From 1194372db6f3c917c9c6f6907e8378cf1076c557 Mon Sep 17 00:00:00 2001 From: Alexander Gordeev Date: Thu, 4 Nov 2021 07:14:45 +0100 Subject: s390/mm: better annotate 2KB pagetable fragments handling Explicitly encode immediate value of pending remove nibble (bits 31-28) and tracking nibble (bits 27-24) of the page refcount whenever these nibbles are tested or changed, for better readability. Also, add some comments describing how the fragments are handled. Reviewed-by: Gerald Schaefer Signed-off-by: Alexander Gordeev Signed-off-by: Heiko Carstens --- arch/s390/mm/pgalloc.c | 127 +++++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 107 insertions(+), 20 deletions(-) (limited to 'arch/s390/mm') diff --git a/arch/s390/mm/pgalloc.c b/arch/s390/mm/pgalloc.c index 457c80924542..5d5549843c5c 100644 --- a/arch/s390/mm/pgalloc.c +++ b/arch/s390/mm/pgalloc.c @@ -176,7 +176,75 @@ void page_table_free_pgste(struct page *page) #endif /* CONFIG_PGSTE */ /* - * page table entry allocation/free routines. + * A 2KB-pgtable is either upper or lower half of a normal page. + * The second half of the page may be unused or used as another + * 2KB-pgtable. + * + * Whenever possible the parent page for a new 2KB-pgtable is picked + * from the list of partially allocated pages mm_context_t::pgtable_list. + * In case the list is empty a new parent page is allocated and added to + * the list. + * + * When a parent page gets fully allocated it contains 2KB-pgtables in both + * upper and lower halves and is removed from mm_context_t::pgtable_list. + * + * When 2KB-pgtable is freed from to fully allocated parent page that + * page turns partially allocated and added to mm_context_t::pgtable_list. + * + * If 2KB-pgtable is freed from the partially allocated parent page that + * page turns unused and gets removed from mm_context_t::pgtable_list. + * Furthermore, the unused parent page is released. + * + * As follows from the above, no unallocated or fully allocated parent + * pages are contained in mm_context_t::pgtable_list. + * + * The upper byte (bits 24-31) of the parent page _refcount is used + * for tracking contained 2KB-pgtables and has the following format: + * + * PP AA + * 01234567 upper byte (bits 24-31) of struct page::_refcount + * || || + * || |+--- upper 2KB-pgtable is allocated + * || +---- lower 2KB-pgtable is allocated + * |+------- upper 2KB-pgtable is pending for removal + * +-------- lower 2KB-pgtable is pending for removal + * + * (See commit 620b4e903179 ("s390: use _refcount for pgtables") on why + * using _refcount is possible). + * + * When 2KB-pgtable is allocated the corresponding AA bit is set to 1. + * The parent page is either: + * - added to mm_context_t::pgtable_list in case the second half of the + * parent page is still unallocated; + * - removed from mm_context_t::pgtable_list in case both hales of the + * parent page are allocated; + * These operations are protected with mm_context_t::lock. + * + * When 2KB-pgtable is deallocated the corresponding AA bit is set to 0 + * and the corresponding PP bit is set to 1 in a single atomic operation. + * Thus, PP and AA bits corresponding to the same 2KB-pgtable are mutually + * exclusive and may never be both set to 1! + * The parent page is either: + * - added to mm_context_t::pgtable_list in case the second half of the + * parent page is still allocated; + * - removed from mm_context_t::pgtable_list in case the second half of + * the parent page is unallocated; + * These operations are protected with mm_context_t::lock. + * + * It is important to understand that mm_context_t::lock only protects + * mm_context_t::pgtable_list and AA bits, but not the parent page itself + * and PP bits. + * + * Releasing the parent page happens whenever the PP bit turns from 1 to 0, + * while both AA bits and the second PP bit are already unset. Then the + * parent page does not contain any 2KB-pgtable fragment anymore, and it has + * also been removed from mm_context_t::pgtable_list. It is safe to release + * the page therefore. + * + * PGSTE memory spaces use full 4KB-pgtables and do not need most of the + * logic described above. Both AA bits are set to 1 to denote a 4KB-pgtable + * while the PP bits are never used, nor such a page is added to or removed + * from mm_context_t::pgtable_list. */ unsigned long *page_table_alloc(struct mm_struct *mm) { @@ -192,14 +260,23 @@ unsigned long *page_table_alloc(struct mm_struct *mm) page = list_first_entry(&mm->context.pgtable_list, struct page, lru); mask = atomic_read(&page->_refcount) >> 24; - mask = (mask | (mask >> 4)) & 3; - if (mask != 3) { + /* + * The pending removal bits must also be checked. + * Failure to do so might lead to an impossible + * value of (i.e 0x13 or 0x23) written to _refcount. + * Such values violate the assumption that pending and + * allocation bits are mutually exclusive, and the rest + * of the code unrails as result. That could lead to + * a whole bunch of races and corruptions. + */ + mask = (mask | (mask >> 4)) & 0x03U; + if (mask != 0x03U) { table = (unsigned long *) page_to_virt(page); bit = mask & 1; /* =1 -> second 2K */ if (bit) table += PTRS_PER_PTE; atomic_xor_bits(&page->_refcount, - 1U << (bit + 24)); + 0x01U << (bit + 24)); list_del(&page->lru); } } @@ -220,12 +297,12 @@ unsigned long *page_table_alloc(struct mm_struct *mm) table = (unsigned long *) page_to_virt(page); if (mm_alloc_pgste(mm)) { /* Return 4K page table with PGSTEs */ - atomic_xor_bits(&page->_refcount, 3 << 24); + atomic_xor_bits(&page->_refcount, 0x03U << 24); memset64((u64 *)table, _PAGE_INVALID, PTRS_PER_PTE); memset64((u64 *)table + PTRS_PER_PTE, 0, PTRS_PER_PTE); } else { /* Return the first 2K fragment of the page */ - atomic_xor_bits(&page->_refcount, 1 << 24); + atomic_xor_bits(&page->_refcount, 0x01U << 24); memset64((u64 *)table, _PAGE_INVALID, 2 * PTRS_PER_PTE); spin_lock_bh(&mm->context.lock); list_add(&page->lru, &mm->context.pgtable_list); @@ -244,19 +321,24 @@ void page_table_free(struct mm_struct *mm, unsigned long *table) /* Free 2K page table fragment of a 4K page */ bit = ((unsigned long) table & ~PAGE_MASK)/(PTRS_PER_PTE*sizeof(pte_t)); spin_lock_bh(&mm->context.lock); + /* + * Mark the page for delayed release. The actual release + * will happen outside of the critical section from this + * function or from __tlb_remove_table() + */ mask = atomic_xor_bits(&page->_refcount, 0x11U << (bit + 24)); mask >>= 24; - if (mask & 3) + if (mask & 0x03U) list_add(&page->lru, &mm->context.pgtable_list); else list_del(&page->lru); spin_unlock_bh(&mm->context.lock); mask = atomic_xor_bits(&page->_refcount, 0x10U << (bit + 24)); mask >>= 24; - if (mask != 0) + if (mask != 0x00U) return; } else { - atomic_xor_bits(&page->_refcount, 3U << 24); + atomic_xor_bits(&page->_refcount, 0x03U << 24); } pgtable_pte_page_dtor(page); @@ -274,43 +356,48 @@ void page_table_free_rcu(struct mmu_gather *tlb, unsigned long *table, page = virt_to_page(table); if (mm_alloc_pgste(mm)) { gmap_unlink(mm, table, vmaddr); - table = (unsigned long *) ((unsigned long)table | 3); + table = (unsigned long *) ((unsigned long)table | 0x03U); tlb_remove_table(tlb, table); return; } bit = ((unsigned long) table & ~PAGE_MASK) / (PTRS_PER_PTE*sizeof(pte_t)); spin_lock_bh(&mm->context.lock); + /* + * Mark the page for delayed release. The actual release will happen + * outside of the critical section from __tlb_remove_table() or from + * page_table_free() + */ mask = atomic_xor_bits(&page->_refcount, 0x11U << (bit + 24)); mask >>= 24; - if (mask & 3) + if (mask & 0x03U) list_add_tail(&page->lru, &mm->context.pgtable_list); else list_del(&page->lru); spin_unlock_bh(&mm->context.lock); - table = (unsigned long *) ((unsigned long) table | (1U << bit)); + table = (unsigned long *) ((unsigned long) table | (0x01U << bit)); tlb_remove_table(tlb, table); } void __tlb_remove_table(void *_table) { - unsigned int mask = (unsigned long) _table & 3; + unsigned int mask = (unsigned long) _table & 0x03U; void *table = (void *)((unsigned long) _table ^ mask); struct page *page = virt_to_page(table); switch (mask) { - case 0: /* pmd, pud, or p4d */ + case 0x00U: /* pmd, pud, or p4d */ free_pages((unsigned long) table, 2); break; - case 1: /* lower 2K of a 4K page table */ - case 2: /* higher 2K of a 4K page table */ + case 0x01U: /* lower 2K of a 4K page table */ + case 0x02U: /* higher 2K of a 4K page table */ mask = atomic_xor_bits(&page->_refcount, mask << (4 + 24)); mask >>= 24; - if (mask != 0) + if (mask != 0x00U) break; fallthrough; - case 3: /* 4K page table with pgstes */ - if (mask & 3) - atomic_xor_bits(&page->_refcount, 3 << 24); + case 0x03U: /* 4K page table with pgstes */ + if (mask & 0x03U) + atomic_xor_bits(&page->_refcount, 0x03U << 24); pgtable_pte_page_dtor(page); __free_page(page); break; -- cgit v1.2.3-59-g8ed1b