define page_check_writable(_page,_pte,_off) do { } while (0) +#define page_reset_writable(_page) do { } while (0) #endif Index: linux-2.6/mm/memory.c =================================================================== --- linux-2.6.orig/mm/memory.c +++ linux-2.6/mm/memory.c @@ -1681,6 +1681,7 @@ static int do_wp_page(struct mm_struct * flush_cache_page(vma, address, pte_pfn(orig_pte)); entry = pte_mkyoung(orig_pte); entry = maybe_mkwrite(pte_mkdirty(entry), vma); + page_check_writable(old_page, entry, 1); if (ptep_set_access_flags(vma, address, page_table, entry,1)) update_mmu_cache(vma, address, entry); ret |= VM_FAULT_WRITE; @@ -1728,6 +1729,7 @@ gotten: flush_cache_page(vma, address, pte_pfn(orig_pte)); entry = mk_pte(new_page, vma->vm_page_prot); entry = maybe_mkwrite(pte_mkdirty(entry), vma); + page_check_writable(new_page, entry, 2); /* * Clear the pte entry and flush it first, before updating the * pte with the new entry. This will avoid a race condition @@ -2147,6 +2149,7 @@ static int do_swap_page(struct mm_struct } flush_icache_page(vma, page); + page_check_writable(page, pte, 2); set_pte_at(mm, address, page_table, pte); page_add_anon_rmap(page, vma, address); @@ -2204,6 +2207,7 @@ static int do_anonymous_page(struct mm_s entry = mk_pte(page, vma->vm_page_prot); entry = maybe_mkwrite(pte_mkdirty(entry), vma); + page_check_writable(page, entry, 2); page_table = pte_offset_map_lock(mm, pmd, address, &ptl); if (!pte_none(*page_table)) @@ -2365,6 +2369,7 @@ retry: entry = mk_pte(page, vma->vm_page_prot); if (flags & FAULT_FLAG_WRITE) entry = maybe_mkwrite(pte_mkdirty(entry), vma); + page_check_writable(page, entry, 2); set_pte_at(mm, address, page_table, entry); if (anon) { inc_mm_counter(mm, anon_rss); Index: linux-2.6/mm/mprotect.c =================================================================== --- linux-2.6.orig/mm/mprotect.c +++ linux-2.6/mm/mprotect.c @@ -21,6 +21,7 @@ #include #include #include +#include #include #include #include @@ -52,6 +53,7 @@ static void change_pte_range(struct mm_s */ if (dirty_accountable && pte_dirty(ptent)) ptent = pte_mkwrite(ptent); + page_check_writable(pte_page(ptent), ptent, 1); set_pte_at(mm, addr, pte, ptent); #ifdef CONFIG_MIGRATION } else if (!pte_file(oldpte)) { Index: linux-2.6/mm/page_alloc.c =================================================================== --- linux-2.6.orig/mm/page_alloc.c +++ linux-2.6/mm/page_alloc.c @@ -637,7 +637,8 @@ static int prep_new_page(struct page *pa page->flags &= ~(1 << PG_uptodate | 1 << PG_error | 1 << PG_readahead | 1 << PG_referenced | 1 << PG_arch_1 | - 1 << PG_owner_priv_1 | 1 << PG_mappedtodisk); + 1 << PG_owner_priv_1 | 1 << PG_mappedtodisk | + 1 << PG_writable); set_page_private(page, 0); set_page_refcounted(page); Index: linux-2.6/mm/page-states.c =================================================================== --- linux-2.6.orig/mm/page-states.c +++ linux-2.6/mm/page-states.c @@ -82,7 +82,7 @@ void __page_make_volatile(struct page *p preempt_disable(); if (!page_test_set_state_change(page)) { if (check_bits(page) && check_counts(page, offset)) - page_set_volatile(page); + page_set_volatile(page, PageWritable(page)); page_clear_state_change(page); } preempt_enable(); @@ -108,7 +108,7 @@ void __pagevec_make_volatile(struct page page = pvec->pages[i]; if (!page_test_set_state_change(page)) { if (check_bits(page) && check_counts(page, 1)) - page_set_volatile(page); + page_set_volatile(page, PageWritable(page)); page_clear_state_change(page); } } @@ -141,6 +141,60 @@ int __page_make_stable(struct page *page EXPORT_SYMBOL(__page_make_stable); /** + * __page_check_writable() - check page state for new writable pte + * + * @page: the page the new writable pte refers to + * @pte: the new writable pte + */ +void __page_check_writable(struct page *page, pte_t pte, unsigned int offset) +{ + int count_ok = 0; + + preempt_disable(); + while (page_test_set_state_change(page)) + cpu_relax(); + + if (!TestSetPageWritable(page)) { + count_ok = check_counts(page, offset); + if (check_bits(page) && count_ok) + page_set_volatile(page, 1); + else + /* + * If two processes create a write mapping at the + * same time check_counts will return false or if + * the page is currently isolated from the LRU + * check_bits will return false but the page might + * be in volatile state. + * We have to take care about the dirty bit so the + * only option left is to make the page stable but + * we can try to make it volatile a bit later. + */ + page_set_stable_if_present(page); + } + page_clear_state_change(page); + if (!count_ok) + page_make_volatile(page, 1); + preempt_enable(); +} +EXPORT_SYMBOL(__page_check_writable); + +/** + * __page_reset_writable() - clear the PageWritable bit + * + * @page: the page + */ +void __page_reset_writable(struct page *page) +{ + preempt_disable(); + if (!page_test_set_state_change(page)) { + ClearPageWritable(page); + page_clear_state_change(page); + } + preempt_enable(); +} +EXPORT_SYMBOL(__page_reset_writable); + +/** * __page_discard() - remove a discarded page from the cache * * @page: the page Index: linux-2.6/mm/rmap.c =================================================================== --- linux-2.6.orig/mm/rmap.c +++ linux-2.6/mm/rmap.c @@ -695,6 +695,7 @@ void page_remove_rmap(struct page *page, __dec_zone_page_state(page, PageAnon(page) ? NR_ANON_PAGES : NR_FILE_MAPPED); + page_reset_writable(page); } } -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/